iii. epitaxial chemistry

7/29/2019 III. Epitaxial Chemistry

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Chemistry of Epitaxy


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Epitaxy is an interface between a thin film and asubstrate

The term epitaxy describes an ordered crystallinegrowth on a monocrystalline substrate

Epitaxial films may be grown from gaseous orliquid precursors

Because the substrate acts as a seed crystal, thedeposited film takes on a lattice structure andorientation identical to those of the substrate


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Epitaxy is different from other thin film depositionmethods which deposit polycrystalline or amorphousfilms, even on single - crystal substrates

If a film is deposited on a substrate of the samecomposition, the process is called homoepitaxy

Otherwise it is called heteroepitaxy

Homoepitaxy is a kind of epitaxy performed with onlyone material in which a crystalline film is grown on asubstrate or film of the same material

This technology is applied to growing a more

purified film than the substrate and fabricatinglayers with different doping levels


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Heteroepitaxy is a kind of epitaxy performed withmaterials that are different from each other in which acrystalline film grows on a crystalline substrate or

film of another material

This technology is often applied to growingcrystalline films of materials of which single crystalscannot be obtained and to fabricating integrated

crystalline layers of different materials

Examples include gallium nitride (GaN) onsapphire or aluminum gallium indium phosphide(AlGaInP) on gallium arsenide (GaAs)

Heterotopotaxy is a process similar toheteroepitaxy except for the fact that thin filmgrowth is not limited to two dimensional growth

In this process, the substrate is similar only instructure to the thin film material


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Epitaxy is used in silicon - based manufacturingprocesses for BJTs and modern CMOS, but it isparticularly important for compound semiconductors

such as gallium arsenide

Manufacturing issues include control of theamount and uniformity of the deposition'sresistivity and thickness, the cleanliness and

purity of the surface and the chamber atmosphere,the prevention of the typically much more highlydoped substrate wafer's diffusion of dopant to thenew layers, imperfections of the growth process,

and protecting the surfaces during themanufacture and handling


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Applications of Epitaxy

Epitaxy has applications in nanotechnology and in

semiconductor fabrication. Epitaxy is the only affordable method of high

crystalline quality growth for many semiconductormaterials, including technologically important

materials as silicon -germanium, gallium nitride,gallium arsenide and indium phosphide

Epitaxy is also used to grow layers of pre - dopedsilicon on the polished sides of silicon wafers, before

they are processed into semiconductor devices.


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Epitaxy is one of the most vital processes insemiconductor device manufacturing

This is especially true in nanotechnology, as itprovides the means of growing very thin films in acontrolled way to achieve the necessary accuracy,purity, and orientation of the film

Chemistry plays an important role in the process ofepitaxial layer growth

The constituents of the film are often presented tothe substrate in the form of compounds with otherelements

They must be extracted from these compoundsand react with the substrate and possibly other

constituents to form the epitaxial layer


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There are many approaches to growing epitaxial films

Vapor Phase Epitaxy (VPE)

Liquid Phase Epitaxy (LPE)

Metallorganic Chemical Vapor Deposition (MOCVD)

Molecular Beam Epitaxy (MBE)

Atomic Layer Epitaxy (ALE)

Several of these methods are based on ChemicalVapor Deposition (CVD)


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Chemical Vapor Deposition (CVD)

CVD is used to produce high - purity, high -performance solid materials, usually in the form of athin film on a substrate

In a typical CVD process, the wafer (substrate) isexposed to one or more volatile precursors, which

react and/or decompose on the substrate surface toproduce the desired deposit

Frequently, volatile byproducts are also produced,which are removed by gas flow through the reaction

chamber


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Microfabrication processes widely use CVD to depositmaterials in various forms, including: monocrystalline,polycrystalline, amorphous, and epitaxial

These materials include: silicon, carbon fiber, carbonnanofibers, filaments, carbon nanotubes, SiO2,silicon-germanium, tungsten, silicon carbide, siliconnitride , titanium nitride, and various high - k

dielectrics

The CVD process is also used to produce syntheticdiamonds


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Types of chemical vapor deposition

A number of forms of CVD are in wide use and arefrequently referenced in the literature

These processes differ in the means by whichchemical reactions are initiated (e.g., activationprocess) and process conditions


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These processes can be classified by operatingpressure

Atmospheric pressure CVD(APCVD) - CVD processes at

atmospheric pressure

Low-pressure CVD(LPCVD) - CVD processes atsubatmospheric pressures

Reduced pressures tend to reduce unwanted gas-

phase reactions and improve film uniformity acrossthe wafer

Most modern CVD process are either LPCVD orUHVCVD

Ultrahigh vacuum CVD(UHVCVD) - CVD processes at avery low pressure, typically below 10 -6 Pa (~ 10 -8 torr)


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Classified by physical characteristics of vapor

Aerosol assisted CVD(AACVD) - A CVD process in whichthe precursors are transported to the substrate by means

of a liquid/gas aerosol, which can be generatedultrasonically.

This technique is suitable for use with nonvolatileprecursors

Direct liquid injection CVD(DLICVD) - A CVD process in

which the precursors are in liquid form (liquid or soliddissolved in a convenient solvent)

Liquid solutions are injected in a vaporizationchamber towards injectors (typically car injectors).

The precursor vapors are then transported to thesubstrate as in classical CVD process

This technique is suitable for use on liquid or solidprecursors

High growth rates can be reached using this technique


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Microwave plasma-assisted CVD(MPCVD)

Plasma-Enhanced CVD(PECVD) - CVD processes that utilize aplasma to enhance chemical reaction rates of the precursors

PECVD processing allows deposition at lowertemperatures, which is often critical in the manufacture ofsemiconductors

Remote plasma-enhanced CVD(RPECVD) - Similar to PECVDexcept that the wafer substrate is not directly in the plasma

discharge region

Removing the wafer from the plasma region allowsprocessing temperatures down to room temperature

Atomic layer CVD(ALCVD) Deposits successive layers of

different substances to produce layered, crystalline films

Hot wire CVD(HWCVD) - Also known as Catalytic CVD (Cat-CVD) or hot filament CVD (HFCVD)

Uses a hot filament to chemically decompose the source

gases


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Metallorganic chemical vapor deposition(MOCVD) - CVDprocesses based on metallorganic precursors

Hybrid Physical-Chemical Vapor Deposition(HPCVD) - Vapordeposition processes that involve both chemical

decomposition of precursor gas and vaporization of solid asource

Rapid thermal CVD(RTCVD) - CVD processes that use heating

lamps or other methods to rapidly heat the wafer substrate

Heating only the substrate rather than the gas or chamberwalls helps reduce unwanted gas phase reactions that canlead to particle formation

Vapor phase epitaxy(VPE)


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Polysilicon

Polycrystalline silicon is widely used as the gate oxide inMOSFETs

Polycrystalline silicon is deposited from silane (SiH4), using thefollowing reaction:

This reaction is usually performed in LPCVD systems, with

either pure silane feedstock, or a solution of silane with 70-80%nitrogen

Temperatures between 600 and 650 C and pressures between25 and150 Pa yield a growth rate between 10 and 20 nm perminute. An alternative process uses a hydrogen - based

solution

The hydrogen reduces the growth rate, but the temperatureis raised to 850 or even 1050 C to compensate

Si H Si H 4 2

2


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Polysilicon may be grown directly with doping, if gases such asphosphine, arsine or diborane are added to the CVD chamber

Diborane increases the growth rate, but arsine and

phosphine decrease it


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TEOS

TEOS is a material commonly used to grow silicondioxide layers on semiconductors

It stands for Tetra - Ethyl - Ortho - Silicate, orequivalently tetra - ethoxy - silane:

TEOS slowlyhydrolyzes into silicondioxide and ethanolwhen in contact withambient moisture


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The key to understanding the difference betweenTEOS and silane is to note that in TEOS the siliconatom is already oxidized

The conversion of TEOS to silicon dioxide isessentially a rearrangement rather than anoxidation reaction, with much reduced changes infree enthalpy and free energy


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While gas phase reactions can occur, particularly atthe high end of the temperature range, deposition isprobably the result of TEOS surface reactions

TEOS chemisorbs onto silanol groups (Si-OH) at thesurface, as well as strained surface bonds


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TEOS will not adsorb onto the resulting alkyl-covered surface, so deposition is probably limited byremoval of the surface alkyl groups

These groups can undergo elimination reactionswith neighboring molecules to form Si-O-Si bridges


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This process proceeds in an inert atmosphere:TEOS can be its own oxygen source, and SiO2 canbe deposited from TEOS in nitrogen

However, addition of oxygen increases thedeposition rate, presumably through providing analternative path for removal of the ethyl groups fromthe surface

TEOS/O2 is generally performed in tube reactors atpressures of a few Torr


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Silicon dioxide

Silicon dioxide (SiO2) is commonly used in integrated circuitsand nanodevices as an insulator and as a capacitor dielectric

Silicon dioxide may be deposited by several different processes

Common source gases include silane and oxygen,dichlorosilane (SiCl2H2) and nitrous oxide (N2O), ortetraethylorthosilicate (TEOS; Si(OC2H5)4)

The reactions are as follows:

SiH4 + O2 SiO2 + 2H2

SiCl2H2 + 2N2O SiO2 + 2N2 + 2HCl

Si(OC2H5)4 SiO2 + byproducts


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The choice of source gas depends on the thermal stability of thesubstrate; for instance, aluminum is sensitive to hightemperature

Silane deposits at temperatures between 300 and 500 C,dichlorosilane at around 900 C, and TEOS between 650 and 750

C, resulting in a layer of Low Temperature Oxide (LTO)

However, silane produces a lower-quality oxide than the othermethods (lower dielectric strength, for instance), and it deposits

nonconformally

Any of these reactions may be used in LPCVD, but thesilane reaction is also done in APCVD

CVD oxide invariably has lower quality than thermal oxide,

but thermal oxidation can only be used in the earliest stagesof IC manufacturing


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Silicon dioxide may also be grown with impurities (alloying or"doping") for one of two purposes

(1) During further process steps that occur at high temperature,

the impurities may diffuse from the oxide into adjacent layers(most notably silicon) and dope them

Oxides containing 5% to 15% impurities by mass are oftenused for this purpose

(2) silicon dioxide alloyed with phosphorus pentoxide ("P-glass") can be used to smooth out uneven surfaces

P-glass softens and reflows at temperatures above 1000 C

This process requires a phosphorus concentration of atleast 6%, but concentrations above 8% can corrode

aluminum

Phosphorus is deposited from phosphine gas and oxygen:

4PH3 + 5O2 2P2O5 + 6H2


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Glasses containing both boron and phosphorus(borophosphosilicate glass, BPSG) undergo viscous flow at lowertemperatures; around 850 C is achievable with glasses containingaround 5 weight % of both constituents, but stability in air can be

difficult to achieve

Phosphorus oxide in high concentrations interacts with ambientmoisture to produce phosphoric acid

Crystals of BPO4 can also precipitate from the flowing glass on

cooling;

These crystals are not readily etched in the standardreactive plasmas used to pattern oxides, and will result incircuit defects in integrated circuit manufacturing


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Besides these intentional impurities, CVD oxide may containbyproducts of the deposition process.

TEOS produces a relatively pure oxide, whereas silane

introduces hydrogen impurities, and dichlorosilane introduceschlorine

Lower temperature deposition of silicon dioxide and doped glassesfrom TEOS using ozone rather than oxygen has also been explored(350 to 500 C)

Ozone glasses have excellent conformality but tend to be

hygroscopic -- that is, they absorb water from the air due to theincorporation of silanol (Si-OH) in the glass

Infrared spectroscopy and mechanical strain as a function of

temperature are valuable diagnostic tools for diagnosing suchproblems


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Silicon Nitride

Silicon nitride is often used as an insulator and chemical barrierin manufacturing ICs

Silicon nitride

The following two reactions deposit nitride from the gas phase:

3SiH4 + 4NH3 Si3N4 + 12H2

3SiCl2

H2

+ 4NH3

Si3

N4

+ 6HCl + 6H2

Silicon nitride deposited by LPCVD contains up to 8% hydrogen.

It also experiences strong tensile stress , which may crackfilms thicker than 200 nm

However, it has higher resistivity and dielectric strength thanmost insulators commonly available in microfabrication (1016cm and 10 MV/cm, respectively)


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Another two reactions may be used in plasma to deposit SiNH:

2SiH4 + N2 2SiNH + 3H2

SiH4 + NH3 SiNH + 3H2

These films have much less tensile stress, but worse electricalproperties (resistivity 106 to 1015cm, and dielectric strength 1 to5 MV/cm)


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Vapor-phase Epitaxy (VPE)

In VPE, one or more materials to be deposited aretransported to the substrate as compounds in vapor

form

In this manner, single materials, doped materials,or compounds may be deposited in single crystalform

Once the materials reach the substrate, they areextracted from the compound and attachthemselves to the surface atoms on the substrate

One of the most common examples of VPE is thegrowth of a doped silicon film on a silicon substrate

This process can be used to fabricate individualtransistors and to fabricate transistors and

isolation regions on integrated circuits


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There are four major chemical sources of silicon forcommercial epitaxial deposition:

1) silicon tetrachloride (SiCl4)

2) trichlorosilane (SiHCl3)

3) dichlorosilane (SiH2Cl2)

4) silane (SiH4)

Each of the chemical sources mentioned above maybe described by an over-all reaction equation thatshows how the vapor phase reactants form the siliconepitaxial film

For example, the over-all reaction for siliconepitaxy by silane reaction may be written asfollows: SiH4 Si + 2H2


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Silicon is most commonly deposited from silicontetrachloride in hydrogen at approximately 1200 C:

SiCl4(g) + 2H2(g) Si(s) + 4HCl(g)

This reaction is reversible, and the growth ratedepends strongly upon the proportion of the twosource gases

Growth rates above 2 m/minute producepolycrystalline silicon, and negative growth rates(etching) may occur if too much hydrogen chloridebyproduct is present

An additional etching reaction competes with thedeposition reaction:

SiCl4(g) + Si(s) 2SiCl2(g)


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The reaction is actually a complex series of reactionsthat ultimately result in the deposition of pure silicon

SiCl4 + H2 SiHCl3 + HCl;SiHCl3 + H2 SiH2Cl2 + HCl;SiH2Cl2 SiCl2 + H2;SiHCl3 SiCl2 + HCl;SiCl

2

+ H2

Si + 2HCl;


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Silicon VPE may also use silane, dichlorosilane, andtrichlorosilane source gases

For instance, the silane reaction occurs at 650 C inthis way:

SiH4 Si + 2H2

This reaction does not inadvertently etch the wafer,

and takes place at lower temperatures than depositionfrom silicon tetrachloride

However, it will form a polycrystalline film unlesstightly controlled, and it allows oxidizing species that

leak into the reactor to contaminate the epitaxial layerwith unwanted compounds such as silicon dioxide


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All reactants in vapor phase, deposited on heated substrate

Halide or hydride process:

I: GaAs (s) + HCl (g) GaCl (g) + As4 (g) + H2 (g)

II: 3 GaCl (g) + As4 (g) 2 GaAs (s) + GaCl3 (g)

III: GaCl (g) + As4 (g) + H2 (g) GaAs (s) + HCl (g)

Advantage: fast rate (.1 - .5 m.min), easy, safe (w/o arsineprocess)

Disadvantage: Al compounds difficult, thickness resolution

VPE (vapor phase epitaxy)

halide: AsCl3, H2, dopants

hydride: AsH3, H2, dopants

Ga metal

HCl

GaAs

I substrate

II, III

As4

halide: AsCl3, H2hydride: HCl, H2

Reducing atmosphere


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Epitaxial growth

LPE (liquid phase epitaxy)

Thermodynamic equilibrium growth

saturated melt (As in Ga)

cool which reduces solubility of As, so GaAs deposits

can do in bath melt, or slider technique

advantage; inexpensive, easy

disadvantages:

no in situdiagnostics

> binaries hard; x = x(t)

surface morphology

thickness control not very precise

direction of sliding

H2 reducing atmosphere


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Metallorganic Chemical Vapor Deposition(MOCVD)

Metallorganic Chemical Vapor Deposition (MOCVD)

is a method of epitaxial growth of materials,especially compound semiconductors, from thesurface reaction of organic compounds ormetallorganics and metal hydrides containing therequired chemical elements

For example, indium phosphide could be grown ina reactor on a substrate by introducingTrimethylindium ((CH3)3In) and phosphine (PH3)


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Formation of the epitaxial layer occurs by finalpyrolisis of the constituent chemicals at thesubstrate surface.

In contrast to molecular beam epitaxy (MBE) thegrowth of crystals is by chemical reaction and notphysical deposition.

This takes place not in a vacuum, but from the gasphase at moderate pressures (2 to 100 kPa)

As such this technique is preferred for the formationof devices incorporating thermodynamically

metastable alloys It has become the dominant process for the

manufacture of laser diodes, solar cells, and LEDs
http://en.wikipedia.org/wiki/Image:MOCVDprocess.jpg


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Growth Processof MOCVD
http://en.wikipedia.org/wiki/Image:MOCVDprocess.jpghttp://en.wikipedia.org/wiki/Image:MOCVDprocess.jpghttp://en.wikipedia.org/wiki/Image:MOCVDprocess.jpghttp://en.wikipedia.org/wiki/Image:GenericMOCVD.jpg


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MOCVD Reactor Block Diagram
http://en.wikipedia.org/wiki/Image:GenericMOCVD.jpg


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Reactor Components

A reactor is a chamber made of a high - temperaturematerial that does not react with the chemicals being

used

The chamber is composed of reactor walls, a liner, asusceptor, gas injection units, and temperaturecontrol units

The reactor walls are typically made from stainlesssteel or quartz

To prevent overheating, cooling water must flow

through the channels within the reactor walls

Special glasses, such as quartz or ceramic, areoften used as the liner in the reactor chamberbetween the reactor wall and the susceptor


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A substrate sits on a susceptorwhich is held at acontrolled temperature.

The susceptor is made from a material resistant

to the metalorganic compounds used, such asgraphite

For growing nitrides and related materials, aspecial coating on the graphite susceptor is

necessary to prevent corrosion by ammonia(NH3) gas


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Gas inlet and switching system

Gas is introduced via devices known as

'bubblers'. In a bubbler a carrier gas (usually nitrogen or

hydrogen) is bubbled through the metallorganicliquid, which picks up some metallorganic vapor

and transports it to the reactor The amount of metallorganic vapor transported

depends on the rate of carrier gas flow and thebubbler temperature

Allowance must be made for saturated vapors


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Gas Exhaust and cleaning System

Toxic waste products must be converted to liquid

or solid wastes for recycling (preferably) ordisposal

Ideally processes will be designed to minimizethe production of waste products


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Basic reaction for GaAs:Ga(CH3)3+AsH3 GaAs+3CH4Al(CH3)3+AsH3 AlAs+3CH4

For GaNGa(CH

3)3+NH

3GaN+3CH

4

Process:

1. MO sources and hydrides mixed inside reactor andtransferred to the substrate

2. high temperature of substrate results in thedecomposition of sources, forming the film precursors.

3. film precursors transport & absorb on the growth surface4. precursors diffuse to the growth site, incorporate5. by-products of the surface reactions absorb from surface

MOCVD Process


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MOCVD System


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I lid MBE lt l t h


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In solid - source MBE, ultra - pure elements such asgallium and arsenic are heated in separate quasi-Knudsen effusion cells until they begin to slowly

sublimate The gaseous elements then condense on the wafer,

where they may react with each other

In the example of gallium and arsenic, single-

crystal gallium arsenide is formed.

The term "beam" simply means that evaporatedatoms do not interact with each other or any othervacuum chamber gases until they reach the wafer,due to the long mean free paths of the atoms

During operation RHEED (Reflection High Energy


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During operation, RHEED (Reflection High EnergyElectron Diffraction) is often used for monitoring thegrowth of the crystal layers

A computer controls shutters in front of eachfurnace, allowing precise control of the thickness ofeach layer, down to a single layer of atoms.

Intricate structures of layers of different materials

may be fabricated in this manner

Such control has allowed the development ofstructures where the electrons can be confined inspace, giving quantum wells or even quantum dots

Such layers are now a critical part of many modernsemiconductor devices, including semiconductorlasers and light-emitting diodes

In systems where the substrate needs to be cooled


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In systems where the substrate needs to be cooled,the ultra-high vacuum environment within the growthchamber is maintained by a system of cryopumps

and cryopanels, chilled using liquid nitrogen or coldnitrogen gas to a temperature close to 77 oK(196 oC)

However, cryogenic temperatures act as a sink for

impurities in the vacuum, and so vacuum levels needto be several orders of magnitude better to depositfilms under these conditions

In other systems, the wafers on which the crystals

are grown may be mounted on a rotating platterwhich can be heated to several hundred oC duringoperation

Molecular beam epitaxy is also used for the


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Molecular beam epitaxy is also used for thedeposition of some types of organic semiconductors

In this case, molecules, rather than atoms, are

evaporated and deposited onto the wafer

Other variations include gas-source MBE, whichresembles chemical vapor deposition

MBE


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MBE


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GaAs

AlAs

AlAs

Growth process


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Growth process

UHV (< 10-8)

Knudsen sources

As flux, sticking coeff. < 0.5

growth ~ JIII; excess JV high As/Ga flux, low T - As stabilized

low As/Gas flux, high T - Ga stabilized

Congruent sublimation Tcs (C)GaAs 650AlAs ~850

AlP >700GaP 670InP 363InAs 380

if T < Tcs, group V stableif T > Tcs, group III stable

C


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Comparison of Epitaxial Methods

Some of the sources

like AsH3 are very toxic.

Use metallorganic

compounds as thesources

1968MOCVD

(Metal-OrganicChemical VaporDeposition)

Hard to grow materialswith high vaporpressure

Deposit epilayer atultrahigh vacuum

1958

1967

MBE

(Molecular BeamEpitaxy)

No Al containedcompound, thick layer

Use metal halide astransport agents to

grow

1958VPE

(Vapor phase

epitaxy

Limited substrate areasand poor control overthe growth of very thinlayers

Growth formsupersaturatedsolution ontosubstrate

1963LPE(Liquid phaseepitaxy)

limitfeaturestimeGrowth method



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Atomic layer epitaxy (ALE), or Atomic LayerDeposition (ALD), is a specialized form of epitaxy thattypically deposit alternating monolayers of twoelements onto a substrate, making it ideal to generatenanostructures

The crystal lattice structure achieved is thin, uniform,and aligned with the structure of the substrate

The reactants are brought to the substrate asalternating pulses with "dead" times in between. ALE

makes use of the fact that the incoming material isbound strongly until all sites available forchemisorption are occupied

The dead times are used to flush the excess material

Atomic layer epitaxy (ALE) or atomic layer deposition


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Atomic layer epitaxy (ALE) or atomic layer deposition(ALD) is a technique mostly used in semiconductorfabrication to grow thin films of thickness of the

atomic order The main approach used for this technique is the use

of a self limiting chemical reaction to control in a veryaccurate way the thickness of the film deposited

Compared to basic CVD for example, chemicalreactants are pulsed alternatively in a reactingchamber and then chemisorb on to the surface of thesubstrate in order to form the monolayer

The reaction is very easy to set up and doesntrequire that many restrictions over the reactants,allowing the use of a wide range of materials

ALE introduces two complementary precursors (e g


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ALE introduces two complementary precursors (e.g.Al(CH3)3 and H2O) alternatively into the reactionchamber.

Typically, one of the precursors will adsorb onto thesubstrate surface, but cannot completely decomposewithout the second precursor.

The precursor adsorbs until it saturates the surface

and further growth cannot occur until the secondprecursor is introduced

Thus the film thickness is controlled by the number ofprecursor cycles rather than the deposition time as is

the case for conventional CVD processes

In theory ALCVD allows for extremely precise controlof film thickness and uniformity

iii. epitaxial chemistry

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