Seminar Report for Svce
-
Upload
shiraz-husain -
Category
Documents
-
view
215 -
download
0
Transcript of Seminar Report for Svce
-
7/31/2019 Seminar Report for Svce
1/9
CRYSTAL GROWTH TECHNIQUES Page 1
SWAMIVIVEKANANDCOLLEGEOFENGINEERING
(RAJIVGANDHIPROUDYOGIKIVISHWAVIDHYALAY, BHOPAL)
SESSION2008-2009
AREPORTON
CRYSTALGROWTHTECHNIQUES
HOD
Mr UDAY CHANDRAWAT
SUBMITTED BY
SHIRAZ HUSAIN
-
7/31/2019 Seminar Report for Svce
2/9
CRYSTAL GROWTH TECHNIQUES Page 2
CERTIFICATE
This is to certified that the seminar report entitled CRYSTAL GROWTH
TECHNIQUES been prepared by SHIRAZ HUSAIN, student of M.E. VLSI DESIGN.
The system has been approved by the department of EC, SWAMI VIKANAND
COLLEGE OF ENGINEERING, INDORE (M.P.) and the work has been done under
my guidance.
The work is up to the mark of satisfaction. We wish him success in every aspect
of life .He has performed this project on his own .He has also put in sufficient
periods for completion. This project has been completed as per rules of RGPV
and can be considered as the fulfillment of the A.I.C.T.E. examination.
Date Signature Seal of the
Institution
-
7/31/2019 Seminar Report for Svce
3/9
CRYSTAL GROWTH TECHNIQUES Page 3
CONTENTS
INTRODUCTIONBRIDGMAN TECHNIQUECZOCRALSKI TECHNIQUEFLOAT ZONE REFINING TECHNIQUECONCLUSION
-
7/31/2019 Seminar Report for Svce
4/9
CRYSTAL GROWTH TECHNIQUES Page 4
INTRODUCTION
BRIDGMANTECHNIQUE
The BridgmanStockbarger technique is named after Harvard physicist Percy
Williams Bridgman and MIT physicist Donald C. Stockbarger (1895 - 1952). They
are two similar methods primarily used for growing single crystal ingots (boules),
but which can be used for solidifying polycrystalline ingots as well.
The methods involve heating polycrystalline material above its melting point and
slowly cooling it from one end of its container, where a seed crystal is located. A
single crystal of the same crystallographic orientation as the seed material is grown
on the seed and is progressively formed along the length of the container. Theprocess can be carried out in a horizontal or vertical geometry.
The Bridgman method is a popular way of producing certain semiconductor
crystals for which the Czochralski process is more difficult, such as galliumarsenide.
The difference between the Bridgman technique and Stockbarger technique is
subtle: while a temperature gradient is already in place for the Bridgman technique,
the Stockbarger technique requires pulling the boat through a temperature gradientto grow the desired single crystal.
When seed crystals are not employed as described above, polycrystalline ingots
can be produced from a feedstock consisting of rods, chunks, or any irregularly
shaped pieces once they are melted and allowed to resolidify. The resultant
microstructures of the ingots so obtained are characteristic of directionally
solidified metals and alloys with their aligned grains.
The schematic diagram is shown below:
-
7/31/2019 Seminar Report for Svce
5/9
CRYSTAL GROWTH TECHNIQUES Page 5
-
7/31/2019 Seminar Report for Svce
6/9
CRYSTAL GROWTH TECHNIQUES Page 6
CZOCRALSKI TECHNIQUE
The Czochralski process is a method of crystal growth used to obtain single
crystals of semiconductors (e.g. silicon, germanium and gallium arsenide), metals
(e.g. palladium, platinum, silver, gold), salts, and synthetic gemstones. The processis named after Polish scientist Jan Czochralski, who discovered the method in 1916while investigating the crystallization rates of metals.
The most important application may be the growth of large cylindrical ingots, or
boules, of single crystal silicon. Other semiconductors, such as gallium arsenide,
can also be grown by this method, although lower defect densities in this case can
be obtained using variants of the Bridgman-Stockbarger technique.
Production of Czochralski silicon
High-purity, semiconductor-grade silicon (only a few parts per million of
impurities) is melted in a crucible, usually made of quartz. Dopant impurity atoms
such as boron or phosphorus can be added to the molten silicon in precise amounts
to dope the silicon, thus changing it into p-type or n-type silicon. This influences
the electronic properties of the silicon. A precisely oriented rod-mounted seed
crystal is dipped into the molten silicon. The seed crystal's rod is slowly pulled
upwards and rotated simultaneously. By precisely controlling the temperature
gradients, rate of pulling and speed of rotation, it is possible to extract a large,
single-crystal, cylindrical ingot from the melt. Occurrence of unwanted instabilities
in the melt can be avoided by investigating and visualizing the temperature and
velocity fields during the crystal growth process. This process is normally
performed in an inert atmosphere, such as argon, in an inert chamber, such asquartz.
Size of crystals
Due to the efficiencies of common wafer specifications, the semiconductor
industry has used wafers with standardized dimensions. In the early days, the
boules were smaller, only a few inches wide. With advanced technology, high-enddevice manufacturers use 200 mm and 300 mm diameter wafers. The width is
controlled by precise control of the temperature, the speeds of rotation and the
speed the seed holder is withdrawn. The crystal ingots from which these wafers are
sliced can be up to 2 metres in length, weighing several hundred kilograms. Larger
wafers allow improvements in manufacturing efficiency, as more chips can be
fabricated on each wafer, so there has been a steady drive to increase silicon wafer
-
7/31/2019 Seminar Report for Svce
7/9
CRYSTAL GROWTH TECHNIQUES Page 7
sizes. The next step up, 450 mm, is currently scheduled for introduction in 2012.
Silicon wafers are typically about 0.20.75 mm thick, and can be polished to great
flatness for making integrated circuits, or textured for making solar cells.
The process begins when the chamber is heated to approximately 1500 degrees
Celsius, melting the silicon. When the silicon is fully melted, a small seed crystal
mounted on the end of a rotating shaft is slowly lowered until it just dips below the
surface of the molten silicon. The shaft rotates counterclockwise and the crucible
rotates clockwise. The rotating rod is then drawn upwards very slowly, allowing a
roughly cylindrical boule to be formed. The boule can be from one to two metres,
depending on the amount of silicon in the crucible.
The electrical characteristics of the silicon are controlled by adding material like
phosphorus or boron to the silicon before it is melted. The added material is called
dopant and the process is called doping. This method is also used withsemiconductor materials other than silicon, such as gallium arsenide.
Monocrystalline silicon grown by the Czochralski process is the basic material in
the production of the large-scale integrated circuit chips used in computers, TVs,cell phones and electronic equipment of all kinds.
When silicon is grown by the Czochralski method, the melt is contained in a silica
(quartz) crucible. During growth, the walls of the crucible dissolve into the melt
and Czochralski silicon therefore contains oxygen at a typical concentration of
1018cm3
. Oxygen impurities can have beneficial effects. Carefully chosen annealing
conditions can allow the formation of oxygen precipitates. These have the effect of
trapping unwanted transition metal impurities in a process known as gettering.
Additionally, oxygen impurities can improve the mechanical strength of silicon
wafers by immobilising any dislocations which may be introduced during device
processing. It was experimentally shown in the 1990s that the high oxygen
concentration is also beneficial for radiation hardness of silicon particle detectors
used in harsh radiation environment (such as CERN's LHC/S-LHC projects).
Therefore, radiation detectors made of Czochralski- and Magnetic Czochralski-
silicon are considered to be promising candidates for many future high-energy
physics experiments.It has also been shown that presence of oxygen in siliconincreases impurity trapping during post-implantation annealing processeD.
-
7/31/2019 Seminar Report for Svce
8/9
CRYSTAL GROWTH TECHNIQUES Page 8
However, oxygen impurities can react with boron in an illuminated environment,
such as experienced by solar cells. This results in the formation of an electrically
active boronoxygen complex that detracts from cell performance. Module outputdrops by approximately 3% during the first few hours of light exposure.
FIG: Czocralski technique
-
7/31/2019 Seminar Report for Svce
9/9
CRYSTAL GROWTH TECHNIQUES Page 9
Float zone refining
Float-zone silicon is very pure silicon obtained by vertical zone melting. The
process was developed at Bell Labs by Henry Theuerer in 1955 as a modification
of a method developed by William Gardner Pfann for germanium. In the verticalconfiguration molten silicon has sufficient surface tension to keep the charge from
separating. Avoidance of the necessity of a containment vessel prevents
contamination of the silicon.
Float-zone silicon is a high-purity alternative to crystals grown by the Czochralski
process. The concentrations of light impurities, such as carbon and oxygen, are
extremely low. Another light impurity, nitrogen, helps to control microdefects and
also brings about an improvement in mechanical strength of the wafers, and is nowbeing intentionally added during the growth stages.
The diameters of float-zone wafers are generally not greater than 150mm due to
the surface tension limitations during growth. A polycrystalline rod of ultra-pure
electronic grade silicon is passed through an RF heating coil, which creates a
localized molten zone from which the crystal ingot grows. A seed crystal is used at
one end in order to start the growth. The whole process is carried out in an
evacuated chamber or in an inert gas purge. The molten zone carries the impurities
away with it and hence reduces impurity concentration (most impurities are more
soluble in the melt than the crystal). Specialized doping techniques like core
doping, pill doping, gas doping and neutron transmutation doping are used to
incorporate a uniform concentration of impurity.
Float-zone silicon is typically used for power devices and detector applications. It
is highly transparent to terahertz radiation, and is usually used to fabricate optical
components, such as lenses and windows, for terahertz applications.