IC TECHNOLOGYCHEMICAL VAPOUR

DEPOSITION AND EPITAXIAL LAYER

GROWTH

By:Kritica Sharma

Assistant Professor (ECE)

CONTENTS

2

CVD for deposition of dielectric and polysilicon thick Layer – a simple CVD system

Chemical equilibrium and the law of mass action Introduction to atmospheric CVD of dielectric low pressure CVD of dielectric semiconductor. Epitaxy Vapour Phase Expitaxy Defects in Epitaxial growth Metal Organic Chemical Vapor Deposition Molecular beam epitaxy.

CHEMICAL VAPOUR DEPOSITION Chemical Vapour Deposition (CVD) is a chemical

process used to produce high purity, high performance solid materials.

In a typical CVD process, the substrate is exposed to one or more volatile precursors which react and decompose on the substrate surface to produce the desired deposit.

During this process, volatile by-products are also produced, which are removed by gas flow through the reaction chamber.

STEPS INVOLVED IN CHEMICAL VAPOUR DEPOSITION

Transport of reactants by forced

convection to the

deposition region

Transport of reactants

by diffusion from the main gas stream to

the substrate surface.

Adsorption of

reactants in the wafer

(substrate) surface.

Chemical

decomposition and other

surface reactions

take place.

Desorption of by-

products from the surface

Transport of by-products by diffusion

Transport of by-products by forced

convection away from

the deposition

region.

STEPS INVOLVED IN A CVD PROCESS (SCHEMATIC)

SCHEMATIC DIAGRAM - THE STEPS INVOLVED IN CVD1. Transport of reactants by forced convection to the deposition region. 2. Transport of reactants by diffusion from the main gas stream through the boundary layer to the

wafer surface3. Adsorption of reactants on the wafer surface. 4. Surface processes, including chemical decomposition or reaction, surface migration to

attachment sites (such as atomic-level ledges and kinks), site incorporation, and other surface reactions.

5. Desorption of byproducts from the surface. 6. Transport of byproducts by diffusion through the boundary layer and back to the main gas

stream. 7. Transport of byproducts by forced convection away from the deposition region.

STEPS INVOLVED IN A CVD PROCESS (SCHEMATIC)

STEPS INVOLVED IN A CVD PROCESS (SCHEMATIC)

STEPS INVOLVED IN A CVD PROCESS (LIMITING PROCESSES)1. Gas phase process (mainly diffusion to substrate).2. Surface process (mainly reaction)

CVD KINETIC GROWTH MODEL

We approximate the flux Fl by the linear formula

F1 = hG(CG –CS)

where CG and CS are the concentrations of the SiCI4 (molecules per cubic centimeter) in the bulk of the gas and at the surface, respectively, and hG is the gas-phase mass-transfer coefficient. The flux consumed by the chemical-reaction taking place at the

surface of the growing film F2 is approximated by the formula F2 = kSCS

where kS is the chemical surface-reaction rate constant.In steady state F1 = F2 = F. Using this condition, we get

GS

GS hk

CC/1

CVD KINETIC GROWTH MODEL-IIWe can now express the growth rate of the silicon film by writing

where N1 is the number of silicon atoms incorporated into a unit volume of the film. Its value for silicon is 5.01022 cm-3. Noting that CG = YCT where CT is the total number of molecules per cubic centimeter in the gas, we get the expression for the growth rate,

11 NC

hkhk

NFv G

GS

GS

YNC

hkhk

NFv T

GS

GS

11

The growth rate at a given mole fraction is determined by the smaller of hG or kS. In the limiting cases the growth rate will be given either by

[surface-reaction control]or by [mass-transfer control].

YkNCv ST

1

YkNCv ST

1

CVD GROWTH MODEL – GAS PHASE MASS TRANSFER

The “Stagnant-film” model of gas-phase mass-transfer

SG

GCCDF

1

G

GDh

Boundary layer theory: δ increases with distance in the direction of gas flow (from Newton’s second low).

DG – diffusivity of reactant species - boundary layer thickness

CVD GROWTH MODEL – GAS PHASE MASS TRANSFER

The flow of reactants F is F DG -1

TYPES OF CVD CVD’s are classified into two types on the basis of

Operating Pressure. 1. Atmospheric Pressure CVD 2. Low Pressure CVD Plasma Enhanced CVD Photochemical Vapour Deposition Thermal CVD

CASE 1 : HIGH TEMPERATUREThis process is used to deposit Silicon and compoundfilms or hard metallurgical coatings like TitaniumCarbide and Titanium Nitride. CASE 2 : LOW TEMPERATUREMany insulating film layers such as Silicon dioxide needto be deposited at low temperatures for effectivedeposition.

ATMOSPHERIC PRESSURE CHEMICAL VAPOUR DEPOSITION

Aluminium oxide films are deposited by this method from aluminium trichloride, argon and oxygen gas mixtures at temperatures ranging from 800-1000 degree Celsius

The films have low chlorine content, which continue to decrease with increasing temperature.

Analysis of the film growth rate on the substrates revealed that, the growth takes place only by diffusion from 800 to 950 degree Celsius and only by gas phase reaction at 1000 degree Celsius.

CONTINUED..

CONVEYOR BELT - APCVD

Film thickness uniformity cannot be maintained. Large number of pinhole defects can occur. Wafer (Substrate) throughput is low due to low

deposition rate. The deposits get contaminated very easily since it

takes place at atmospheric pressure. Maintaining stochiometry is extremely difficult.

LIMITATIONS OF APCVD

The deposition of Silicon carbide thin film is performed using low pressure CVD of Dichlorosilane / Acetylene / Hydrogen reaction system.

The Silicon carbide film deposited at three different temperatures has three different properties.

LOW PRESSURE CVD

1023 K AMORPHOUS1073 K MICROCRYSTALLINE1173 K PREFERENTIALLY

ORIENTED

This technique permits either horizontal or vertical loading of the wafers into the furnace and accommodates a large number of wafers for processing.

The process results in the deposition of compounds with excellent purity and uniformity.

However the technique requires higher temperatures and the deposition rate is low.

CONTINUED..

Plasma-enhanced chemical vapor deposition (PECVD) is a process used to deposit thin films from a gas state (vapor) to a solid state on a substrate.

Chemical reactions are involved in the process, which occur after creation of a plasma of the reacting gases.

The plasma is generally created by RF (AC) frequency or DC discharge between two electrodes, the space between which is filled with the reacting gases.

The helping hand of the Plasma helps in increasing the film quality at low temperature and pressure.

PLASMA ENHANCED CVD

PECVD uses electrical energy which is transferred to the gas mixture.

This transforms the gas mixture into reactive radicals, ions, neutral atoms and molecules, and other highly excited species.

These atomic and molecular fragments interact with a substrate and, depending on the nature of these interactions, either etching or deposition processes occur at the substrate.

Some of the desirable properties of PECVD films are good adhesion, low pinhole density and uniformity.

CONTINUED..

SCHEMATIC DIAGRAM OF PECVD

REINBERG TYPE REACTOR (DIRECT): Reactants, by-products, substrates and plasma are in

the same space. Capacitive-coupled Radio Frequency plasma. Rotating substrates are present. DOWNSTREAM REACTOR (INDIRECT): Plasma is generated in a separate chamber and is

pumped into the deposition chamber. Allows better control of purity and film quality when

compared to the Direct type.

REACTORS USED IN PECVD

Al thin films are deposited via photochemical vapour deposition on catalytic layers of Ti, TiO2, and Pd, using dimethyl aluminum hydride.

Deposition is carried out at low gas pressures to induce a surface reaction based on adsorption and subsequent decomposition of adsorbates.

Of these three layers Ti is so effective as a catalyst that the Al films are thermally deposited even at a low substrate temperature of 60°C with a growth rate of 0.5 nm/min.

PHOTOCHEMICAL VAPOUR DEPOSITION

The UV light generated by a deuterium lamp helped increase the growth rates. On the other hand, Al could be deposited on TiO2 layers only under irradiation at a substrate temperature of 120°C It takes several minutes to cover the TiO2 surface with Al and initiate the Al film's growth.

Here, the UV light inhibited the Al growth on the surface, whereas the films are deposited thermally.

X-ray photoelectron spectroscopy showed the formation of a photolytic production of the adsorbate, which acts presumably as a center that inhibits further Al growth.

CONTINUED..

In thermal CVD process, temperatures as high as 2000 degree Celsius is needed to deposit the compounds.

There are two basic types of reactors for thermal CVD.1. Hot wall reactor2. Cold wall reactorA hot wall reactor is an isothermal surface into whichthe substrates are placed. Since the whole chamber isheated, precise temperature control can be achievedby designing the furnace accordingly.

THERMAL CVD

A disadvantage of the hot wall configuration is that deposition occurs on the walls of the chamber as well as on the substrate.

As a consequence, hot wall reactors must be frequently cleaned in order to reduce contamination of substrates.

In a cold wall reactor, only the substrate is heated. The deposition takes place on the area of the highest

temperature, since CVD reactions are generally endothermic.

CONTINUED..

The deposition is only on the substrate in cold wall reactors, and therefore contamination of particles is reduced considerably.

However, hot wall reactors have higher throughput since the designs can easily accommodate multiple wafer (substrate) configurations.

CONTINUED..

Variable shaped surfaces, given reasonable access to the coating powders or gases, such as screw threads, blind holes or channels or recesses, can be coated evenly without build-up on edges.

Versatile –any element or compound can be deposited.

High Purity can be obtained. High Density – nearly 100% of theoretical value. Material Formation well below the melting point Economical in production, since many parts can be

coated at the same time.

ADVANTAGES OF CHEMICAL VAPOUR DEPOSITION

CVD has applications across a wide range of industries such as:

Coatings – Coatings for a variety of applications such as wear resistance, corrosion resistance, high temperature protection, erosion protection and combinations thereof.

Semiconductors and related devices – Integrated circuits, sensors and optoelectronic devices

Dense structural parts – CVD can be used to produce components that are difficult or uneconomical to produce using conventional fabrication techniques. Dense parts produced via CVD are generally thin walled and maybe deposited onto a mandrel or former.

APPLICATIONS OF CHEMICAL VAPOUR DEPOSITION

Optical Fibres – For telecommunications. Composites – Preforms can be infiltrated using CVD

techniques to produce ceramic matrix composites such as carbon-carbon, carbon-silicon carbide and silicon carbide-silicon carbide composites. This process is sometimes called chemical vapour infiltration or CVI.

Powder production – Production of novel powders and fibres

Catalysts Nanomachines

CONTINUED..

Epitaxy: “arranged upon”Definition: Epitaxy means the growth of a single crystal film on

top of a crystalline substrate. For most thin film applications (hard and soft coatings,

optical coatings, protective coatings) it is of little importance.

However, for semiconductor thin film technology it is crucial

EPITAXY

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

Si

Si

Name Silicon Bond length in singleCrystal Si

2.352 Ao

Symbol Si Density of solid 2.33 gm/cm3

Atomic Number 14 Molar Volume 12.06 cm3

Atomic Weight 28.0855 Velocity of sound 2200 m/s

Discoverer Jons Jacob Berzelius Electrical resistivity 1000,000 µΩcm

Discovered at Sweden Reflectivity 28%

Discovery Year 1824 Melting point 1414oC

Origin of name From Latin Word“silices”

Boiling Point 2900oC

Si

Si

Why Si 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 twomaterials eg. GaAs and AlAs both have zincblende structure

1.43eV

In .53Ga.47 As

0.36eV

5.65 6.06

Gases used inSilane (SiH4) Pyrolysis :

Dichlorosilane (DCS) Tricholorosilane (TCS)Silicon tetrachlorideDisilane

Silicon Epitaxya)

b) c) d) e)f)

SiH4 (H2)SiH2Cl2

SiHCl3

SiCl4

Si2H6

Si + 2H2

Dopant gases –Diborane (B2H6)– Phosphine (PH3)– Arsine (AsH3)

TYPES OF EPITAXY Homoepitaxy – The film and the substrate are the same material. – Often used in Si on Si growth. – Epitaxially grown layers are purer than the substrate

and can be doped independently of it. Heteroepitaxy – Film and substrate are different materials. – Eg: AlAs on GaAs growth – Allows for optoelectronic structures and band gap

engineered devices.

HETEROEPITAXY Trying to grow a layer of a different material on top of

a substrate leads to unmatched lattice parameters. This will cause strained or relaxed growth and can

lead to interfacial defects. Such deviations from normal would lead to changes in

the electronic, optic, thermal and mechanical properties of the films.

LATTICE STRAINS For many applications nearly matched lattices are

desired to minimize defects and increase electron mobility.

As the mismatch gets larger, the film material may strain to accommodate the lattice structure of the substrate. This is the case during the early stages of film formation (pseudomorphic growth) and with materials of the same lattice structure. The Si-Ge system is an example.

If strain accommodation is not possible then dislocation defects at the interface may form leading to relaxed epitaxy and the film returns to its original lattice structure above the interface.

LATTICE MISFITS AND DEFECTS If the lattice mismatch is less than ~9%, the initial

layers of film will grow pseudomorphically. Therefore very thin films strain elastically to have the

same inter-atomic spacing as the substrate. As film thickness increases, the rising strain will

eventually cause a series of misfit dislocations separated by regions of relatively good fit. As such they are equilibrium theories.

There is a critical film thickness, dc, beyond which dislocations are introduced.

DEFECTS GE SI FILM The GeSi/Si system has a large lattice misfit built in

and as such is not an equilibrium system. This results in a large number of dislocations with few

regions of good fit and the theory breaks down. Rippled surfaces and pyramidal tips are typical.

TYPES AND SOURCES OF DEFECTS Defects reduce electron mobility, carrier concentration

and optical efficiency. Current levels in Si are 1-10 defects/cm 2. Defects can propagate from the substrate as a screw

dislocations. Dopants and impurities can cause edge and point

dislocations. Another type of defect is the stacking faults where the

stacking order of successive layers do not follow a specific order.

FORMATION OF MISFIT DISLOCATIONS They generally originate from threading dislocations at

the film-substrate interface. The dislocation pierces through the substrate and the

film. As it grows, it glides and bends in a slip plane. • Above the critical thickness ( dc) the increasing strain

allows a break and the film dislocation separates from the originating defect, leaving behind a stable misfit dislocation.

Types of EpitaxyLiquid phase epitaxy- III-V epitaxial layer GaAs- Refreeze of laser melted siliconMolecular beam epitaxy- Crystalline layer grows in vacuum- 500o CVapor phase epitaxy- It is performed by chemical vapor deposition

(a)

(b)

(c)

(CVD)

- Provides excellent control of thickness, doping andcrystallinity- High temperature (800o C – 1100oC)

Liquid phase epitaxy Growing crystals from

melting point .Melting point of GaAs

a liquid solution below their

is 1238oC whereas a mixtureof GaAs with Ga metal has considerably lowermelting point

Single crystal GaAs layer can be grown fromGa+GaAs melt.

The solution becomes richer in Ga melting point.

and thus lower

Low temperature eliminates many problems ofimpurity introduction.

LIQUID PHASE EPITAXYGrowth of AlGaAs and GaAs layer on GaAs substrate

Wafer held on carbon slider

Moves into a pocket containing melt

Slider moves the substrate to the next chamber.

Molecular beam

Substrate is held in high vacuum

epitaxy (MBE)

10-10in the range torr

in separateComponents along with dopants, are heatedcylindrical cells.

Collimated beams of these escape into directed into the surface of a substrate

the vacuum and are

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

Equipment

Equipment 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 of400oC to 1100oC.

Advantages of MBELow temperature processing (400oC-800oC)Precise control of dopingNo chemical reactions along with high thermal velocities results in properties rapidly changing with sourceA wider choice of dopantsMostly used dopants are Sb, Ga, Al

Vapor phaseepitaxy

Crystallization from vapor phase

Better purity and crystal perfectionOffers great flexibility in the actual fabrication of devices

Epitaxial layers are generally grown on Si substrates by thecontrolled deposition of Si chemical vapor containing atoms if Si

e.g. SiCl4 + 2H2 Si + 4HCl(for deposition as well as for etching)

Vapor Phase EpitaxyusedFour silicon sources have been

Silicon (Si)Silicon tetrachloride (SiCl4) Dichlorosilane (SiH2Cl2) Trichlorosilane (SiHCl3) Silane (SiH4)Four species in a reaction

for growing epitaxial

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

at 1200o C were detected

Concentration of species at different

positions along a horizontal reactor

Overall reaction in VPE SiCl4 concentration decreases while the other three

constituents (SiHCl3, SiH2Cl2, HCl) increase

SiCl4 + H2

SiHCl3 + H2

SiH2Cl2 + H2

SiHCl3

SiCl2 + H2

SiHCl3 + HCl ……….. (1)SiH2Cl2 + HCl……….. (2)

SiCl2 + H2 …………(3)

SiCl2 + HCl …………. (4)Si + 2HCl ……………(5)

Equipment

Weight 2000 KgOccupy 2m2 or more of floor space. Quartz reaction chamber withsusceptors

Graphite susceptors for physical supportA coating of silicon carbide (50 to500 µm) applied by CVD process on susceptors.Rf heating coil or tungsten halogen lamps.Radiant heatingWater cooling

A radiant barrel reactor

Three basic reactor configurations

VPE 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 Inentional 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

2AsH3 (gas) 2As (solid) + 3H2 (gas)2As (solid) 2As+ (solid) +

2e-

Doping: Schematic representation ofarsine doping and growth processes

2AsH3 (gas) 2As2As

(solid) + 3H2 (gas)

2As+ (solid) + 2e–(solid)

Doping: Impurity concentration Interaction between doping process & growth process Growth rate influences the amount of dopant incorporated in Si Equilibrium established at low growth rates.

AutodopingOutdiffusion from heavily doped substrate Impurity incorporation from dopant in gas phase Autodoping limits the minimum layer thicknessGeneralized doping profile of an epitaxial layer detailing various regions of autodoping

Minimizing Autodoping

• Fast growth to minimize outdiffusion.• Low temperature deposition reduces boron

autodoping (not As however).

• Seal backside of substrate withpolyoxide.

• Avoid the use of HCl etching.• Reduced pressure epitaxy.

highly doped

Silicon 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 sapphire

SiH4 Si + 2H2 (low temperature)

• Temperature 1000o C – 1050oC• Growth rate 0.5 µm/min• Film thickness 1 µm or less

• Doping range 1014 1016 atoms/cm3to

• High defect density permits only majority carrier devices• Carrier mobility is reduced.

Buried layerThe 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 Detector

APPLICATIONS

The driving force today is the fabrication of advanced electronic and optoelectronic devices.

Transistors (HEMT,HBT):

Microwave devices (IMPATT)

Optoelectronic devices (MQW) laser

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