Seminar Report for Svce

7/31/2019 Seminar Report for Svce

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CRYSTAL GROWTH TECHNIQUES Page 1

SWAMIVIVEKANANDCOLLEGEOFENGINEERING

(RAJIVGANDHIPROUDYOGIKIVISHWAVIDHYALAY, BHOPAL)

SESSION2008-2009

AREPORTON

CRYSTALGROWTHTECHNIQUES

HOD

Mr UDAY CHANDRAWAT

SUBMITTED BY

SHIRAZ HUSAIN


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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


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CONTENTS

INTRODUCTIONBRIDGMAN TECHNIQUECZOCRALSKI TECHNIQUEFLOAT ZONE REFINING TECHNIQUECONCLUSION


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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:


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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


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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.


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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


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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.

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