Epitaxy

From Ingot to Wafers

Shaping

Grinding

Sawing or Slicing

Edge Rounding

Lapping

Etching

Polishing

Cleaning

Inspection

Packaging

Shipping

Epitaxy is a kind of interface between a thin film and a

substrate. The term epitaxy (Greek; epi "above" and taxis "in

ordered manner") describes an ordered crystalline growth on

a monocrystalline substrate. Epitaxial films may be grown

from gaseous or liquid precursors. Because the substrate

acts as a seed crystal, the deposited film takes on a lattice

structure and orientation identical to those of the substrate.

This is different from other thin-film deposition methods

which deposit polycrystalline or amorphous films, even on

single-crystal substrates. If a film is deposited on a substrate

of the same composition, the process is called homoepitaxy;

otherwise it is called heteroepitaxy.

Epitaxy

This technology is quite similar to what happens in

CVD processes, however, if the substrate is an

ordered semiconductor crystal (i.e. silicon, gallium

arsenide), it is possible with this process to continue

building on the substrate with the same

crystallographic orientation with the substrate acting

as a seed for the deposition. If an

amorphous/polycrystalline substrate surface is used,

the film will also be amorphous or polycrystalline.

Epitaxial and polysilicon film growth

Film deposited on a

<111> oriented wafer

<111> orientation

The presence of SiO2

Layer cause depositing

atoms have no

structurepolysilicon

Homoepitaxy is a kind of epitaxy performed with only one

material. In omoepitaxy, a crystalline film is grown on a

substrate or film of the same material. This technology is

applied to growing a more purified film than the substrate and

fabricating layers with different doping levels.

Heteroepitaxy is a kind of epitaxy performed with materials

that are different from each other. In heteroepitaxy, a

crystalline film grows on a crystalline substrate or film of

another material. This technology is often applied to growing

crystalline films of materials of which single crystals cannot be

obtained and to fabricating integrated crystalline layers of

different materials. Examples include gallium nitride (GaN) on

sapphire or aluminium gallium indium phosphide (AlGaInP) on

gallium arsenide (GaAs).

Heterotopotaxy is a process similar to

heteroepitaxy except for the fact that thin

film growth is not limited to two dimensional

growth. Here the substrate is similar only in

structure to the thin film material.

Applications

• It has applications in Nanotechnology and in

Semiconductor Fabrication. Indeed, epitaxy is the only affordable method of High Crystalline Quality Growth for many semiconductor materials, including technologically important materials as silicon-germanium, gallium nitride, gallium arsenide and indium phosphide.

• Epitaxy is also used to grow layers of PRE-DOPED SILICON on the polished sides of silicon wafers, before they are processed into semiconductor devices. This is typical of power devices, such as those used in pacemakers, vending machine controllers, automobile computers, etc.

Applications of epitaxial layers

1. Discrete and power devices

2. Integrated circuits

3. Epitaxy for MOS devices

1. Discrete and power devices • Technology change: junction transistors diffused

planar structure – Requires a material structure that are not achieved by diffusion

of dopants from the surface

• Si epitaxy was developed to enhance the electrical performance of discrete bipolar transistors

• Breakdown voltage of the discrete transistor was limited by the field avalanche breakdown of the substrate material – Use higher resistivity substrates produced higher breakdown

voltages but increased collector series resistance

• Structure needed: thin, lightly doped and single crystal layer of high perfection upon more heavily doped Si substrate – But, the use of a more heavily doped substrate reduces the

collector series resistance while the base-collector breakdown voltage is governed by the lighter doping in the near surface region

• Epitaxial deposition of a lightly doped P+ epitaxial

layer on a N+ substrate make the desired properties

are achievable

• Epitaxial grows also allows accurate control of doping

levels and advantages which arises from a generally

low oxygen and carbon levels in epitaxial layer

• Epitaxial technique was developed to 2 and 3 layers

epitaxial structure

– For lightly doped area of collector

– Based region was also grown epitaxially

• E.g. of multilayer structures: Si-Controlled Rectifier

(SCR), Triac, high voltage or high power discrete

products

Mesa discrete transistor fabricated in an epitaxial

layer on a heavily doped N+ substrate

Transistors

Diodes

2. Integrated circuit (IC)

• Development of planar bipolar IC caused the requirement for

devices built on the same substrate to be electrically isolated

– The use of opposite typed substrate and epitaxial layer met

part of the requirement

– Device isolation was completed by the diffusion of “isolation”

region through the epitaxial layer to contact the substrate

between active areas

• In planar bipolar circuits, common to employ a heavily doped

diffused (or implanted) region under the transistor

– Usually called ‘buried layer’ or ‘DUF’ for diffusion under film

– The buried layer

• serves to lower the lateral series resistance between collector area

below the emitter and the collector contact

• produce uniform planar operation of the emitter, avoiding current

crowding which leads to hot spots near edges of the emitter

Integrated circuits

3. Epitaxy for MOS devices

• Unipolar devices such as junction field-effect

transistors (JFETs), VMOS, DRAMs technology

also use epitaxial structures

• VLSI CMOS (complimentary metal-oxide-

semiconductor) devices have been built in thin

(3-8 micron) lightly doped epitaxial layers on

heavily doped substrates of the same type (N or

P)

– That epitaxial structure reduces the “latch up” of high

density CMOS IC by reducing the unwanted

interaction of closely spaced devices

(a) A junction isolated bipolar

device fabricated as part of an

integrated circuit using a buried

layer subcollector and a lightly

doped n-epitaxial layer

(b) An N-Well CMOS structure

fabricated in a lightly doped p-

epitaxial layer

Advantages of epitaxy

• Ability to place a lightly oppositely doped

region over a heavily doped region

• Ability to contour and tailor the doping

profile in ways not possible using diffusion

or implantation alone

• Provide a layer of oxygen free material

that is also contained low carbon

Techniques for silicon epitaxy

1. Chemical Vapour Deposition (CVD)

2. Molecular Beam Epitaxy (MEB)

3. Liquid Phase Epitaxy (LPE)

4. Solid phase regrowth

1. Chemical Vapour Deposition (CVD)

• The most common technique in Si epitaxy

• In the CVD technique

– Si substrate is heated in a chamber: sufficient heat

to allow the depositing Si atoms to move into

position to

– Reactive Si containing gaseous compounds are

introduced

– Gaseous react on the hot surface of the substrate

and deposit a Si layer

– The deposit will take on Si substrate structure if the

substrate is atomically clean and the temperature

is sufficient for atoms to have surface mobility

Schematic drawing of a simple horizontal flow, cold

wall, CVD reactor

Schematic CVD reactor geometries for

(a) True vertical reactor

(b) Classic horizontal flow reactor

(c) Modified vertical (or pancake) reactor

(d) Downflow cylinder reactor

CVD processes and products

CVD for silicon devices

CVD reactions 1. Pyrolysis: chemical reaction is driven by heat alone, e.g. silane

decomposes with heating

SiH4 Si + 2H2

2. Reduction: chemical reaction by reacting a molecule with hydrogen, e.g. silicon tetrachloride- reduction in hydrogen ambient to form solid silicon

SiCl4 + 2H2 Si + 4HCl

3. Oxidation: chemical reaction of an atom or molecule with oxygen, e.g. SiH4 decomposes at lower temperature

SiH4 + O2 SiO2 + 2H2

4. Nitridation: chemical process of forming silicon nitride by exposing Si wafer to nitrogen at high temperature e.g. SiH2Cl2 readily decomposes at 1050C

3SiH2Cl2 + 4NH3 Si3N4 + pH + 6H2

CVD

CVD film growth steps

1. Nucleation

• Dependent on substrate quality

• Occurs at first few atoms or molecules deposit on a surface

2. Nuclei growth

• Atoms or molecules form islands that grow into larger islands

3. Island coalescence

• The islands spread , and coalescing into a continuous film

• This is the transition stage of the film growth, thickness several

hundreds Angstroms

• Transition region film possesses different chemical and

physical properties for thicker bulk film

4. Bulk growth

• Bulk growth begins after transition film is formed

CVD film growth steps

Types of film

structure

Basic CVD subsystem

Amorphous

Polycrystalline

Single crystal

• CVD Process steps:

• Pre-clean: remove particulates and mobile

ionic contaminants

• Deposition:

• Evaluation: thickness, step coverage, purity,

cleanliness and composition

Pre-clean Deposition Evaluation

Load wafer into

chamber, inert

atmosphere

Heat

Introduce

chemical

vapour

Flush excess

chemical

vapour source

Remove vapour

Vapor Phase Epitaxy • Silicon is most commonly deposited from silicon

tetrachloride in hydrogen at approximately 1200 °C:

SiCl4(g) + 2H2(g) ↔ Si(s) + 4HCl(g)

• This reaction is reversible, and the growth rate depends strongly upon the proportion of the two source gases. Growth rates above 2 micrometres per minute produce polycrystalline silicon, and negative growth rates (etching) may occur if too much hydrogen chloride byproduct is present. (In fact, hydrogen chloride may be added intentionally to etch the wafer.) An additional etching reaction competes with the deposition reaction:

SiCl4(g) + Si(s) ↔ 2SiCl2(g)

• Silicon VPE may also use silane, dichlorosilane, and trichlorosilane source gases. For instance, the silane reaction occurs at 650 °C in this way:

SiH4 → Si + 2H2

• This reaction does not inadvertently etch the wafer, and takes place at lower temperatures than deposition from silicon tetrachloride. However, it will form a polycrystalline film unless tightly controlled, and it allows oxidizing species that leak into the reactor to contaminate the epitaxial layer with unwanted compounds such as silicon dioxide.

• VPE is sometimes classified by the chemistry of the source gases, such as hydride VPE and metalorganic VPE.

VPE Advantages/Disadvantages

• Low temperature process

• High purity (low defect density) material

• Readily automated for mass production

• Ability to grow thin layers with precise

composition, doping density, thickness O

on an atomic scale for advanced systems)

• Well suited to research has opened “new”

physics

Disadvantages

• Toxic gases are used – must have gas monitors and stainless steel plumbing.

• The exhaust pump system includes a ‘scrubber’ that breaks down toxic end products before atmospheric release.

• Research systems are expensive, as are many of the precursors (purchased as pressurized gases in cylinders or as ‘bubbler’s’ VPE works well with Si and GaAs (usually not used) and related elemental and compound semiconductors

Liquid Phase Epitaxy (LPE)

• LPE technique is widely used for preparation of epitaxial layers on compound semiconductors and for magnetic bubble memory films on garnet substrate

• In films growth by LPE from solution melts, low cooling rates, when the surface reaction (growth)

• Kinetics are rapid compare to the mass transport of Si to the seed, epitaxial layer thickness will vary in proportion to the temperature drop

• Increase cooling rates, mass transport rate will increase and the growth rate will increase with cooling rate until growth rate becomes limited by surface reaction kinetics

Liquid Phase Epitxy

• LPE deposits a monocrystalline film from the liquid

phase, typically at a rate of 0.1 to 1 μm/minute.

• Liquid phase epitaxy (LPE) is a method to grow semiconductor crystal layers from the melt on solid substrates. This happens at temperatures well below the melting point of the deposited semiconductor. The semiconductor is dissolved in the melt of another material. At conditions that are close to the equilibrium between dissolution and deposition the deposition of the semiconductor crystal on the substrate is slowly and uniform. The equilibrium conditions depend very much on the temperature and on the concentration of the dissolved semiconductor in the melt. The growth of the layer from the liquid phase can be controlled by a forced cooling of the melt. Impurity introduction can be strongly reduced. Doping can be achieved by the addition of dopants.

• The method is mainly used for the growth of compound semiconductors. Very thin, uniform and high quality layers can be produced. A typical example for the liquid phase epitaxy method is the growth of ternery and quarternery III-V compounds on Galliumarsenid GaAs substrates. As a solvent quite often Gallium is used in this case. Another frequently used substrate is Indiumphosphide InP. However also other substrates like glass or ceramic can be applied for special applications. To facilitate nucleation, and to avoid tension in the grown layer the thermal expansion coefficient of substrate and grown layer should be similar.

LPE growth rate increasing with

cooling rate up to about 1 micron per

minute

• Growth rate increases with

cooling rate up to about 1

degree/min while growth rate

above 2 degree/min occurred

under kinetically limited

conditions

Schematic drawing of a typical silicon liquid

phase epitaxy (LPE)

Schematic of fabrication steps in the fabrication

vertical field effect transistors by etch and LPE refill

techniques

•Fabrication sequence for a vertical

channel field effect transistor

•N and N+ epitaxial structure can be

built using liquid or vapour phase

epitaxial growth

•Preferential etching can be used to

open areas part way through the N

type epitaxial layer

• In this figure, LPE is used to fill the

etched out gate areas which control

current flow vertically from the top

side source to the N+ substrate drain

region

ADVANTAGES OF LPE

1. Simple

2. Inexpensive

3. Rather non-hazardous

4. Suitable for selective growth

5. Al and Sb compounds possible

6. Highly suitable for simple structures

DISADVATAGES OF LPE

1. Too simple to grow quantum structures

2. Thickness control and composition control

difficult

3. Redissolution of the grown material

4. High growth temperatures for certain

compounds (e.g. GaAs at ~ 800-900 oC but

InP at ~ 600 oC)

5. Fe doping (for semi-insulation) difficult

because of low distribution coefficient

2. Molecular Bean Epitaxy (MBE)

• Uses an evaporation method

• MBE is carried out at a lower temperature than 1000-

1200C (typical CVD temperature)

– Reduces outdiffusion of local areas of dopant diffused

into substrates and reduce autodoping which is

unintentionally transfer of dopant into epitaxial layer

• MBE is favourable

– preparation of sub-micron thickness epitaxial layers or

– high frequency devices requiring hyper-abrupt transition in the

doping concentration between the epitaxial layer and the

substrate

Molecular beam Epitaxy (MBE)

Molecular beam epitaxy (MBE) is basically a

sophisticated form of Vacuum Evaporation.

Molecular Beams of the constituent elements are

generated from sources and travel without Scattering to

a substrate where they combine to form an epitaxial film.

In solid source MBE, material is Evaporated from solid

ingots by heating or with an electron beam.

The Rate of growth depends on the flux of material

in the molecular beams which can be controlled by

the evaporation rate and, most importantly, switched

on and off with shutters in a fraction of the time

required to grow one monolayer (ML).

• MBE epitaxial process involves the reaction of one or more thermal beams of atoms or molecules with crystalline surface under ultrahigh vacuum conditions.

• Single crystal Multilayer of the order of atomic layers can be made using MBE.

• MBE growth rates are quite low.(1micrometer per hour)

• Separate effusion ovens made of pyrolytic boron nitride are used for Ga, As and the dopants.

• The substrate holder rotates to obtain uniform epitaxial layers.

• An overpressure of As is maintained to grow GaAs.

• Effusion ovens behave like small area sources and exhibit a cosθ emission.(θ is angle b/w normal of wafer surface and source).

Typical growth rates are 1 ML per second, or 1 micron per hour, which is equivalent to a pressure of 10-6mbar arriving at the substrate in the molecular beams.

Great trouble is taken to ensure that negligible quantities of Impurity atoms are introduced into the material: substrates are carefully prepared and cleaned; only ultra pure sources are used.

The reaction chamber is evacuated to <10-11mbar and the walls of the chamber cooled with liquid nitrogen.

Even so the highest mobility layers are only grown after an extended run when the machinery has completely cleaned itself.

• The basic principle of epitaxial growth is that atoms on a clean surface are free to move around until they find a correct position in the crystal lattice to bond.

• Growth occurs at the step edges formed since at an edge an atom experiences more binding forces than on the free surface.

• In practice there will be more than one nucleation site on a surface and so growth is by the spreading or islands.

• In high quality material these islands will be large with height differences of less than a ML.

• In MBE, – Si and dopant(s) are evaporated in an ultra high vacuum

(UHV) chamber

– The evaporated atoms are transported at relatively high velocity in a straight line from the source to the substrate

– They condense on the low temperature substrate

– The condensed atoms of Si or dopant will diffuse on the surface until they reach a low energy site that they fit well the atomic structure of the surface

– The “adatom” then bonds in that low energy site, extending the underlying crystal by a vapour to solid phase crystal growth

– Usual temperature range of the substrate is 400-800C. Higher than 800C is possible but it will increase outdiffusion or lateral diffusion of dopants in the substrate

Schematic drawing of a molecular beam

epitaxial system

• Insitu cleaning of the substrate – Can be done by high temperature bake at 1000-1250C for

several minutes under high vacuum to decompose the native surface oxide and to remove other surface contaminants

– Other technique is by using a low energy beam of inert gas to sputter clean the substrate

– Difficult to remove carbon but will decrease at the surface by diffusion into the substrate during short anneal at 800-900C

• Wider range of dopants for MBE than CVD epitaxy: – Typical dopants: Antimony, Sb (N-type), aluminum, Al or gallium

(Ga) for P-type

– N-type dopant: As and P, evaporate rapidly even at 200C. Difficult to control

– P-type dopant: Boron, evaporate slowly even at 1300C

Schematic drawing of a multiple chamber MBE

system

MBE Equipment

• The mobility of an atom on the surface will be greater at higher substrate temperature resulting in smoother interfaces, but higher temperatures also lead to a lower "sticking coefficient" and more migration of atoms within the layers already grown.

• In practice the beams do not contain individual atoms, but molecular species like As2 or As4. These are cracked at the surface and the efficiency of the process is also temperature dependent.

• Clearly there will be a compromise temperature to achieve the best results.

• For GaAs this is ~600o C, while for SiGe alloys a much higher temperature is used, up to 950o C.

Metal Organic CVD (MOCVD)

• MOCVD is also a VPE process based on Pyrolytic Reactions.

• MOCVD is important for those elements that do not form stable hydrides or halides but form stable Metal Organic compounds with reasonable vapour pressure.

• MOCVD has been extensively applied in the Hetero-epitaxial Growth Of III-V And II-VI Compounds.

• To grow GaAs, metal organic compounds such as trimethylgallium Ga(CH3) for the gallium component and aresine AsH3 for Arsenic component.

• The overall reaction is

• For Al-containing compounds , such as AlAs, we can use Trimethylgallium Ga(CH3)3.

• During Epitaxy, the GaAs is doped by introducing dopants in vapour form.

• Diethylzine(Zn(C2H5)2) is a p-type dopant and Silane is the n-type dopant for III-V compounds.

• The metal organic compound is transported to quartz reaction vessel by hydrogen carrier gas, where its is mixed with AsH3

• Temperature is between 600˚C-800˚C and is used to heat gases above a substrate placed on graphite susceptor during radio frequency heating.

4333 3)( CHGaAsCHGaAsH

RF Power

Quartz reactor

Wafer

4. Solid Phase Re-growth

i. Re-growth of amorphous layers

• Surface layers subjected to high dose ion implants are

in amorphous structure due to the heavy damage

inflicted on the lattice as the energetic ions are

absorbed

• Annealing above 600C amorphous layer re-

crystallize

• Re-crystallisation occurs from interface moves toward

the surface and results in solid phase epitaxial re-

growth

ii. Re-crystallisation of thin films

• Involves re-crystallisation of a deposited amorphous or

polysilicon film

• Si film is deposited on a Si substrate or more commonly SiO2

heated using a strip heater passed over the surface or by a

scanned pulsed laser to crystallise the film to single crystal or

large grain polysilicon

• This fabrication technique is used to produce a stacked n-channel

device in re-crystallised polysilicon on a thermally grown or

deposited oxide

• Oriented epitaxial growth can be obtained by making series of

holes in the oxide to allow points of contact between the

underlying substrate and the deposited polysilicon

• The contact points become “seeds” areas for establishing re-

growth orientation

Re-crystallisation solid phase

epitaxy using a moving strip heater

A stacked MOS structure over an

insulating oxide fabricated in a re-

crystallised polysilicon layer

Structure and defects in epitaxial layer

• Surface morphology of Silicon epitaxial deposits is

affected by growth and substrate parameters

• Growth parameters:

– Temperature

– Pressure

– Concentration of Si containing gas

– Cl : H2 ratio

• Substrates parameters

– Substrate orientation

– Defects in the substrate

– Contaminants on the surface of the substrate

Typical defects in epitaxial layers

1. Substrate orientation effects

2. Spikes and epitaxial stacking faults

3. Hillocks and pyramids in epitaxial layers

4. Dislocations and slip

5. Microprecipitates (S-pits)

1. Substrate orientation effect

• Growth of smooth epitaxial films can be obtained on

(100) and (110) oriented Si substrates

• Epitaxial growth on substrate surface on oriented on

(111) plane results in facetted “alligator skin” surface

– (111) surfaces contain no atomic steps to provide a density of

growth sites

– Without atomic steps, the growth produces pyramids and

terraces

• Misorientation of the surface by 0.5 degree

introduces a sufficient density of steps for growth of

smooth planar films

2. Spikes and epitaxial stacking faults

i. Growth spike

• Originate from Si particle on the surface not removed

by the pre-epitaxial cleaning process

• Si Chips may expose faster growing crystal planes

than the plane of the substrate

• Chips nucleate and produce polysilicon nodule. The

chips then protrude above the substrates surface into

a region of richer supply of gaseous reactants

• Results in nodule grows at 2-10 times the rate of

epitaxial film on the substrate.

• May be removed mechanically before the next step

but will leave a region unusable for functional

materials

ii. Epitaxial stacking faults

• Crystallographic in nature and arise from defects in

atomic arrangement during film growth

• Could result from an extra atomic layer (extrinsic fault)

or a missing atomic layer (intrinsic fault) along {111}

type plane

Epitaxial growth spike Stacking fault on <111> Si

3. Hillocks and pyramids in epitaxial layers

• Hillocks: Small oval mounds on the surface of the

epitaxial

• Pyramids: Faceted regions on the epitaxial surface

• Density of hillocks and pyramids is dependent on

growth parameters such as type and concentration of

Si source and deposition temperature

4. Dislocations and slip

• Non-uniform heating of a substrate results in non-uniform thermal

expansion of the substrate which produces elastic stresses

• The thermal stress can cause bowing which may lift the edge of

the substrate away from the substrate in response to the thermal

stress

• At lower temperature (< 900C) the yield point of the Si lattice is

sufficiently high that the substrate behaves elastically. During

cooling, the thermal stress is removed and the substrate returns

to its original shape

• If the stress exceeds a critical values, the substrate will yield

plastically occurs due to generation and motion of dislocations

which are atomic level line defects which glide along slip planes

of the crystal

• The passage of one dislocation offsets the material above and

below the slip plane by a unit known as “Berger’s vector” of the

dislocations

• Dislocations normally propagate from near the edge of the

substrate (highest stress), and glide towards the centre of the

substrate and produce plastic deformation of the substrate which

relieves the thermal stress

• Dislocation motions is slow because dislocation moves to a

region of lower shear stress

• The continuous slow motion of the dislocations produces “creep”

deformation of the crystal

• Device impact from slip normally comes from rapid “pipe

diffusion” of dopant along the core of the dislocations

Typical wafer edge slip as a

result of excessive within

wafer temperature

gradients during heating or

during epitaxial film growth

Crystal slip

5. Microprecipitates (S-pits)

• Microprecipitates may come from metallic elements

such as copper, nickel, iron and chromium

• This is due to their solubility in Si at high temperatures

and fast diffusion rates through the Si

• The metal contaminants may exist in the starting

substrates or being pick up during handling in the

loading operation or from metal parts or susceptors

within the epitaxial reactor itself