CLASS 3

EPITAXY

What is Epitaxy?• Epitaxy: Deposition and growth of

monocrystalline structures/layers.

• Epitaxial growth results in monocrystalline layers differing from deposition which gives rise to polycrystalline and bulk structures.

• Epitaxy types:– Homoepitaxy: Substrate & material are

of same kind.

(Si-Si)

– Heteroepitaxy: Substrate & material are of different kinds. (Ga-As)

3

MBE growth mechanism *

MBE growth mechanism

• Epitaxy is the type of silicon depositionthat results in single crystal growth due tocontact with a suitable crystalline lattice.

• Epitaxy usually performed using thewafer, for economic reasons.

• Mechanism of epitaxial growth

• Methods of epitaxial deposition

• Properties of epitaxial layers

• Applications of epitaxial layers

Epitaxial Growth

• Deposition of a layer on a substrate which matches the crystalline order of the substrate

• Homoepitaxy– Growth of a layer of the same

material as the substrate

– Si on Si

• Heteroepitaxy– Growth of a layer of a

different material than the substrate

– GaAs on Si

Ordered, crystalline growth; NOT epitaxial

Epitaxial growth:

Why Epitaxy?

• Epitaxial growth is useful for applications that place stringent demands on a deposited layer:

– High purity

– Low defect density

– Abrupt interfaces

– Controlled doping profiles

– High repeatability and uniformity

– Safe, efficient operation

• Can create clean, fresh surface for device fabrication

General Epitaxial Deposition Requirements

• Surface preparation– Clean surface needed– Defects of surface duplicated in epitaxial layer– Hydrogen passivation of surface with water/HF

• Surface mobility– High temperature required heated substrate– Epitaxial temperature exists, above which deposition is ordered – Species need to be able to move into correct crystallographic

location– Relatively slow growth rates result

• Ex. ~0.4 to 4 nm/min., SiGe on Si

Thin film

Thin Film Growth

Why thin or thick films ?

What are thin and thick films ?

Methods of Preparation of films

physical routes

chemical routes

Formation of the film

Need for vacuum and how to achieve vacuum

Methods of determination of film thickness

Characterization of film

Thick film 500 nm and above

Thin Film 2 nm – 500 nm

Ultra thin films a few atomic layer

Thick and Thin films

No clear cut off for each region.

For different materials the physical properties determine the nature of the film

GLOW-DISCHARGE PROCESSES

Sputtering Plasma Processes

Diode sputtering Plasma-enhanced CVD

Reactive sputtering Plasma oxidation

Cluster beam deposition (CBD) Cathodic arc deposition

Reactive ion plating Microwave ECR plasma CVD

Magnetron sputtering Plasma polymerization

Ion beam sputter deposition Plasma reduction

Ion beam deposition Plasma nitridation

Bias sputtering (ion plating) Plasma anodization

GAS-PHASE CHEMICAL PROCESSES

•

Chemical Vapor Deposition (CVD) Thermal Forming Processes

CVD epitaxy Thermal oxidation

Atmospheric-pressure CVD (APCVD) Thermal nitridation

Low-pressure CVD (LPCVD) Thermal polymerization

Metalorgainc CVD (MOCVD)

Photo-enhanced CVD (PHCVD)

Laser-induced CVD (PCVD)

Electron-enhanced CVD Ion implantation

LIQUID-PHASE CHEMICAL TECHNIQUES

Electro Processes Mechanical Techniques

Electroplating Spary pyrolysis

Electroless plating Spray-on techniques

Electrolytic anodization Spin-on techniques

Chemical reduction plating

Chemical displacement plating

Electrophoretic deposition Liquid phase epitaxy

Steps in Film Formation

1.Thermal accommodation

2.binding

3.surface diffusion

4.nucleation

5.island growth

6.coalescence

7.continued growth

How do nuclei grow initially ?

Substrates are NOT flatsteps, kinks, etc. have higher Edes barrier => longer residence time on surface => preferred sites for nucleation

1. Island growth (Volmer - Weber)

•form three dimensional islands

•source:

•film atoms more strongly bound to each other than to

substrate

•and/or slow diffusion

2. Layer by layer growth (Frank - van der Merwe)

generally highest crystalline quality

•source:

•film atoms more strongly bound to substrate

than to each other

•and/or fast diffusion

3. Mixed growth (Stranski – Krastanov)

•initially layer by layer

•then forms three dimensional islands

•=> change in energetics

Structures

• matched

• Common in homoepitaxy, sometimes in heteroepitaxy

Structures•strained (pseudomorphy)

film grows with structure different from bulk

• not stable • at some thickness film will convert to bulk structure

• example: Co is hcp• can deposit as fcc up to one micron thick

• example: strained layer superlattices• can make materials with unusual properties

Structures

•relaxed •form edge dislocations

• strained vs. relaxed depends on minimizing energy of system

• strain energy vs. dislocation energy

Pump

Major pump types for vacuum applications.

Type Mechanical non- Mechanical

gas transfer rotary

turbomolecular

sorption

diffusion

entrapment cryo ion

sublimation

Gas transfer pumps remove gas permanently from the chamber,

entrapment do not.

Terminology

Pumping Speed (S) L/s

Pumping Rate (Q) mbar L/s

Leak Rate (QL) mbar L/s

Outgassing Rate (QO) mbar L/s

When designing a vacuum system, one of the most basic considerations is the time required to evacuate a vessel to a given pressure

Vacuum Pumps

Vacuum Pumps

Type of Pump range (mbar)

rotary 1000 - 0.001

sorption 1000 - 10–5

turbo (1000) - UHV

diffusion 10–5

- UHV

ion (1000) - UHV

cryro 10–5

- UHV

sublimation 10–7

- UHV

Range for different pumps

Film Thickness

• Quartz crystal frequency monitoring technique

• Multiple beam interference technique

• Ellipsometric technique

• Stylus technique

• Optical density measurement technique

We must take care to keep a good vacuum.

• use materials and samples with low outgassing rates

(low vapor pressures)

• maintain pumps at operation conditions

• change gaskets or o-rings according to requirements

• avoid contamination from “finger grease” in chamber

and on samples

Maintenance

Heats the chamber to temperatures between 100oC to 200

oC

for an extended period of time (1 - 2 days)

rapidly removes adsorbed gases from the chamber walls at high temperatures in order to lower the outgassing rates at room temperature

Bake Out

What are the mechanical properties of the sample ?

• internal stress in films /

substrates

• friction

• adhesion

• stress curvature

measurements

• adhesion tests

• …..

Epitaxy Techniques

• Vapor-Phase Epitaxy (VPE)– Modified method of chemical

vapor deposition (CVD). – Undesired polycrystalline layers– Growth rate: ~2 µm/min.

• Liquid-Phase Epitaxy (LPE)– Crystal layers are from the melt

existent on the substrate.– Hard to make thin films

– Growth rate: 0.1-1 µm/min.

• Molecular Beam Epitaxy (MBE)– Relies on the sublimation of

ultrapure elements, then condensation of them on wafer

– In a vacuum chamber (pressure: ~10-11 Torr).

– “Beam”: molecules do not collide to either chamber walls or existent gas atoms.

– Growth rate: 1µm/hr.

December 17, 2008MASE 570 Micro and Nanofabrication,

Fall 200831

Vapor Phase Epitaxy

• Specific form of chemical vapor deposition (CVD)• Reactants introduced as gases• Material to be deposited bound to ligands• Ligands dissociate, allowing desired chemistry to reach

surface• Some desorption, but most adsorbed atoms find proper

crystallographic position• Example: Deposition of silicon

– SiCl4 introduced with hydrogen– Forms silicon and HCl gas– Alternatively, SiHCl3, SiH2Cl2– SiH4 breaks via thermal decomposition

Precursors* for VPE

• Must be sufficiently volatile to allow acceptable growth rates

• Heating to desired T must result in pyrolysis

• Less hazardous chemicals preferable– Arsine highly toxic; use t-butyl arsine instead

• VPE techniques distinguished by precursors used

( precursor is a compound that participates in the chemical reaction that produces another compound)

(pyrolisis decomposition brought about by high temperatures)

Varieties of VPE

• Chloride VPE– Chlorides of group III and V elements

• Hydride VPE– Chlorides of group III element

• Group III hydrides desirable, but too unstable

– Hydrides of group V element

• Organometallic VPE– Organometallic group III compound

– Hydride or organometallic of group V element

• Not quite that simple– Combinations of ligands in order to optimize

deposition or improve compound stability

– Ex. trimethylaminealane gives less carbon contamination than trimethylalluminum

Other Methods

• Liquid Phase Epitaxy

– Reactants are dissolved in a molten solvent at high temperature

– Substrate dipped into solution while the temperature is held constant

– Example: SiGe on Si

• Bismuth used as solvent

• Temperature held at 800°C

– High quality layer

– Fast, inexpensive

– Not ideal for large area layers or abrupt interfaces

– Thermodynamic driving force relatively very low

• Molecular Beam Epitaxy– Very promising technique

– Elemental vapor phase method

– Beams created by evaporating solid source in UHV

Properties of Epitaxial Layer

• Crystallographic structure of film reproduces that of substrate

• Substrate defects reproduced in epi layer

• Electrical parameters of epi layer independent of substrate

– Dopant concentration of substrate cannot be reduced

– Epitaxial layer with less dopant can be deposited

• Epitaxial layer can be chemically purer than substrate

• Abrupt interfaces with appropriate methods

Applications

• Engineered wafers

– Clean, flat layer on top of less ideal Si substrate

– On top of SOI structures

– Ex.: Silicon on sapphire

– Higher purity layer on lower quality substrate (SiC)

• In CMOS structures

– Layers of different doping

– Ex. p- layer on top of p+

substrate to avoid latch-up

More applications

• Bipolar Transistor

– Needed to produce buried layer

• III-V Devices

– Interface quality key

– Heterojunction Bipolar Transistor

– LED

– Laser

Summary

• Deposition continues crystal structure• Creates clean, abrupt interfaces and high

quality surfaces• High temperature, clean surface required• Vapor phase epitaxy a major method of

deposition• Epitaxial layers used in highest quality wafers• Very important in III-V semiconductor

production

Liquid-Phase Epitaxy

• Molten semiconductor material is poured directly onto wafer

• After allowing material to cool for a specified time the non-bonded material is wiped away

• Wafer must then be reground and polished for further processing

Drawbacks to Liquid-Phase Epitaxy

• Considered as economically undesirable due to the costs incurred to repolish the wafer after each step

• Also it is difficult to accurately control the thickness of the epi layer in this process

Low Pressure Chemical Vapor Deposition (LPCVD Method)

• Wafers are mounted on an inductively heated block and a mixture of Dichlorosilane and hydrogen gas is passed over the wafers. These gases react at the wafer surface to create a slow growing layer monocrystalline silicon.

LPCVD Systems

Advantages of LPCVD Method

• The rate of silicon growth can be regulated by varying the temperature, pressure, and gas mixture.

• No polishing is required as the vapor deposited silicon will faithfully reproduce the structure of the underlying lattice.

• The epitaxial film can also be doped by adding small amounts of gaseous dopants such as phosphine or diborane.

Advantages / Disadvantages of Epitaxy

• ADVANTAGES

• Create stacks of differently doped layers useful in the creation of bipolar transistors

• Create buried layers

• DISADVANTAGES

• Time required to grow silicon layers

• High cost of equipment used in process

Vapor-phase epitaxy• Layer formation by chemical reaction between

gaseous compounds.

• Also known as CVD.

• Can be performed at atmospheric pressure (APCVD) or low pressure(LPCVD).

• Susceptors are made from graphite blocks.

Vapor Phase epitaxy

Steps involved:

• Reactants transported to substrate region.

• Transferring the reactants to the substrate surface where they are adsorbed.

• Chemical reaction followed by growth.

• Gaseous products are desorbed into the main gas stream.

• Reaction products are transported out of the reaction chamber.

Optimizing different process parameters

• Parameters:– Temperature– Pressure

• Temperature:– High temperature => higher mobility– But increases thermal stress

• Pressure:– At low pressure, increase in velocity of gas stream can

be attained for the same amount of gas at normal pressure.• LPCVD process

Chemical Vapor Deposition

• “Chemical vapour deposition(CVD) is a process where oneor more volatile precursors aretransported via the vapourphase to the reaction chamber,where they decompose on aheated substrate”

• Many materials may bedeposited using CVD andrelated techniques. Metals,oxides, sulfides, nitrides,phosphides, arsenides,carbides, borides, silicides…

Example: Preparation of TiB2, melting point 3325˚C. May be deposited by CVD at 1000˚C: CVD was first used for hard coatings (cutting tools etc.) Microelectronics, 3D-structures Glass (SnO2, TiN, SiO2, TiO2) Solar cells, catalysis, membranes, waveguides, mirrors, ”synthetic gold” (TiNx)

The ideal precursor

• Liquid rather than solid or gaseous• Good volatility • Good thermal stability in the delivery system, during evaporation and

transport • Decompose cleanly and controllably on the substrate without

incorporation • Give stable by-products which are readily removed from the reaction

zone • Readily available in consistent quality and quantity at low cost • Non-toxic and non-pyrophoric

Impossible to meet all criteria.

Industrially important precursors: Hydrides: AiH4, AsH3 ... Metal alkyls: AliBu3, GaEt3 Metall halides: WF6, TiCl4

Silicon CVD ( Chemical Vapor Deposition)

• Main sources:• Silicon tetrachloride

• Dichlorosilane

• Trichlorosilane

• Silane

• Commonly uses SiCl4high temp. process.

• Others used because of lower temperature.

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

Accompanying reaction:

SiCl4 + Si (solid) 2SiCl2 (gas)

CVD overview

• • Velocity ratio (molecules/s, not meters/s!):

• ▫ Mass transport velocity

• Depends on pressure

• ▫ Surface reaction velocity

• Does not depend on pressure •

• Low ratio -> pure; well-controlled thickness

• • High ratio -> contaminants; poorly-controlled thickness

CVD overview

• • Atmospheric-pressure CVD (APCVD) velocity ratio too high: ~1:1

• • Mass transport velocity proportional to 1/pressure[2]

• • 1 atm ~= 100 kPa

• LPCVD

• • LPCVD typical pressure: 10-1000 Pa

• • Ratio 1:100–1:10,000!

• • Reduced film variation

• • Increased purity

• LPCVD

• • Substrate inserted

• • Tube evacuated to 0.1 Pa

• • Process gas (“working gas”) added at 10-1000 Pa

• • Reaction performed

• • Substrate removed S

LPCVD

• Advantages: :

• • Excellent uniformity of thickness & purity

• • Simple

• •Reliable/reproducible • Homogenous layer

• Disadvantages

• Slows down deposition rate

• • Requires high temperatures, <600°C

• Advantages– Low deposition temp

– Precise control of layer thickness and doping profile (excellent uniformity)

– Versatile (used for fabricating heterostructures, quantum wells, etc)

– In-situ cleaning and characterization• High temp. baking to decompose native oxygen.

• Low energy ion beam of inert gas to sputter impurity.

• Disadvantage:– Expensive (UHV), very slow deposition

Chemical Vapor Deposition

• Gases dissociate on surfaces at high temperature

• Typically done at low pressure (LPCVD) rather than atmospheric (APCVD)

• LPCVD pressures around 300mT (0.05% atmosphere)

• Moderate Temperatures– 450 SiO2

– 580-650 polysilicon

– 800 SixNy

• Very dangerous gases– Silane: SiH4

– Arsine, phosphine, diborane: AsH3, PH3, B2H6