MCG 5138 Project_Electronic_Group 6

8/19/2019 MCG 5138 Project_Electronic_Group 6

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Surface Engineering in Electronic Industry Semiconductor Doping

UNIVERSITY OF OTTAWA

DEPARTMENT OF MECHANICAL ENGINEERING

SURFACE Engineering

Coatings and Thin Films Technologies MCG 5138

Submitted by

SUBMITTED BY- PROJECT GROUP 6

Akshay Makhija (uOttawa ID: 7904761)

Ratandeep Pandey (uOttawa ID: 8411658)

Harshul Patel (uOttawa ID: 8371068)

Dhrumil Prajapati (uOttawa ID: 8493686)


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MCG 5138 Project Electronic Industry Semiconductor Doping Page 1

TABLE OF CONTENTS

Sr. No. Topic Page No.

1. Introduction 2

1.1 History & Development 2

1.2 Vacuum tube 3

2. Semiconductor 5

2.1 Types of Semiconductor 5

2.2 Material used in Semiconductor 7

3. Techniques in Semiconductor doping 9

3.1 Ion Implantation 9

3.1.1 Characteristics of Ion Implantation 11

3.1.2 Advantages & Disadvantages 12

4. Diffusion 13

4.1 Diffusion Methods 14

4.1.1 Characteristics of Diffusion 15

4.1.2 Advantages & Disadvantages 16

5. New Experimental Method – Gas Immersion Laser Doping 16

6. Table of comparative study 18

7. Conclusion 198. Reference 20


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Surface Engineering in Electronic Industry Semiconductor Doping

Ratandeep Pandeya, Akshay Makhijaa, Harshul Patela, Dhrumil Prajapatia,

aMaster’s Student Mechanical Engineering, University of Ottawa, 75 Laurier Ave E, Ottawa, ON

K1N 6N5 Canada

1. INTRODUCTION:

In electronic and I.T sector, semiconductor are extensively used in manufacturing of thermistor,

IR sensors, diodes, transistors, and so forth. The reason of such an extensive use of semiconductor

in electronic equipment is due properties such as slope of fixation for a dopant in a substrate gives

distinctive properties, for example, variable conductivity, light emanation, warm vitality

transformation, and so on. The most popular materials used in substrate are Silicon (Si) and

Germanium (Ge), which are both the elements in group IV of the table and have 4 valence electrons

in their outer shell.

1.1 History & Development in Electronic and Semiconductor Industry:

With technological improvements in the telegraph industry during late 19th century industrial era

and in the radio and telephone industries during the early 20th century. World War I pushed this

development to an unprecedented scale and a lot of technological leaps were made during this era.

The modern electronic industry was born out of telephone-, radio-, and television-equipment

development and the large amount of electronic-systems development during World War II of

radar, sonar, communication systems, and advanced munitions and weapon systems. In the

interwar years, the subject was known as radio engineering. The word electronics began to be used

in the 1940s. The electronic laboratories (Bell Labs in the United States for instance) created and

subsidized by large corporations in the industries of radio, television, and telephone equipment,

began churning out a series of electronic advances. Invented in 1904 by John Ambrose Fleming,

vacuum tubes were a basic component for electronics throughout the first half of the twentieth

century.


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1.2 What is vacuum tube?

In electronics, a vacuum tube or electron tube is device that controls electric current between

electrodes in an evacuated container. Vacuum tubes mostly rely on thermionic emission of

electrons from a hot filament or a heated cathode by the filament. This type of tube is called as a

thermionic tube or thermionic valve. A phototube, though, achieves electron emission through the

photoelectric effect only. Gas-filled tubes are similar devices containing a gas, typically at low

pressure, which exploit phenomena related to electric discharge in gases, usually without a heater.

How it works?

All modern vacuum tubes are established on the concept a heated "cathode" boils off electrons into

a vacuum; electrons pass through a grid or many grids, which control the electron current; the

electrons then strike on the anode (plate) and are absorbed into it. The tube will make a small AC

signal voltage into a larger AC voltage by designing the cathode, grid(s) and plate suitably. Thus

we can amplify it.

Figure shows a characteristic modern vacuum tube. It is a glass bulb with wires passing through

its bottom, and connecting to the various electrodes inside. A powerful vacuum pump sucks all the

air and gases out before the bulb is sealed. To make a good tube, the pump must make a vacuum

with no more than a millionth of the air pressure at sea level. The "harder" the vacuum, the better

the tube will work and it will have long life.


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Figure 1: Schematic Diagram of vacuum tube

Advantages of vacuum tube:

1) Higher sound quality.

2) Highly linear without negative feedback, specifically for small-signal types.

3) Easily tolerate large overloads and voltage spikes.

4) Characteristics are highly independent of temperature, greatly simplifying biasing.

5) Wider dynamic range available than transistors circuits, because of higher operating

voltages and overload tolerance.

6) Device capacitances differ only slightly with signal voltages.

7) Capacitive coupling can be done with small, high-quality film capacitors, due to inherently

high-impedances of tube circuits.

8) Tubes can be comparatively easily replaced by user.


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Disadvantages of vacuum tube:

1) Bulky, hence less suitable for portable products.

2) Higher operating voltages generally required.

3)

High power consumption; needs heater supply that generates waste heat and produceslower efficiency, particularly for small-signal circuits.

4) Cathode electron-emitting materials are used up in operation.

5) Sometimes higher cost than equivalently powered transistors.

In 1948 came the transistor and in 1960 the integrated circuit, which would revolutionize the

electronic industry. Semiconductors used in integrated circuits facilitated the development of many

technologies including wireless telegraphy, radio, television, radar, computers and

microprocessors.

2. SEMICONDUCTOR:

A semiconductor is defined as a solid chemical element or compound that can conduct electricity

under some conditions, making it a decent medium for the control of electrical current. Depending

on the current or voltage applied to a control electrode its conductance varies, or on the intensity

of irradiation by infrared (IR), visible light, ultraviolet (UV), or X rays. Most semiconductors are

crystals made of certain materials, most commonly silicon.

A semiconductor is capable of functioning of a vacuum tube which has hundreds of times its

volume. A single integrated circuit (IC) can do the work of a set of vacuum tubes that would fill a

large building and require its own electric generating plant.

2.1 TYPES OF SEMICONDUCTOR DOPING:

N-type semiconductor:

It is created when the dopant is an element that consists of five electrons in its valence layer.Most commonly used for this purpose is phosphorus.

The phosphorus atoms bond with the crystal structure of the silicon, each one bonding with

four adjacent silicon atoms just like a silicon atom. As phosphorus atom has five electrons in


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its outermost shell, and since four of them are bonded to adjacent atoms, the fifth valence

electron is unable to form a bond.

The extra valence electrons in the phosphorous atoms then start to behave like the single

valence electron in a regular conductor such as copper. They are free to move about. Sincethis type of semiconductor has extra electrons, it's called an N-type semiconductor.

Figure 2: Microstructure arrangement of P-type semiconductor

P-type semiconductor:

This type of semiconductor is created when the dopant (such as boron) is left with three

electrons in its valence shell. When small amount of dopant is added into the crystal, the atom

is able to bond with four silicon atoms and since it has only three electrons to offer, a hole is

created. This hole behaves like a positive charge, hence semiconductors doped in this way are

called P-type semiconductors.

Just like a positive charge, holes attract electrons. But when an electron moves into a hole, a

new hole is formed at its previous location. Hence, in a P-type semiconductor, holes move

around continuously within the crystal as electrons constantly try to fill them up.


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Figure 3: Microstructure arrangement of N-type semiconductor

As voltage is applied to either an N-type or a P-type semiconductor, current flows just like it flows

in a regular conductor. The negative side of the voltage pushes electrons, and the positive side

pulls them. The outcome is that the random electron and hole movement which is always presentin a semiconductor becomes organized in one direction thereby creating measurable electric

current.

2.2 MATERIALS USED FOR SEMI CONDUCTOR DOPING:

By far, silicon is the most widely used material in semiconductor devices. Its combination of low

raw material cost, relatively simple processing and a useful temperature range make it currently

the best compromise among the various competing materials. Silicon used in manufacturing

semiconductor device is currently fabricated into boules that are large enough in diameter to allow

the production of 300mm (12 inch) wafers.

Germanium (Ge) was widely used early as a semiconductor material but it is less useful than

silicon due to its thermal sensitivity. Today, Germanium-Silicon alloy is formed for use in very

high speed devices of which IBM is the largest producer. Silicon is abundantly available in earth's


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crust so it is easily available resulting in low cost. It had a limited usable temperature range, lower

voltage operation with greater current leakage. With advancement in technology, silicon proved

to be superior, though at the highest operating frequencies, elements of group III and group VI

prove to be superior – eg: Gallium Arsenide. With better physical and chemical properties and

being cheaper, Silicon ensured its progress. The only germanium components in the age of silicon

was for germanium signal diodes, where their lower voltage drop was often a desirable feature.

Germanium is also used to overcome the losses which occur due to silicon. Finfets are currently

being used in the scaling industry (14nm) to reduce the leakage current and to develop integration

density and performance of chips. It has been established that germanium finfets give better

performance when compared to silicon and helps in reducing short channel effects to a greater

extent.

The most important reason why Si is preferred over Ge is that it forms Silicon Dioxide due to

oxidation. SiO2 has very good passivizing qualities, have high dielectric constant and can protect

the chip from hostile surroundings. It can be etched only by HF acid. On the other hand,

Germanium oxide can be easily washed away by water. In addition, Si is easily and abundantly

available and can be purified to a high degree.

Figure 4: Table describing valance electronic configuration of dopants


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POPULAR DOPING IN SEMICONDUCTOR:

Doping is a process of introducing impure atoms into a semiconductor in a controllable manner to

improve its electrical properties. The doping with donors and acceptors allows to alter the electron

hole concentration in semiconductor in a very large range from 10 cm-3 up to 10 cm . The

carrier concentration can also be varied quite accurately which is used to produce PN-junctions

and built-in electric fields.

3. TECHNIQUES IN SEMICONDUCTOR DOPING:

3.1 ION IMPLANTATION

In the ion implantation process, charged dopants (ions) are accelerated in an electric field and

irradiated onto the wafer. The depth of penetration can be set precisely either by reducing or

increasing the voltage needed to accelerate the ions. As the process is taking place at room

temperature, previously added dopants cannot diffuse out. Masking photo resist layer can be used

to cover the regions that need not to be doped. An implanter consists of the following components:

Ion source: To ionize the dopants which are in gaseous state (e.g. boron trifluoride BF3).

Accelerator: The ions are drawn with approximately 30 kilo electron volts out of the ion

source

Mass separation: the charged particles are deflected at 90 degrees under the influence of

magnetic field. Very light particles are deflected more and too heavy particles are deflected

less than the desired ions and are trapped with screens behind the separator

Acceleration lane: the particles are accelerated to their final velocity (200 keV accelerate

ions up to 2.000.000 m/s)

Lenses: lenses are used to focus the ion beam

Distraction: the ions are deflected with electrical fields to irradiate the desired destination

Wafer station: Wafers are held into the ion beam after placing them on large rotating

wheels.


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Figure 5: Illustration of an ion implanter

Penetrating depth of ions in the wafer: In contrast to diffusion processes, the particles do not

penetrate into the crystal due to their high velocity. The particles are slowed down by collisions

with silicon which causes damage to the lattice. Since silicon atoms are knocked from their places,

the dopants themselves are mostly placed interstitial. There, they are not electrically active as there

are is no bonding with other atoms that may give rise to free charge carriers.

Recovery the crystal lattice and activation of dopants: Immediately after the implantation process, about 5 % of the dopants are still bonded in the lattice. At temperatures about 1000 °C,

the dopants move on lattice sites. The lattice damage due to the collisions were already cured at

about 500 °C. These steps are carried out only for a very short time because the dopants move

inside the crystal during high temperature processes

Channeling: The substrate is present as a single crystal due to which the silicon atoms are

regularly arranged and form "channels". The dopant atoms injected by ion implantation can move

parallel to these channels and are slowed down slightly, hence penetrate deeply into the substrate.

In order to overcome this, some of the possibilities are as follows:

Wafer alignment: Regarding the ion beam, the wafers would be deflected at angle of 7

degrees. Thus the radiation is not parallel to the channels and the ions are decelerated

immediately by collision.


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Scattering: A parallel arrival is prevented by a thin oxide layer which deflects the ions

Figure 6: Ion Implantation Channeling Process

3.1.1 Characteristic:

The precision of ion implantation is very high.

the outward diffusion of other dopants is prevented by carrying out the process at room

temperature

spin coated photo resist as a mask is sufficient, an oxide layer, , is not necessary

ion implanters are very expensive, the costs per wafer are relatively high

the dopants do not spread laterally under the mask (only minimally due to collisions)

Wide range of elements can be implanted in the crystal with highest purity

Dopants which were previously used to deposit on walls or screens inside the implanter

and later be carried to the wafer.

three-dimensional structures (e.g. trenches) cannot be doped by ion implantation

the implantation process takes place under high vacuum, which must be produced with

several vacuum pumps

There are several types of implanters for small to medium doses of ions (1011 to 1015 ions/cm2) or

for even higher doses of 1015 to 1017 ions/cm2. The ion implantation has replaced the diffusion

mostly due to its advantages.


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3.1.2 Advantage and Disadvantage

Advantages:

1) It is a low temperature process.

2) The dose of the ions can be controlled.

3) It is possible to control the precise depth.

4) This process can be used to implant ions through thin layers of oxide.

5) This method can be used to obtain extremely low as well as high dope.

6) It is a fast process.

Figure 7: Different parts of ion implantation equipment

Disadvantages:

1) The major disadvantage is that ion implantation process causes physical damage to the surface.

2) Annealing is required to relieve the stresses and remove physical damage to the material.

3) Amorphous regions are formed in the crystal lattice.

4) Channeling causing irregular distribution of ions.

5) It is an expensive process.

6) It is one of the most hazardous process tools available in the semiconductor industry.


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4. DIFFUSION

Diffusion is defined as the molecular movement from a region of higher concentration to one of

lower concentration by random motion of molecules. The result of diffusion is a gradual mixing

of materials. For example, a drop of ink in a glass of water is evenly distributed after a certain

amount of time. The diffusion can be performed in different ways:

Empty space diffusion: the empty spaces in the crystal are filled by the impurity atoms which

are always present, even in perfect single crystals

Inter lattice diffusion: the impurity atoms move in-between the silicon atoms in the crystal

lattice.

Changing of places: the impurity atoms are located in the crystal lattice and are exchanged

with the silicon atoms.

Figure 8: Illustration of Diffusion Process

The dopant will continue to diffuse till concentration gradient is balanced, or the temperature was

lowered, so that the atoms can no longer move. The speed of the diffusion process depends on

several factors:

Dopant

Concentration gradient

Temperature

Substrate


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Crystallographic orientation of the substrate

4.1 DIFFUSION METHODS:

Diffusion with an exhaustible source

It means that the dopant is in limited quantity. As the diffusion process continues, the concentration

at the surface decreases, hence the depth of penetration into the substrate increases. The diffusion

coefficient of a substance indicates how fast it moves in the crystal. Arsenic with a low diffusion

coefficient penetrates slower into the substrate.

Diffusion with an inexhaustible source

It means the dopant is in unlimited quantity, hence the concentration at the surface remains

constant during the process which results in continuous replenishment of the particles that have

penetrated into the substrate. In the subsequent processes the wafers are placed in a quartz tube

that is heated to a certain temperature.

Diffusion from the gas phase

A carrier gas (nitrogen, argon etc.) that is mixed with the desired dopant (also in gaseous form,

e.g. phosphine PH3 or diborane B2H6) leads to the silicon wafers, on which the concentration

balance can take place.

Diffusion with solid source

Slices containing the dopants are placed between the wafers. As the temperature in the quartz tube

is raised, the dopant from the source discs diffuses into the atmosphere. With a carrier gas, the

dopant will be distributed uniformly, hence reaches the surface of the wafers.

Diffusion with liquid source

Boron bromide BBr 3 or phosphoryl chloride POCl3 are used as liquid sources. A carrier gas is led

through the liquids transporting the dopant in gaseous state. Certain areas can be masked with

silicon dioxide since the entire wafers should not be doped. The dopants cannot penetrate through


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the oxide, and therefore no doping takes place at these locations. To avoid tensions or even

fractions of the discs, the quartz tube is gradually heated (e.g. +10 °C per minute) till 900 °C.

Subsequent the dopant is led to the wafers. To set the diffusion process in motion, the temperature

is then increased up to 1200 °C.

4.1.1 CHARACTERISTICS:

Since many wafers can be processed simultaneously, this method is quite favorable

If there already are dopants in the silicon crystal, they can diffuse out in later processes due

to high process temperatures

Dopants can deposit in the quartz tube, and be transported to the wafers in later processes

Dopants in the crystal are spreading not only in perpendicular orientation but also laterally,

so that the doped area is enlarged in a unwanted manner

4.1.2 Advantages and Disadvantages

Advantages:

1) The diffusion process doesn’t damage the surface of the parent semiconductor.

2) Batch fabrication is possible.

3) Low overall cost of the process.

4) Being an isotropic process, the properties in the whole Silicon crystal is same.

Disadvantages:1) Diffusion process is based upon solid solubility of the parent material and the impurity.

This makes the process limited to a narrow range of materials.

2) Shallow junctions are difficult to fabricate.

3) The process cannot be carried at room temperature.

4) Low dose doping is difficult to carry out.


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5. NEW EXPERIMENTAL METHODS:

A lot of research has been carried out in field of manufacturing semiconductors to reduce the cost

of production and increase quality of semiconductor. A lot of new experimental methods have

been discovered. Once Such method which is gaining popularity is Gas immersion laser doping

(GILD).

Gas Immersion Laser Doping (GILD):

In this process a thin Silicon wafer is immersed in Boron gas while a pulsed laser repeatedly melts

and cools the wafer. The Boron atoms in the gas diffuse into the molten parts of the Silicon and

stay there when the Silicon solidifies, thus producing a P-type Silicon wafer with Boron impurities.

Figure 10: Illustrates Laser Induced Diffusion Process

Process: GILD is performed in a high vacuum chamber (10-7 mbar) on Si and SOI wafers, using

homogenized XeCl excimer laser (308 nm, 30 ns, 200 mJ per pulse, 1 – 25 Hz).After cleaning and

removing native oxide the substrate is introduced in the chamber. The dopant precursor gas (BCl3)

is injected and chemisorbed on the substrate before each laser pulse.

Figure 11: Stages of GILD process.


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GILD process could be an alternative to ion implantation as it uses rapid annealing for making

ultra-shallow junctions. Quantity Boron atoms incorporated per laser shot depends on the laser

energy density and temperature gradient.

Advantages:

1. GILD process can be used for large scale manufacturing and low cost manufacturing.

2. GILD provides process control over concentration as well as depth of doping process.

Future Scope of Work:

GILD still requires a lot of research to precisely determine the laser energy density and the number

of laser shots to make an ultra-thin junction. The use of GILD technique has only been tested on a

select group of semiconductor materials. This technique still requires a lot of research to make it

usable for large scale production of semiconductor


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6. Table of Comparative Study

Parameter: Diffusion Ion Implantation

Cost: It is relatively cheaper It is expensive

Batch Formation: Possible Not Possible

Reproducibility: Not Possible Possible

Very High

Concentration

Doping:

Not Possible Possible

Temperature:It is a high temperature

process (900-

10000C)

It is relatively a low temperature

process

Process Type: It is a natural process It is a forced process

Driving Force: ConcentrationDifference

Electric Field (acceleration)

Shallow Junction: Not Possible Possible

Doping

Concentration:Cannot be controlled Can be controlled precisely

Doping Depth: Cannot be controlled Can be controlled easily

Parent Material

Surface:Doesn’t undergo any

damage

Damage in form of distortion may

occur

Directional: Isotropic Process Anisotropic Process

Post doping process: No Annealing isrequired

Annealing is required


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7. CONCLUSION:

The driving force in the diffusion process is the difference between the concentrations of the

materials involved and is carried out at high temperature. It is a non-destructive process and causes

no damage to the material surface. Though relatively cheaper the diffusion process has its own

drawbacks which include inability to control the junction depth and difficulty in maintaining

precise doping concentration. The Ion Implantation process offers better doping concentration

control, precise junction depth control, and easy reproducibility and doesn’t require high

temperature for being carried out

In semiconductors, the doping depth and concentration decide the quality of the semiconductor.

Ion implantation process not only offers excellent doping uniformity but also precise control over

depth and profile of the ion distribution of doping. Ion implantation can also be used for both high

and low concentration doping and is carried out in a closed and controlled environment, reducing

the possibility of any unwanted contamination due to impurities. In comparison to ion implantation

process, diffusion process offers peak concentration of the dopants near surface. Easy

reproducibility of the product is also an advantage of the ion implantation process. Although ion

implantation is expensive but it produces better quality semiconductor as compared to Diffusion

Process. Ion implantation has some drawbacks like formation of amorphous regions, physical

distortion of the substance but it is preferred over diffusion process as these damages can be

removed by annealing the product. An improvement to the ion implantation process is the plasma

immersion ion implantation (PIII).


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8. REFRENCES:

1. Carrier gas diffusion (for gas phase)

URL: http://www.powerguru.org/doping-silicon-wafers/

2. Diffusion of dopants(for gas phase and liquid)

URL: http://www.circuitstoday.com/diffusion-of-impurities-for-ic-fabrication

3.

Masking oxide(ion implantation)

URL: http://81.161.252.57/ipci/courses/technology/inde_195.htm

4. Doped semiconductor

URL: http://www.physics.udel.edu/~watson/scen103/98w/clas0128b.html

5. Ion implantation system URL:http://www.circuitstoday.com/ion-implantation

6. G.Kerrien, T.Sarnet, D.Débarre, J.Boulmer, M.Hernandez, C.Laviron, M.-N.Semeria: Gas

immersion laser doping (GILD) for ultra-shallow junction formation; Proceedings of

Symposium H on Photonic Processing of Surfaces; Volumes 453 – 454, 1 April 2004, Pages

106 – 109.

7.

J D Plummer, M D Deal and P B Griffin, “Silicon VLSI Technology: fundamentals, practiceand modelling”, Pearson Edu. Inc., 2001

8. Razeghi and Manijeh, “Technology of Quantum Devices”, Springer, 2010

9. Bose D.N., “Semiconductor Material and Devices”, New Age Publishers, 2012

10. F.G, Tseng, “IC Fabrication Process 2: Diffusion, Ion Implantation, Film Deposition,

Interconnection and contacts”. Lecture conducted from National Tsing Hua University,

Taiwan. Available [online]: http://oz.nthu.edu.tw/~d9511818/10ess5850Lec%203-1.pdf

11. John (2010, June 1), “Diffusion of impurities for IC fabrication” [online].

Available:http://www.circuitstoday.com/diffusion-of-impurities-for-ic-fabrication

12. R.C. Jaeger (Vol.5), Introduction to Microelectronic fabrication, Pearson Education,Inc.,New

Jersey,USA ,200213. Advancements in ion implantation Modelling for Doping of Semiconductors, Sivaco, Inc.

Available [Online] : http://www.silvaco.com/content/kbase/ion_implantation.pdf

14. Plummer D. James, Deal Michael , Griffin D. Peter, “Silicon VLSI Technology:

Fundamentals, Practise and Modelling”, Prentice Hall , 2000

15. Gupta Dushyant (2011), “Plasma Immersion Ion Implantation Process- Physics and

Technology”, International Journal of Advancements in Technology, 2(4), (472-474).

Available [Online] : http://omicsonline.org/open-access/plasma-immersion-ion-implantation-

piii-process-physics-and-technology-0976-4860-2-471-490.pdf

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MCG 5138 Project_Electronic_Group 6