2. Introduction
• Epitaxy comes from Greek words:
– Epi: upon
– Taxis: ordered
• The term epitaxial comes from the Greek word meaning ‗arranged upon‘.
• Epitaxial growth: single crystal growth of a material in which a substrate serve as a seed
• In semiconductor technology, it refers to the single crystalline structure or a film deposited
over the silicon wafer.
• Epitaxy or epitaxial growth is the process of depositing a thin layer (0.5 to 20 microns) of
single crystal material over a single crystal substrate usually through chemical vapor
deposition (CVD).
• In the same material as the substrate, and the process is known as homoepitaxy, or simply,
epi. An example of this is silicon deposition over a silicon substrate.
• Two types of epitaxy:
1. Homoepitaxy – material is grown epitaxially on a substrate of the same material. E.g.
growth of Si on Si substrate
2. Heteroepitaxy – a layer grown on a chemically different substrate. E.g. Si growth on
sapphire
• Similar crystal structures of the layer and the substrate, BUT
– The shift of composition causes difference in lattice parameters
– Limit the ability to produce epitaxial layers of dissimilar materials
Epitaxy or epitaxial growth is the process of depositing a thin layer (0.5 to 20 microns) of single
crystal material over a single crystal substrate, usually through chemical vapor deposition (CVD).
3. Applications of epitaxial layers
1. Discrete and power devices
2. Epitaxy for MOS devices
3. Integrated circuits
Silicon Epitaxy is done to improve the performance of bipolar devices.
By growing a rightly doped epi layer over a heavily-doped silicon substrate:
• a higher breakdown voltage across the collector-substrate junction is achieved
• while maintaining low collector resistance.
• Lower collector resistance allows a higher operating speed with the same current.
Epitaxial technique is used to developed 2 and 3 layers epitaxial structure
• For lightly doped area of collector
• Base region was also grown epitaxially
Example of multilayer structures: Si-Controlled Rectifier (SCR), Triac, high voltage or high power discrete products
Epitaxy has also recently been used in CMOS VLSI circuits.
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
•Aside from improving the performance of devices, epitaxy also allows better control of doping concentrations of
the devices
•The layer can also be made oxygen- and carbon-free.
4. Epitaxy in Integrated circuit (IC)
• Development of planar bipolar IC require devices built on the same substrate to be electrically
isolated
– The use of opposite type 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‘ ( 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
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.
5. 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 (MBE)
3. Liquid Phase Epitaxy (LPE)
4. Solid phase regrowth
6. 1. Chemical Vapour Deposition (CVD)
• It is a chemical process for depositing thin films of various materials.
• The most common technique in Si epitaxy
• Chemical Vapor Deposition is the formation of a non-volatile solid film on a substrate by the
reaction of vapor phase chemicals (reactants) that contain the required constituents.
• The reactant gases are introduced into a reaction chamber and are decomposed and reacted at a
heated surface to form the thin film.
• In the CVD technique
– Si substrate is heated in a chamber: sufficient heat to allow the depositing Si atoms to move
into position
– Si is exposed to one or more volatile gases, which react and/or decompose -on the hot
substrate surface to produce the desired deposit.
– Gases react on 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
• Frequently, volatile byproducts are also produced.
Examples of CVD films
8. • 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
9. Explanation of Steps involved in a CVD process
3. 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.
1. 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 surface.
11. 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
12. CVD of Si - Epitaxy
•When SiH4 gas is used in a CVD reactor, a Si layer is deposited on the wafer surface.
•The size of the crystallites depends on the deposition temperature.
•At high enough temperature, the ad-atoms have enough kinetic energy to move on the surface and
align themselves with the underlying Si.
•This is an epitaxial layer, and the process is called Epitaxy instead of CVD.
•At lower deposition temperatures, the layer is poly-crystalline Si (consisting of small crystallites)
Si Epitaxy
The chemical reaction that produces the
Si is fairly simple:
SiCl4(g)+2H2(g)=(1000-1200oC)=Si(s)+4HCl(g)
Instead of SiCl4 you may
want to use SiHXCl4-X
The chemical vapor deposition of silicon
epitaxy is usually achieved using an
epitaxial reactor (Fig. 1) that consists
of a quartz reaction chamber into
which a susceptor is placed.
The susceptor provides two things:
1) mechanical support for the wafers
2) an environment with uniform thermal
distribution.
Epitaxial deposition takes place at a high
temperature as the required process
gases flow into the chamber.
(Fig. 1)
Si Epitaxy:
can form very thick doped structures (30-100 um) not
possible with implantation or diffusion.
Such thick, pure layers are often used in power devices while
thinner, 1-5 um, are commonly used for many CMOS and
bipolar technology.
14. 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
15. CVD film growth steps
Types of film structure
Basic CVD subsystem
Amorphous
Polycrystalline
Single crystal
16. Advantages of CVD processes
CVD processes are ideally suited for depositing thin layers of materials on
some substrate. In contrast to some other deposition processes which we
will encounter later, CVD layers always follow the contours of the
substrate: They are conformal to the substrate as shown below.
Disadvantages of CVD processes
The two most important ones (and the only ones we will
address here) are:
1. They are not possible for some materials; there simply is no
suitable chemical reaction.
2. They are generally not suitable for mixtures of materials.
17. A number of forms of CVD are in wide use and a‘
frequently referenced in the literature
• Plasma Enhanced CVD (PECVD) CVI
processes that utilize a plasma to enhanc chemical
reaction rates of the precursorf PECVD processing allows
deposition t‘Llower tern er res, which Is often critical i I e
manufacture of semiconductors.
• Rapid Thermal CVD (RTCVD) - CVI processes that use
heating lamps or oth methods to rapidly heat the wafer
substrat‘ Heating only the substrate rather than th gas or
chamber walls helps reduce& unw h e reactiofls that cãii1
oc1ii&ma -
• A(mosIieric Pressure CVD (APCVD) CVD processes at
atmospheric pressure.
• Low Pressure CVD (LPCVD) • CV processes at
subatmospheric pressure Reduced pressures tend to redL
unwanted gas phase reactions and irnpro{ film
uniformity across the wafer. M.t. modern CVD process
are either LPCVD UHVCVD.
• Ultra-High Vacuum CVD (UHVCVD) - C processes at
very low pre5sures, typically the range of a few to a
hundred millltorr5.
-.--- —-- —ii -
Tillat 11
18.
19. Molecular Beam Epitaxy
• In MBE, a source material is heated to produce an evaporated beam of particles.
• These particles travel through a very high vacuum (10-8 Pa ; practically free space) to the substrate, where
they condense.
• Means that Molecular beam epitaxy takes place in high vacuum or ultra high vacuum (10−8 Pa).
• The most important aspect of MBE is the slow deposition rate (1 to 300 nm per minute), which allows the
films to grow epitaxially.
• However, the slow deposition rates require proportionally better vacuum in order to achieve the same
impurity levels as other deposition techniques.
• MBE has lower throughput than other forms of epitaxy.
• This technique is widely used for growing III-V semiconductor crystals. Wiki
• Molecular beam epitaxy (MBE) was developed in the early 1970s as a means of growing high-purity epitaxial
layers of compound semiconductors.
• Since that time it has evolved into a popular technique for growing Ill-V compound semiconductors as well as
several other materials.
• MBE can produce high- quality layers with very abrupt interfaces and good control of thickness, doping, and
composition.
• Because of the high degree of control possible with MBE, it is a valuable tool in the development of
sophisticated electronic and optoelectronic devices.
• The term ‗beam‖ simply means that evaporated atoms do not meet each other or any other gases until they
reach the wafer.
Tilat
• In solid source MBE, ultra-pure elements such as gallium and arsenic are heated in separate furnaces until they each slowly begin to evaporate.
• (The material sources, or effusions cells, are independently heated until the desired material flux is achieved.
• Changes in the temperature of a cell as small as 0.5°C can lead to flux changes on the order of one percent).
• The evaporated elements (the arsenic in this case is actually in a molecular form) condense on the wafer, where they react with each other forming, in this case, gallium arsenide.
Tilat =done this para else where
20. 2. Molecular Bean Epitaxy (MBE)
• MBE is a non-CVD epitaxial process that 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 auto-doping which is
unintentionally transfer of dopant into epitaxial layer
• MBE is favourable in
– 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
Principle:
• 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
Offers the highest purity material (due to UHV conditions) and the best layer control (almost any fraction of an
atomic layer can be deposited and layers can be sequenced one layer at a time (for example Ga then As then Ga
etc…).
21. • Conventional temperature range
– for MBE is from 400-800oC
– Higher temperature are feasible, but the advantages of reduced outdiffusion and auto-doping are
lost.
• Growth rates in the range 0.01 to 0.3µm/min have been reported.
• In-situ cleaning of the substrate is done in two ways
– First technique: by high temperature bake at 1000-1250C for several(30) minutes under high
vacuum to decompose the native surface oxide and to remove other surface contaminants such as
carbon
– Other technique: is by using a low energy beam of inert gas to sputter clean the substrate
– Difficult to remove carbon but short anneal at 800-900C will reorder the surface
– Wider range of dopants for MBE than CVD epitaxy:
– Typical dopants: Antimony, Sb (N-type), aluminium, 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, evaporates slowly even at 1300C
Molecular Beam Epitaxial System
Important Parameters:
22. Fig.1. Schematic of an elementary MBE System
Molecular Beam Epitaxial System
• In contrast to CVD processes, MBE is not complicated by boundary-layer transport effects, nor are there
chemical reactions to consider.
• The essence of the process is an evaporation of silicon and one or more dopants, as depicted in Fig.
• The evaporated species are transported at a relatively high velocity in a vacuum to the substrate.
• The relatively low vapor pressure of silicon and the dopants ensures condensation on a low-temperature
substrate.
• Usually, silicon MBE is performed under ultra-high vacuum (UHV) conditions of 10-8 to 10-10 Torr, where the
mean free path of the atoms is given by
where L is the mean free path in cm, and P is the system pressure in Torr.
At a system pressure of 10-9 Torr, L would be 5 x 106 cm.
• The mean free path is very long (can be hundreds of meters)
• because ultra high purity materials are evaporated in an UHV
chamber and because of the very low pressure.
• Thus, the evaporated material travels in a straight line (a
molecular beam) toward a hot substrate.
• Once on the substrate, the atom or molecule moves around
until it finds an atomic site to chemically bond to.
System Equipment:
• An elementary MBE system is shown in Fig.1.
• It is, in essence, a UHV chamber where furnaces holding
electronic-grade silicon and dopant direct a flux of
material to the heated surface.
23.
24. • Fig. 2 illustrates the many components of a comprehensive system.
• A distinguishing feature of MBE is the ability to use sophisticated
analytical techniques in situ to monitor the process.
• In contrast to the CVD process, MBE does not require extensive safety
precautions, although solid arsenic dopant must be handled carefully.
• The vacuum system is the heart of the apparatus.
• To consistently attain a vacuum level in the 10-10 Torr range, materials
and construction choices must be carefully considered.
• Materials should have low vapor pressure and low sticking coefficients.
Sze83
Fig.2 -Schematic of practical MBE system.
• Repeated exposure to air is detrimental to a UHV system because of the long bakes needed to desorb
atmospheric species from the system walls.
• A load lock system minimizes this problem.
• Consistently low base pressure is needed to ensure overall film perfection and purity.
• These needs are best met with an oil-free pump design, such as a cryogenic pump.
• Because of its high melting point, silicon is volatilized not by heating in the furnace, but by electron-beam
heating.
• Dopants are heated in a furnace.
• A constant flux is assured by the use of closed-loop temperature control.
• Baffles and shutters shape and control the flux, so uniformity of doping and deposition can he attained
without boundary layer effects.
• Substrates are best heated when they are placed in proximity to a resistance heater with closed-loop
temperature control.
• Resistance heating generates temperatures over the range of 400 to 1100°C. A wide choice of temperature-
sensing methods is available, including thermocouples, optical pyrometry, and infrared detection.
25. • The ultra-high vacuum environment within the growth camber- is—maintained by a system of cryopumps and
cryopanels, chilled using liquid nitrogen to a temperature of 77 Kelvin (−196 degrees Celsius).
• The wafers on which the crystals are grown are mounted on a rotating platter which can be heated to several
hundred degrees C during operation.
• For improved layer uniformity, the sample holder is designed for continual azimuthal rotation of the sample,
and is thus commonly termed the ‗CAR‘.
• The ‗CAR‘ also has an ion gauge mounted on the side opposite the sample which can read the chamber
pressure, or be placed facing the sources to measure beam equivalent pressure (BEP) of the material sources.
Tilat pp.10
toroidal (azimuthal) field
• Shutters can be used to turn the beam flux on and off
• The flux of atoms/molecules is controlled by the temperature of the ―effusion cell‖ (evaporation source).
•A computer controls the shutter in front of each furnace, allowing precise control of the thickness of each layer,
down to a single layer of atoms.
•During operation, RHEED (Reflection High Energy Electron Diffraction) is often used for monitoring the
growth of the crystal layers.
• In solid source MBE:
• ultra-pure elements such as gallium and arsenic are heated in separate
furnaces until they each slowly begin to evaporate.
• (The material sources, or effusions cells, are independently heated until the
desired material flux is achieved.
• Changes in the temperature of a cell as small as 0.5°C can lead to flux
changes on the order of one percent).
• The evaporated elements (the arsenic in this case is actually in a molecular
form) condense on the wafer, where they react with each other forming, in
this case, gallium arsenide.
Tilatpp. 9 more to from pp.10
26. RHEED (Reflection High Energy Electron Diffraction):
•One of the most useful tools for in-situ monitoring of the growth is reflection high-energy electron diffraction
(RHEED).
•It can be used to calibrate growth rates, observe removal of oxides from the surface.
•calibrate the substrate temperature, monitor the arrangement of the surface atoms, determine the proper arsenic
overpressure and provide information about growth kinetics.
•The RHEED gun emits ~1OKeV electrons which strike the surface at a shallow angle (~0. 5-2 degrees), making it a
sensitive probe of the semiconductor surface.
•Electrons reflect from the surface and strike a phosphor screen forming a pattern consisting of a spectral reflection
and a diffraction pattern which is indicative of the surface crystallography.
•A camera monitors the screen and can record instantaneous pictures or measure the intensity of a given pixel as a
function of time.
27. • Present since 1960 but was not in use due to absence of industrial equipment and quality was not suitable for
device needs.
• Equipment is now commercially available, but the process has low throughput and is expensive.
• MBE, however, does have a number of inherent advantages over CVD techniques.
• Its main advantage for VLSI use is low- temperature processing.
• Low-temperature processing minimizes outdiffusion and auto-doping, a limitation in thin layers prepared by
conventional CVD.
• Another advantage is the precise control of doping that MBE allows.
• Because doping in MBE is not affected by time-constant considerations as is CVD epitaxy, complicated doping
profiles can be generated.
• Presently, these advantages are not being exploited for IC fabrication, but they have found application in
discrete microwave and photonic devices.
• For example, the C-V characteristic of a diode with uniform doping is nonlinear with respect to reverse bias.
• Varactor diodes used as FM modulators could advantageously employ a linear dependence of capacitance on
voltage.
• This linear voltage—capacitance relationship can be achieved with a linear doping profile, which is easily
obtained with MBE.
Sze-pp80
Advantages and Disadvantages of MBE
28. 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
Ref:3-epitaxy growth-2=USMalaysia
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)
Details in Ref:3-epitaxy growth-2=USMalaysia
