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UNIT 1 By: Ajay Kumar Gautam

Asst. Prof.

Electronics & Communication Engineering

Dev Bhoomi Institute of Technology & Engineering, Dehradun

Saturday, August 04, 2012 UNIT 1

0

Syllabus

• Introduction to VLSI Technology: Classification if ICs,

Scale of integration, semiconductor and hybrid ICs

Features of ICs

• Crystal Growth: monolithic and hybrid ICs, crystal

growth, Czochralski technique of crystal growth, wafer

preparation and specifications, testing, measurements of

parameters of crystals, Fabrication steps

• Oxidation: Theory of growth of Silicon di oxide layer,

calculation of SiO2 thickness and oxidation kinetics, Dry

wet and high pressure oxidation, plasma oxidation

properties of oxidation, defects induced due to oxidation


1

Module 1: Introduction

to VLSI Technology Saturday, August 04, 2012 UNIT 1

2

What is a circuit?

• It is the combination of active & passive elements.

• Active elements are those, which can produce gain or can

amplify signals, like transistors or amplifiers.

• Passive elements doesn’t have this kind of capability

without active elements.

• This kind of circuit is known as discrete circuit.


3

What is an Integrated

Circuit?

• It is an electronic circuit, used almost in all electronic

equipment's.

• It is the combination of active & passive elements, when

placed on the same substrate, usually Si.

• 4 MB DRAM chip is the best example of IC.

• Active elements in an IC can be transistors (BJT or

MOSFET) & are most important part of any circuit.


4

Moore’s Law

• Gordon Moore, in 1965 an integrated circuit pioneer and

co-founder of Intel corporation predicted that, the number

of transistors on chip will be double by every 18 months.

• In 1965, Intel fabricated its first processor on 25 µm

technology.

• In 2000, Intel fabricated processors on 0.18 µm technology.

• Now-a-days technology used for IC fabrication is 90 nm to

40 nm.

• 90 nm for computer processor

• 40 nm for smart phone.


5

Classification of ICs

• There are 5 ways to classify Integrated Circuits.

1. By fabrication

2. By complexity

3. By packing density

4. By application

5. By technology


6

1. By Fabrication

a) Monolithic IC – IC which are entirely a single chip of

semiconductor usually Si, are known as monolithic circuit.

b) Hybrid IC – IC which contain one or more monolithic

circuit or individual transistors bounded to an insulating

substrate with passive elements.


7

2. By Complexity

a) SSI – components 1 – 10, flip flops & gates.

b) MSI - components 10 – 100, registers & counters.

c) LSI – components 100 – 10,000, ROM, decoders.

d) VLSI – more than 10,000 components, microprocessors

& high capacity memories.

e) ULSI – more than 3 million components.


8

3. By Packing Density

• Packing density is defined as no. of components contained

in the unit area of the chip, & is given by;

P = log Q,

• where P = packing density & Q = Scale of integration.

a) SSI P < 1

b) MSI 1 < P < 2

c) LSI 2 < P < 4

d) VLSI 4 < P < 6

e) ULSI P > 6


9

4. By Application

a) Analog IC – it performs amplification & other functions

on analog signals linearly. Ex: OP – AMP.

b) Digital IC – it performs operations with discrete

quantities, i.e., information for digital IC is represented by

binary digit. Ex: computer calculator & microprocessor.


10

5. By Technology

1. BJT Technology – it is used for high speed.

2. MOSFET Technology - it is used for high packing

density.

3. BiCMOS Technology – it is used for high speed as well as

high packing density.


11

Advantages of ICs

1. Increased reliability, due to lesser no. of connections

2. Extremely small size, due to fabrication of various circuits

onto a single chip of semiconductor.

3. Lesser weight & space requirements.

4. Low power requirement.

5. Lower cost.


12

Disadvantages of ICs

1. ICs are not flexible.

2. It is not possible to produce high power ICs greater than

10 watt.

3. Fabrication of high value capacitors, inductors and

transformers is not possible.

4. If any component in an IC goes out of order, the whole

IC has to be replaced with new one.


13

Features of ICs

1. An IC performs complex functions, whereas elementary

devices ( transistors ) can perform same functions only in

combination with other components.

2. Reliability and cost are not affected by increase in

functional complexity of ICs.

3. The size of most of the ICs are comparable with discrete

transistors, but the function performed by an IC is much

more complex.


14

Cond….

4. In an IC, the adjacent components are spaced mostly 50 –

100 µm apart, but the components doesn’t change their

electrical, chemical & physical properties. Whereas the

parameters are changed in different. Ex: Change in

temperature coefficient due to variations in temperature.

5. In an IC, interconnection of individual elements is made

through metallization ( i.e. without soldering and welding).

Hence it is more reliable in comparison to discrete

components in which large number of soldering and

welding is used.


15

Silicon Vs. Germanium

1. Si devices can operate upto 1500 C as compared to 1000 C

for Ge.

2. Si grows a stable oxide, which is important in fabrication

if IC as compared to Ge oxide which is unsuited for

device applications.

3. Silicon is cheaper than Ge.

4. Silicon is easily available near sea beaches as raw material

for silicon wafer preparation.

5. Silicon has low leakage current as compared to that of

Ge.


16

Module 2:

Crystal Growth Saturday, August 04, 2012 UNIT 1

17

Production of Single Crystal Silicon (Si) Saturday, August 04, 2012 UNIT 1

18

Raw

Material

MGS

(Si)

SiHCl3 +

H2 (Gas)

EGS (Si) Si + HCL

(Gas)

CVD

Reactor Single Crystal

Silicon (Si)

Submerged-

electrode arc

furnace

Cz Technique

Cond….

• Silicon occurs naturally in form of silica and silicates.

• It is most important semiconductor for the electronic

industry.

• More than 95% semiconductor devices are made of silicon.

• It is easy to grow SiO2


19

Electronic Grade Silicon

• It is polycrystalline material of high purity.

• EGS is the raw material for the preparation of single crystal

silicon.

• EGS is the one of the purist material available for

electronics industry.

• To obtain EGS, we require a multistep process.

• MGS ( Metallurgical Grade Silicon ) is produced in

submerged – electrode.

• The furnace is charged with quartzite and carbon.


20

Fig: Submerged – electrode arc furnace

for production of MGS Saturday, August 04, 2012 UNIT 1

21

Cond….

• Quartzite is the pure form of sand i.e., SiO2.

• In furnace large number of reactions takes place.

• The overall reaction is,

SiC (Solid)+SiO2 (solid) -> Si (liquid) + SiO (gas) + CO (gas).

• This process requires 13 Kwh/kg of power.

• MGS available in this process is then solidified at a purity

of 98%.

• The next step is to pulverize the silicon mechanically.


22

Cond….

• Now react it with anhydrous hydrogen chloride to form

trichlorosilane (SiHCl3)

Si (solid) +3HCl -> SiHCl3 (gas)+H2 (gas)+Heat

• The reaction takes place at temperature of 300oC using a

catalyst.

• EGS is prepared from the purified SiHCl3 in a Chemical

Vapor Deposition (CVD) process.


23

Fig: CVD Reactor used for production of EGS


24

Cond….

• The chemical reaction is a hydrogen reduction of

trichlorosilane.

SiHCl3 (gas) + H2 (gas)-> Si (Solid) + 3HCl (gas)

• A slim rod of silicon serves as nucleation point for the

deposition of silicon.

• This process takes many hours & will give polycrystalline

silicon (EGS) on slim rod of silicon.

• The polycrystalline silicon (EGS) is 0.2 m or greater in

diameter & several meter in length.

• This EGS can be cut from these rod as single chunks (solid

piece).


25

Czochralski (Cz)technique

of crystal growth

• Polycrystalline silicon may contain many defects.

• So in order to fabricate an IC, it must be nearly perfect.

• Therefore we need to produce single crystal silicon.

• Cz process is the basic technique to produce the single

crystal silicon.

• Cz process is a liquid-solid mono-component system.


26

Fig: Cz Crystal Grower Saturday, August 04, 2012 UNIT 1

27

Cond….

• In the Cz grower, we have big quartz chamber, with gas

inlet and outlet & some pumps.

• Pumps are required to maintain the pressure in quartz

chamber.

• There is one another opening through which a pull rod is

passed.

• There is a big cup (crucible), in which the poly crystalline

silicon is placed in order to grow single crystal silicon.

• Crucible is usually made of quartz which is SiO2.


28

Cond….

• Some oxide may come to the crystal from the crucible, so

this crucible can be usually used once.

• This is because when, Cz grower get cooled down, then

because of the thermal mismatch the quartz crucible

cracks.

• This quartz crucible is usually placed inside a graphite

susceptor, which is outer jacket to the quartz crucible.

• The susceptor, is surrounded by RF heating elements.


29

Cond….

• Cz crystal grower has mainly four subsystem.

1) Furnace

2) Crystal pulling mechanism

3) Ambient Control

4) Control system


30

Cond….

• Furnace is the most important part of the Cz crystal

grower.

• The furnace is heated to a temperature of 1690 K, which is

slightly greater than melting point (1685 K)of silicon.

• Inside the furnace, we have a quartz crucible, a quartz

susceptor, a heater and cooling for outer quartz chamber.


31

Cond….

• The crystal pulling mechanism includes pull rod, a seed

crystal and rotation mechanism (CCW).

• At the end of the pull rod, the small seed crystal is fixed in

a chuck.

• The pull rod is rotated (CCW) and pulled up very slowly at

the same time, the quartz crucible is rotated in the opposite

direction (CW).


32

Cond….

• The ambient control is very important in the Cz crystal

grower.

• There must not be oxygen inside the quartz crucible,

because it will react with the polycrystalline silicon (EGS).

• So, the quartz chamber is filled up with an inert ambient,

i.e., organ helium or nitrogen.


33

Cond….

• The control system provides control of temperature,

crystal diameter, pull rate and the rotation speed.

• The control system may have sensors and outputs.


34

Cond….

• For the crystal growth, the polycrystalline silicon is placed

inside the quartz crucible & the ambient condition is

created.

• The furnace is heated to a temperature of 1690 K, the

polycrystalline silicon (EGS) gets heated.

• A controlled amount of dopant (boron or phosphorus) is

added to the melt to make the silicon either P type or N

type.

• Initially the seed crystal is not in contact with the melt.


35

Cond….

• The pull rod is rotated and lowered gradually till the seed

crystal touches the melt surface.

• While the crucible is rotated in the opposite direction.

• When molten silicon is attached to the seed crystal and it

becomes identical to the seed, & get solidified.

• After that the solidified silicon crystal can be pulled up.

• So, using the Cz technique, we have cylindrical single crystal

silicon known as ingot.


36

Cond….

• By controlling temperature and pulling rate, desired

diameter of the silicon ingot can be obtained.

• The ingot is cooled and is removed to make wafers.

• Usually the ingot has diameter of 200 mm and length 1000

mm.

• The pull rate must be adjusted very carefully.


37


38


39


40


41

How the Pull Rate is

adjusted

• The pull rate should me adjusted carefully, so that we can

get good quality of crystal.

• If the pull rate and thermal conditions are not adjusted, the

seed crystal can also get melted.

• So, the Cz technique is much more complicated technique.

• Since, the Cz technique is liquid solid monocomponent

system, there is some heat transfer.

• So, we need to consider the Heat transfer Equation.


42

Cond…. Saturday, August 04, 2012 UNIT 1

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Cond… Saturday, August 04, 2012 UNIT 1

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Cond… Saturday, August 04, 2012 UNIT 1

45

Dopant incorporation

during the crystal growth

• Dopants are added to the melt in order to provide a

controlled N or P doping level in the wafer.

• Dopant incorporation process is accomplished by dopant

segregation.

• Generally impurities are added to the melt.

• This process is known as segregation.

• The degree of segregation is characterized by segregation

coefficient k0, & is given by:


46

0s

l

Ck

C

Cond…

• Where, Cs & Cl are the equilibrium concentrations of the

impurities of the impurities in the solid (crystal) & liquid

(melt) respectively near the solid-liquid interface.

• For most of the common impurities k0 < 1, i.e., Cs < Cl.

• It means impurities are not coming into solid, they are

rejected by the solid.

• Note that purpose of incorporating impurities to the melt

is only to change its electrical properties, i.e., to make it

either N or P type.


47

wafer preparation Saturday, August 04, 2012 UNIT 1

48

Crystal Growth

Shaping

Slicing

Lapping and Edge

Grind

Etching

Polishing

Cleaning

Testing

Packaging

Shaping

• The silicon crystal obtained after Cz process is called

“ingot”.

• The first shaping operation removes the seed and tang ends

from the ingot.

• The ingot surface is then grounded throughout to an exact

diameter.


49

Fig: Shaping Saturday, August 04, 2012 UNIT 1

50

Flat grind

Diameter grind

Preparing crystal ingot for grinding

Wafer Slicing

• Once the ingot surface is made smooth, ingot is sliced in

wafers by high speed “diamond saw”.

• Slicing is done by inner diameter sawing using a diamond

saw.

• It is rotated at a high speed and then moved across the

ingot to obtain wafer slices.

• Slicing is done very slowly.

• Thicker wafers are usually preferred.


51

Lapping & Edge Grind

• After slicing there is a variation in the thickness of the

sliced wafer.

• So, it can’t be used directly for IC fabrication.

• The wafers a mechanically lapped on both the sides.

• Lapping is done in order to remove the cracked or damaged

surface of the wafer.

• After lapping edge grinding takes place.


52

Fig: Lapping Machine Saturday, August 04, 2012 UNIT 1

53

Etching

• Etching is the process of selectively removing unwanted

semiconductor material from the wafer.

• Wet etching: chemical solution is used.

• Dry Etching: sputtering method is used.

• Chemical etching removes the remaining damaged and

contaminated regions of the wafer.

• A mixture of HF & HNO3, is used for chemical etching.


54

Wafer Polishing

• The purpose of polishing is to provide smooth surface of

the wafer.

• The wafer surface is polished to mirror like finish.


55

Fig: Wafer Polishing Saturday, August 04, 2012 UNIT 1

56

Cleaning

• Wafers are cleaned and dried for use in IC fabrication.

• The final wafer thickness is about one third of the sliced

wafer.


57

Testing

• Wafer testing is a step performed during semiconductor

device fabrication.

• During this step, performed before a wafer is sent to die

preparation.

• All individual integrated circuits that are present on the

wafer are tested for functional defects.

• The wafer testing is performed by a piece of test

equipment called a wafer prober.


58

Packaging

• Packaging is done in order to protect the wafers from

moisture & contaminants.

• The completed packages are inspected, sealed, and marked

with a special ink to indicate product type, date, package

code, and speed.


59

IC Fabrication steps Saturday, August 04, 2012 UNIT 1

60

Wafer

preparation

oxidation

Diffusion

Metallization Photolithography

Etching Packaging

Epitaxy Testing

Epitaxy

• Epitaxy is a combination of two Greek words, epi means

upon and taxis means ordered.

• It is a process of growing thin crystalline layer on a

crystalline substrate.

• 2 kind of epitaxial process CVD & MBE.

• CVD is a chemical process whereas in MBE, there is no

chemical reaction involved.

• 2 kind of epitaxy, homoepitaxy & heteroepitaxy.


61

Oxidation

• Process of growing film on the surface of silicon.

• Takes place at 1000 – 12000 C.

• It serves as mask against diffusion of dopants.

• It is used as isolation between two device.

• It is used as Gate oxide for MOS devices.

• It provides electrical isolation of multilevel metallization.

• It as used as surface passivation.


62

Photolithography

• Process of transferring geometrical pattern on the glass

plate to the surface of the wafer.

• To transfer the pattern, the wafer is coated with light

sensitive material called photoresist.

• The photoresist is then placed on the wafer.

• This is exposed to ultraviolet light.

• The photoresist under the opaque regions of the glass plate

is unaffected.


63

Diffusion

• Process of introducing impurities into selecting regions of

a wafer to form junctions.

• Diffusion allows the formation of source and drain regions.


64

Etching

• Etching is the process of selectively removing unwanted

semiconductor material from the wafer.

• Wet etching: chemical solution is used.

• Dry Etching: sputtering method is used.

• Chemical etching removes the remaining damaged and

contaminated regions of the wafer.

• A mixture of HF & HNO3, is used for chemical etching.


65

Metallization

• Process of interconnecting various components to form

desired IC.

• Initially metal is deposited over the entire surface of the

wafer.

• After that etching takes place.

• Metallization is usually done by sputtering method.


66

Testing

• Wafer testing is a step performed during semiconductor

device fabrication.

• During this step, performed before a wafer is sent to die

preparation.

• All individual integrated circuits that are present on the

wafer are tested for functional defects & electrical

properties of the device.

• The wafer testing is performed by a piece of test

equipment called a wafer prober.


67

Packaging

• Packaging is done in order to protect the wafers from

moisture & contaminants.

• The completed packages are inspected, sealed, and marked

with a special ink to indicate product type, date, package

code, and speed.


68

Measurements of

parameters of crystals

• Resistivity measurements are performed on flat ends of the

crystal by four-probe technique.

• Current I (mA) is passed through the outer probes and

voltage V (mV) measured between the inner probes.

• The resistivity ρ (Ω-cm) is measured by the formula;

ρ=(V/I) 2πS


69

Fig: Four-point probe measurement on crystal end


70

Module 3: Oxidation Saturday, August 04, 2012 UNIT 1

71

Introduction

• Process of growing film on the surface of silicon.

• Takes place at 1000 – 12000 C.

• Oxides are used as

1. Masking element against diffusion of dopants.

2. Gate oxide in MOS devices

3. Isolation between devices

4. Electrical isolation of multilevel metallization

5. Used as surface passivation


72

Cond…

• Since silicon has great affinity for oxygen, so it is very easy

to oxidize silicon.

• There are a lot of methods for oxidation.

• If silicon is boiled in nitric acid, oxide layer will be formed,

it is called chemical oxidation.

• Thermal oxidation is another method & is widely used in

VLSI technology.


73

Thermal Oxidation

• For thermal oxidation, take Si sample.

• Put it inside a furnace.

• Raise the temperature.

• Allow either oxygen or water vapor.

• Silicon will react either with oxygen or water to form SiO2.

• Si+O2 -> SiO2 Dry Oxidation

• Si+2H2O -> SiO2+2H2 Wet Oxidation


74

Oxidation Growth

Kinetics Saturday, August 04, 2012 UNIT 1

75

Cond…

• Consider the Groove model for thermal oxidation of the

silicon.

• Initially SiO2 layer is not present.

• We have oxidizing species in the gas.

• Once the oxidizing species has reached the gas-oxide

interface, it must diffuse through the existing oxide layer in

order to reach the Si-SiO2 interface.

• Once these oxidizing species reach the Si-SiO2 interface, it

must react with silicon.


76

Cond…

• We have three fluxes F1, F2 & F3.

• Flux is the number of molecules or atoms crossing an unit

area in an unit time.

• CG is the concentration of the oxidizing species in the gas.

• CS is the concentration of the oxidizing species next to the

gas-oxide interface.

• C0 is the concentration at outer surface of the oxide.

• Ci is the concentration at inner surface of the oxide.

• C* is the equilibrium concentration of the oxidizing species

in the oxide layer.


77

Cond…

• Where,

F1=hG(CG- CS)

F1= flux of the oxidizing species in the gas.

hG= gas phase mass transfer coefficient.

CG = PG/kT

CS = PS/kT


78

Cond…

• According to Henry’s Law “the concentration of a species

within a solid is proportional to the partial pressure of that

species in the surrounding gas”.


79

Cond…

•


80

Cond…

•


81

Cond…

•


82

Cond…

•


83

Cond…

•


84

Calculation of SiO2

Thickness

• SiO2 thickness can be calculated from the flux equations.

• Equating Eq. (2) & (3), we get,


85

0

(8)

/(9)

0 i S i

0 S i

0 S i

0 S

S

0 S

S

i

i

s

DC -C = K C = F

x

D DC = K C

x x

DC = D xK C

FDC = D xK

K

DC KF =

D xK

DC CF dx

C dtDx

K

Cond…

• Eq. (9) is known as Growth rate of the oxide layer

thickness.

• Initially at t=0, x=d0., where d0 is initial oxide thickness.

• Eq. (9) can be solved using initial conditions.

• So, integrating Eq. (9), we will have.

• Where,


86

2 02 2 ( ) (10)S i

CDx x= D t

K C

2 00

0

2 (11)2

i

S

C dd D

C D K

Cond…

• The oxide thickness after an oxidizing thickness t is given

by:

• For small values of t,

• For lager values of t,


87

2

02 ( )1 1 (12)S

S i

C K tDx

K DC

0 ( ) (13)S

i

Cx K t

C

02 ( ) (14)i

Cx D t

C

Cond…

• For smaller values of t, surface reaction is rate-limited, the

oxide thickness varies linearly with time.

• As, the oxide layer becomes thicker, the oxidant must

diffuse through the oxide layer to react at the Si-SiO2,

interface, & the reaction becomes diffusion limited.

• For larger values of t, the oxide growth than becomes

proportional to the square root of the oxidizing time,

which results in a parabolic growth rate.


88

Cond…

• Eq. (10) can be written in more compact form;

• Where,

• So, Eq. (13) & (14) becomes;


89

2 ( ) (15)x Ax B t

0 02 ; 2 & S

S i i

C CD BA B D K

K C A C

( ) (16)

( ) (17)

tan

tan

Bx t Linear region

A

x B t Parabolic region

BLinear RateCons t

A

B Parabolic RateCons t

Cond…

• So, the Eq. (12) can be written as;


90

1/2

21 1 (18)

/ 2 / 4

x t

A A B

Oxidation Techniques

1. Dry Oxidation

2. Wet Oxidation

3. High Pressure Oxidation

4. Plasma Oxidation


91

1. Dry Oxidation

• O2 & HCl are used to grow oxide layer.

• It is also known as HCl dry oxidation.

• It has lower growth rate than wet oxidation.

• HCl is used to improve the electrical characteristics of the

oxide layer.

• It has excellent dielectric properties.

• During the oxidation Cl is concentrated on the Si-SiO2

interface.

• Cl increases the life of the oxide layer.


92

Cond…

• The oxide layer formed by HCl dry oxidation is used as gate

oxide in MOS devices.

• This oxide layer has very stable threshold voltage property.

• Reaction involved:


93

2 2 2

2 2 2

4 2 2

2 2

HCl O H O Cl

Si H O SiO H

2. Wet Oxidation

• Also known as pyrogenic (producing heat) wet oxidation.

• Pure H2 & O2 are directly supplied to the furnace.

• Vaporized H2O is obtained by increasing the H2 & O2 in

the furnace.

• The oxide layer is grown by vaporized H2O.

• Advantage of wet oxidation is that, it can employ various

partial pressures of H2O.


94

2 2 2

2 2 2

2 2

2 2

H O H O

Si H O SiO H

3. High Pressure

Oxidation

• The oxidation rate can be enhanced at lower temperature by

increasing the pressure of the oxidizing species.

• The increased pressure will allow the oxidizing species to

penetrate the growing oxide layer more rapidly.

• At high partial pressure, the oxidation rate becomes limited

by the reaction rate at the Si-SiO2 interface instead of the

oxidants in SiO2.

• The oxidation rate varies directly with pressure P.


95

Cond…

• High pressure oxidation technique has been used mostly on

bipolar devices to increase speed of the devices.

• So for any case, high pressure oxidation can be performed

at lower temperature instead of oxidation at Atmospheric

pressure.

• For example, under 10 atm of steam, the oxidation rate at

8500 C is comparable to that above 12000 C and 1 atm of

steam.


96

Fig: Higher growth rate achieved at lower temperature


97

4. Plasma Oxidation

• Plasma oxidation process offers the possibility of growing

high quality oxides at temperature even lower than those

achieved with the high pressure technique.

• It is low temperature vacuum process.

• The plasma is produced by high frequency discharge or a

DC electron source.

• Place the wafer in a uniform density region of plasma.

• Biasing it positively below the plasma potential.

• Allow it to collect active charged oxygen species.


98

Cond…

• The growth rate of oxide increases with increasing

substrate temperature, plasma density and substrate dopant

concentration.

• The quality of oxide grown by this process can be as good

as that of thermally grown SiO2.

• Anodic plasma oxidation can grow thick oxides on the

order of 1µm at lower temperature (< 6000 C) at growth

rate upto about 1µm/h.


99

Oxide properties

• There 3 properties of oxides;

1. Masking properties

2. Oxide charges

3. Oxide stress


100

1. Masking properties

• SiO2 layer is used as mask.

• During the process of IC fabrication, the temperature of

the Si wafer is very high to derive in dopants into the

silicon.

• These dopants are near the Si surface and occurs because

of the process of ion-implantation & diffusion.

• So, it is very important that dopants should not enter into

the Si wafer and not into SiO2.

• So, the SiO2 layer is responsible for the dopants are blocked

by SiO2 and should enter into Si.


101

Cond…

• Usually 0.5 µm to 0.7 µm oxides are used to mask.

• The common impurities used are phosphorus, antimony

and arsenic for n type doping and boron for p type doping.

• All the impurities must have small diffusion coefficients in

oxide so that SiO2 serves as effective mask for them.


102

2. Oxide charges

• At the Silicon-Oxide interface, there is a transition region

between crystalline silicon and amorphous silica.

• Various charges and traps are associated with the thermally

oxidized silicon.

• A charge at the interface can induce a charge of the

opposite polarity in the underlying silicon.

• This charge affects the ideal characteristics of the MOS

devices.

• See, figure on next slide.


103

Fig: Charges in thermally oxidized silicon Saturday, August 04, 2012 UNIT 1

104

Cond…

• The net number of charges at Si-SiO2 interface is given by:

N=Q/q; where;

• N is net number of charges per unit area (number/cm2) at

Si-SiO2 interface.

• Q is net charge per unit area (Coulombs/cm2) at Si-SiO2

interface.

• q is electric charge in Coulombs.


105

Cond…

• There are four kind of oxide charges.

1. Interface trapped charges

2. Fixed oxide charges

3. Mobile ionic charges

4. Oxide trapped charges.


106

Cond...

• Interface-trapped charges Qit can interact electrically

with the underlying silicon.

• These charges results from several sources, like structural

defects, metallic impurities or bond-breaking process.

• So, in order to neutralize the Interface-trapped charges Qit,

a low temperature (450 0C)hydrogen anneal is required.


107

Cond…

• The fixed oxide charges Qf are located in the oxide

within 30 0A of the Si-SiO2 interface.

• Qf can’t be charged or discharged. It’s density varies from

1010/cm2 to 1012/cm2.

• Qf is related to the oxidation process.

• For electrical measurements Qf can be considered as a

charge sheet at Si-SiO2 interface.


108

Cond…

• Mobile ionic charges QM is formed due to the alkali ions

such as Na, K & Li as well as –ve ions and heavy metals.

• The alkali ions are mobile even at room temperature.

• When electric fields are present, density of QM, varies from

1010/cm2 to 1012/cm2 or higher and are related to

processing materials, chemicals ambient or handling.


109

Cond…

• Oxide trapped charges Qot may be +ve or –ve due to

holes or electrons trapped in the bulk oxide.

• The charge associated with defects in the SiO2, may occur

from ionizing radiation or higher currents in the oxide.

• It can be annealed out by low temperature.

• Density of Qot, varies from 1009/cm2 to 1013/cm2.


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3. Stress in Oxide Layer

• Stress occurs because of temperature variations during the

oxidation.

• Stress associated with a film is important, because high

stress levels can contribute to wafer warpage, film cracking

and defect formation in the underlying silicon.

• At room temperature stress value is about 3x109 dyne/cm2

is obtained.

• So, stress-free oxidation is usually preferred.


111

Defects Induced Due to

Oxidation

• The defects may be introduced in silicon wafer while

oxidation takes place.

• These defects may degrade the performance of integrated

circuits.

• So, care must be taken during the process of oxidation.

• 2 kind of defects occurs basically;

1. Defects due to oxide isolation

2. Staking faults due to oxidation.


112

1. Defects due to oxide

isolation

• In VLSI, junctions are usually isolated with oxide-isolation

technique.

• The stress along the edge of oxidizing area may damage the

silicon substrate.

• Due to this stress, the leakage current of nearby or adjacent

devices get increased.

• So in order to prevent defects due to oxide isolation, the

temperature should be sufficiently large (9500 C)during

oxidation.


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2. Staking faults due to

oxidation

• The stacking faults may occur because of thermal oxidation

of silicon.

• These faults are structural defects of the silicon lattice.

• During the thermal oxidation, if the oxidation temperature

is high the oxidant species penetrate silicon at very high

speed.

• So, some of the silicon atoms in the silicon lattice may be

displaced.

• This creates structural defects in the silicon lattice.


114

Cond…

• The stacking fault is temperature dependent.

• For higher oxide thickness, shorter stacking faults are

grown, if the oxidation temperature is lower.

• For oxide thickness of 1 µm, stacking fault is completely

suppressed when temperature is below 950 0C.


115

Review Questions

1. What is IC’s? Describe the types of IC’s?

2. What are the features of ICs?

3. What are the advantages & disadvantages of ICs?

4. Describe IC fabrication Steps.

5. How pull rate is adjusted during CZ crustal growth?

Explain how dopant incorporation takes place during Cz

crystal growth process?

6. What is single crystal growth? Describe CZ crystal growth

technique.

7. What is the Pull Rate in CZ technique? How the Pull Rate

is controlled during the CZ crystal growth process?


116

8. Explain wafer processing techniques used in IC

technology.

9. Why oxidation is done? Explain the chemistry of

oxidation and kinetic of oxide growth.

10. Compare dry and wet process of silicon oxidation.

11. What is High Pressure oxidation? Describe the wafer

processing steps.

12. What are disadvantages of IC`s over discrete circuits?

13. What are monolithic and hybrid ICs.

14. How resistivity of the crystal is measured? A boron-

doped crystal is measured at its seed end with a four-

probe of spacing 1mm. The (V/I) reading is 10 ohms.

What is the resistivity at seed end?


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15. Describe the preparation method for metallurgical grade

silicon (MGS).

16. Describe the preparation method for electronic grade

silicon (EGS).

17. What are seed end and tang end of silicon boule. How

they are obtained.

18. Describe how the shaping of ingot is done?

19. What is the need of wafer etching and polishing? Explain

the process.

20. What are Oxide-Induced defects?


118

21. What are the properties of Oxide Layer?

22. Write short note on masking property of oxidation layer.

23. What is oxidation? Explain kinetics behind the oxide

growth.

24. What are linear and digital IC?

25. Explain the process in which CVD reactor is used.

26. Calculate oxide thickness. Also show that,

reduces to for short time and to for long

time, where x = oxide thickness


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1/2

21 1

/ 2 / 4

x t

A A B

( )B

x tA

( )x B t

27. Discuss different steps in preparing wafers from the raw

silicon.

28. Explain the application of SiO2 layer in IC fabrication.

29. Show that to grow an oxide layer of thickness x, a

thickness of 0.44x of silicon is consumed.

30. State Moore’s law.

31. Why SiO2 is an important component in electronics?

32. A silicon ingot with 0.5x1016 boron atoms/cm3 is to be

grown by Cz method. What should be the concentration

of boron in the melt to obtain the required doping

concentration? The segregation coefficient of the boron

is 0.8.


120

References

1. VLSI Technology/S. M. SZE/2nd Ed./TMH

2. VLSI Fabrication Principles Silicon and Gallium

Arsenide/Sorab K. Gandhi/2nd Ed./Wiley

3. The Science and Engineering of Microelectronic

Fabrication/S. A. Campbell/2nd Ed./Oxford

4. Silicon VLSI Technology – Fundamentals, Practice and

Modeling/J. D. Plummer, M. D. Deal & P. D. Griffin/1st

Ed./Pearson

5. Semiconductor Devices: Physics & Technology/2nd

Ed./S. M. Sze/Wiley


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