Assignment 1
Table of contents:
1. Introduction……………………………………………………………………………2
2. Czochralski method………………………………………………………………..3
2.1. Introduction to Czochralski method…….………………..…….3
2.2 Steps of Czochralski method………………………………………...4
2.3 Applications of Czochralski method………………………………5
2.4 Advantages and disadvantages of Czochralski method.…6
3. Float zone method………………………………………………………………….7
3.1. Introduction to float zone method……………………………….7
3.2 Steps of Float zone method…………………………………………..8
3.3 Applications of Float zone method………………………………..9
3.4 Advantages and disadvantages of Float zone method….10
4. Comparison between Czochralski and float zone methods…….12
5. Conclusion………………………………………………………………………..…..13
6. Resources……………………………………………………………………………..14
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1. Introduction
Due to its distinct electrical and thermal characteristics, silicon plays a
significant role in the semiconductor industry. Silicon is the most
abundant solid element on earth, being second only to oxygen and it
makes up more than 25% of the earth’s crust. However, it rarely occurs
in elemental form, virtually all of it is existing as compounds, like sand
as shown in Figure (1). Several growth techniques are used to produce
high-quality silicon crystals for electronic devices.
Figure 1: Silicon is a fundamental element in earth crust.
The goal of silicon growth techniques is to produce uniformly shaped
crystals of silicon with a low defect density, as shown in Figure (2). The
performance of electronic devices is dependent on the quality of the
silicon crystals used in their manufacture, so this is significant in the
semiconductor industry.
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The Czochralski (CZ) and Float Zone (FZ)
methods, as well as other techniques like
Bridgman-Stockbarger (BS) growth, epitaxial
growth, and zone melting (ZM) growth, are
used to accomplish this. With these methods,
high-purity silicon crystals are created by
melting a silicon source material and letting it
slowly solidify under controlled
circumstances. [1]
Figure 2: silicon wafers are the final products
of these processes.
The FZ method involves melting a small portion of a silicon rod and
letting it slowly solidify, whereas the CZ method involves slowly
removing a silicon seed crystal from a molten silicon melt. Both
techniques yield silicon crystals of high quality and with a low defect
density, but the method to use depends on the particular application.
Other methods, like epitaxial growth, use chemical vapor deposition to
grow a thin layer of silicon on top of a silicon wafer (CVD). This method
is frequently used to produce thin silicon layers for integrated circuits.
In summary, the goal of silicon growth techniques is to use carefully
controlled melting and solidification processes to create high-quality
silicon crystals with a uniform structure and low defect density. These
methods are essential to produce high-performance electronic devices
in the semiconductor industry.
2. Czochralski method
2.1. Introduction to Czochralski method
The Czochralski method, also known as the Czochralski technique or
Czochralski process, is a method of crystal growth used to produce
single crystals of metals, such as palladium, platinum, silver, and gold,
salts, and artificial gemstones. The technique was developed in 1915
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while a Polish scientist named Jan Czochralski was looking into the rates
at which metals crystallised. By mistake, he dipped his pen in molten tin
instead of his inkwell and drew a
filament of tin, which later turned out to
be a single crystal. [2]
The growth of large cylindrical ingots –
as the one shown in figure (3) - , or
boules, of single crystal silicon used in
the electronics sector to produce
Figure 3: Ingots produced by CZ method.
semiconductor products like integrated
circuits, may be the most significant
application. This technique can also be used to grow other
semiconductors, such as gallium arsenide, though in this case lower
defect densities can be obtained by modifying the BridgmanStockbarger technique.
2.2 Steps of Czochralski method
With this technique, the charge melts and is kept at a temperature just
above its melting point. First, a thin layer of melt is reached by lowering
the pulling rod. The pulling rod's point tip melts because the rod is at a
lower temperature than the rest of the rod.
The crystal is then gradually pulled out. The rate of pulling depends on
several variables, including thermal conductivity, latent heat of charge
fusion, and rate of pulling rod cooling. To maintain the grow crystal's
uniformity and cylindrical shape, the seed is rotated.
A rod with a seed crystal attached to it is slowly rotated. A melt that is
kept at a temperature just above the melting point is used to submerge
the seed crystal. By cooling the rod and gradually removing it from the
melt (the environment is cooler than the melt), a temperature gradient
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is created. The quality of the crystals improves (fewer flaws), but the
growth rate is slowed down, when the crystal is removed from the melt
more slowly.
A summary of the steps in shown in Figure (4).
Figure 4: Summary of the steps of Czochralski method
2.3 Applications of Czochralski method
Numerous manufacturing processes use the Czochralski Process, which
involves the controlled crystallization of molten materials. The
production of large cylindrical ingots, also known as boules and made
of single crystal silicon, may be the process' most significant
application. The fabrication of a wide variety of semiconductor devices,
including integrated circuits, like the shown in figure(5), which are the
foundation of contemporary electronics, depends on these boules.
Additional semiconductors that can be grown using the Czochralski
Process include gallium arsenide, which has special qualities that make
it valuable in a range of electronic applications. It is possible to create
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semiconductors with specific properties, such as
electrical conductivity and optical properties, that
are tailored to meet the needs of various
industries by carefully controlling the growth
conditions during the Czochralski Process.
Overall, the Czochralski Process is essential to the
creation of modern society's indispensable
Figure 5: Example of integrated
electronic devices. This process has revolutionized
circuits that can be produced using
the electronics industry and paved the way for
CZ method.
countless technological advancements by enabling
the growth of high-quality, large-scale single crystals. [3]
Monocrystalline Czochralski silicon is a common name for
monocrystalline silicon (mono-Si) grown using this technique (Cz-Si). It
serves as the building block for the production of integrated circuits,
which are used in a variety of electronic devices like semiconductors,
televisions, computers, and mobile phones. The photovoltaic industry
also makes extensive use of monocrystalline silicon to create traditional
mono Si solar cells. For silicon, the highest light-to-electricity
conversion efficiency is produced by the nearly ideal crystal structure.
2.4 Advantages and disadvantages of Czochralski method
There are several advantages in using Czochralski method including:
1. High purity: The Czochralski method yields crystals with a low
impurity content that are of high purity. This is due to the process' use
of an inert gas or vacuum to melt the raw material, which minimises
contamination.
2. Large crystal size: The Czochralski method can yield large single
crystals with a diameter of up to several inches. This is helpful for
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applications like those in the electronics sector that call for large single
crystals.
3. Precisely control : Reproducible and uniform crystal growth are
made possible by the Czochralski method's ability to precisely control
crystal growth parameters like temperature, pressure, and pulling rate.
3. Float zone method
3.1. Introduction to float zone method
Very pure silicon is produced by vertical zone melting and is known as
float-zone silicon. The method was modified from one created by
William Gardner Pfann for germanium and created at Bell Labs by
Henry Theuerer in 1955. Molten silicon in the vertical configuration has
enough surface tension to prevent the charge from separating. The
main benefit of crucibleless growth is that
it shields silicon from contamination from
the vessel itself, making it a naturally highpurity substitute for boule crystals grown
using the Czochralski method.
Light impurities like carbon (C) and oxygen
(O2) are present in very small amounts.
Nitrogen (N2), another light impurity, is
now purposefully added during the
Figure 6: wafers produced by float zone method.
growth stages because it helps to
control microdefects and improves the mechanical strength of the
wafers. [4]
Due to surface tension restrictions during growth, the diameters of
float-zone wafers are typically no larger than 200 mm. An RF heating
coil is used to produce a localized molten zone from which a
polycrystalline rod of ultrapure silicon of electronic grade grows. The
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growth is initiated at one end by a seed crystal. The entire procedure is
carried out in an inert gas purge or an evacuated chamber.
The impurities are carried away by the molten zone, which lowers the
concentration of impurities (most impurities are more soluble in the
melt than the crystal). To incorporate a uniform concentration of
desired impurity, specialised doping techniques like core doping, pill
doping, gas doping, and neutron transmutation doping are used.
3.2 Steps of Float zone method
High-quality single-crystal materials are purified and grown using a
method called the Float Zone Method. In this process, a tiny piece of
the polycrystalline rod is melted, and the molten zone is then
recrystallized as a single crystal. The Float Zone Method involves the
following steps:
Choosing the initial material: A polycrystalline rod that has first been
grown using another method is typically the starting material for the
Float Zone Method. The initial substance must have the desired crystal
structure and a sufficient level of purity.
Adding the content: The
required heating and cooling
components are loaded into a
vacuum-sealed chamber
along with the polycrystalline
rod. After that, the chamber
is vacuumed to produce a
high vacuum.
Melting of the rod: Using a
focused heating source, such
as a laser or radiofrequency
Figure 7: The melting steps in the FZ method.
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coil, a small portion of the polycrystalline rod is melted. With exact
movements, the molten zone is precisely controlled and moved down
the length of the rod.
Crystal growth: The material behind the molten zone solidifies to form
a single crystal as it travels along the rod. The temperature gradient,
pulling speed, and dopant concentration are just a few of the methods
used to carefully monitor and manage the crystal growth. Repeat the
procedure on the newly formed crystal to further boost its purity and
perfection after the crystal growth is finished.
Finishing and annealing: The crystal is typically annealed to reduce
internal stresses and enhance its mechanical and electrical properties
after it has been grown. To correct any surface flaws and prepare the
crystal for additional processing and use, it may also be polished or
finished in another way.
Overall, the Float Zone Method is a sophisticated and tightly controlled
process that can create single crystals of exceptionally high quality with
a variety of uses in optics, electronics, and other fields.
3.3 Applications of Float zone method
Power devices use these particular silicon crystals that are produced.
This is due to the fact that they are semiconductor materials, which
shield people from electric shock. Additionally, it has detector
applications. Float zone wafers are frequently used in high efficiency
solar products, including chips, as the one shown in figure (8).
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There are numerous optical applications for
float zone wafers. This is because it is
employed in the production of lenses and
windows, which are typically employed in
terahertz applications.
The number of impurities, such as oxygen
and carbon, is incredibly low in float zone
silicon. For this reason, float zone silicon is
utilized instead of czochalski grown silicon. It
can go through a lighter doping process
Figure 8: Example of solar products produced by
Float zone method
thanks to the purity of the silicon in the float
zone. In some cases, silicon from the float
zone results in measurements of high resistivity.
The most common material used in the production of high efficiency
solar panels and discrete power devices is FZ silicon. Based on scientific
research, silicon is a semiconductor. As a result, it is ideal for electric
equipment because it lessens the hazards related to electricity. [5]
3.4 Advantages and disadvantages of Float zone method
There are several benefits and drawbacks to the Float Zone Technique.
Some of the main benefits are as follows:
High purity: Crystals made using the Float Zone Technique can have
impurity levels as low as 1 part per million (ppm).
High perfection: The Float Zone Technique can produce crystals with
very low defect densities, which makes them appropriate for uses
where good crystal quality is crucial.
Customized doping: The Float Zone Technique enables fine control
over dopant concentrations, enabling the crystal's characteristics to be
tailored to applications.
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Huge crystals: Using traditional techniques, it can be challenging to
develop crystals as large as several centimeters in diameter. The Float
Zone Technique makes this possible.
Continuous growth: Continuous crystal growth is achievable using the
Float Zone Technique, resulting in the production of long,
homogeneous crystals.
Also, there are several disadvantages of Float zone method including:
Time-consuming: Crystal growth rates for the Float Zone Technique are
typically only a few millimeters per hour. This implies that the growth
of a single crystal may need several days or even weeks.
Expensive: The Float Zone Technique is an expensive approach when
compared to other crystal growth techniques since the equipment
required is complicated and expensive.
Restricted accessibility: The Float Zone Technique is a highly specialized
procedure that can only be used by people with a high level of skill. It
might not be publicly accessible or available to researchers outside of
specialist facilities as a result.
Restricted scalability: The Float Zone Technique is more suited for
laboratory-scale research than industrial-scale manufacture because it
is not readily scalable to large-scale production.
Restricted crystal shapes: Cylindrical crystals are commonly grown
using the Float Zone Technique, which may not be appropriate for all
applications.
Overall, the Float Zone Technique is an effective method for producing
single crystals of high quality, but it necessitates a high level of
knowledge and specialized equipment, and it may not be appropriate
for many applications.
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4. Comparison between Czochralski and float zone methods
Method
Principle
Crystal purity
Crystal quality
Control
Dopant
concentration
Equipment
complexity
Crystal shape
Growth rate
Applications
Float Zone
Melting and
solidification of a
small portion of a
rod, which is
progressively
moved along its
length
Extremely high
Excellent
Precise control over
the zone of melting
and solidification
Can be precisely
controlled
High
Czochralski
Growth of a single
crystal by pulling it
out of a melt
Cylindrical
Conical or
cylindrical
Moderate to fast
Production of large
single crystals for
use in
semiconductor and
optical applications
Slow
Research,
production of highpurity crystals for
semiconductor and
solar cell
applications
High
Good to excellent
Control over the
melt composition
and temperature
Can be difficult to
control
Moderate
Table 1: Comparison between FZ and CZ
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5. Conclusion
In conclusion, the growth of high-quality single crystals for a variety of
applications is largely carried out using the Float Zone and Czochralski
processes, particularly in the semiconductor and solar cell sectors.
While Czochralski can make larger crystals at a faster rate and is known
for producing exceptionally pure and high-quality crystals with fine
dopant control, Float Zone is a more complicated and time-consuming
procedure. In the end, the decision between these strategies is based
on the particular needs of the application, as well as the tools and
resources accessible.
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6. Resources
[1] Hu, X. (2021). Research on silicon wafer manufacturing process and
physical properties testing using high-purity polysilicon. Journal of
Physics: Conference Series, 2083(2), 022050.
https://doi.org/10.1088/1742-6596/2083/2/022050
[2] Silicon Wafer Manufacturing: The process of growing silicon ingots.
Wafer Manufacturing. (n.d.).
https://www.waferworld.com/post/silicon-wafer-manufacturingthe-process-of-growing-silicon-ingots
[3] Crystal growth in the process of modified Czochralski. (2006).
Magnetohydrodynamics, 42(4), 451–468.
https://doi.org/10.22364/mhd.42.4.12
[4] Lüdge, A., Riemann, H., Wünscher, M., Behr, G., Löser, W.,
Muiznieks, A., & Cröll, A. (2010). Floating zone crystal growth.
Crystal Growth Processes Based on Capillarity, 203–275.
https://doi.org/10.1002/9781444320237.ch4
[5] Basics of Solar Cell. Basics of solar cell, solar photovoltaic modules.
(n.d.).
https://www.leonics.com/support/article2_13j/articles2_13j_en.p
hp#:~:text=Solar%20Cell%20or%20Photovoltaic%20(PV,created%2
0at%20positive%2Fnegative%20junctions.
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