CHAPTER 1

Introduction

1

Chapter: I

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IntroductionIntroductionIntroductionIntroduction

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1.1 Introduction:

‘Nanotechnology’ is not merely working with matter at the nanoscale, but

also research and development of materials, devices, and systems that have novel

properties and functions due to their nanoscale dimensions or components.

The ever- increasing demand for thin film coatings stems from their ability

to continue to offer high technological solutions to material limiting problems. The

implementation of certain thin film for specific applications depends on the

material’s properties. The optical coating materials are chemical compounds such

as metal oxide, metal fluoride and certain sulfides that transmit, absorb or reflect

the selected portion of the spectrum. The transmittance region has great practical

importance. Among the foresaid compounds, oxide thin films are of the special

interest because under proper deposition conditions they are fully transparent and

environmentally stable. In an increased number of applications these coatings not

only modify the optical properties of surface but also have to serve as protective

coatings that can withstand adverse chemical and physical environment.

In particular, magnesium oxide and aluminium oxide thin films are

characterized by low refractive indices nearer to glass, large optical band gap,

have higher adherence to glass substrates, higher secondary electron emission

coefficient etc. [1-5] properties that endear them for use as optical coatings. Due to

a combination of these properties they also find wide range of recent technological

applications in optoelectronics such as plasma display panel, optical waveguide.

2

On the other hand, successful preparations of films for these applications in cost

effective ways with improved performance and device lifetime are still

challenging.

‘Integrated optics’ is a branch of physics which provides functionality of

photonic devices to transmit the information in the form of optical signals

typically in the visible spectrum. In this integrated optical circuits interconnector

plays a major role during the signal transmission between two components. To

make possible various optical interconnection architectures, optical waveguides

are needed which are integrable with various electronic and optoelectronic devices

and systems [6]. For integrated optics applications, it is desirable to fabricate

waveguides with low propagation losses and desired refractive index profile.

Alternating current plasma display panels (ac-PDPs) [1-5] are now

established as large-scale flat-panel display system and have good characteristics,

such as high luminance, large angle of visibility and high resolution. A plasma

display panel (PDP) is essentially a matrix of submillimetre fluorescent lamps

which are controlled in a complex way by electronic drivers. Each pixel of a PDP is

composed of three elementary visible light emitting discharge cells. The UV light is

converted into visible light by phosphors in the three primary colors (red, green &

blue). Durable coatings for the PDP’s have also become a topic for research.

Looking into the various applications, the work in this thesis have aim to

obtain the cost effective, durable coating for PDP and optical waveguide purposes

by using vapor chopping technique. MgO and Al2O3 thin films have been used.

The co-deposited MgO-Al2O3 thin films have also been studied. The films have

been deposited by thermal oxidation of vacuum evaporated metal films (Mg &

Al).

1.2 Thin film deposition techniques:

Thin film technology is a modern tool of today’s advanced technology

because of their vast application in various fields. The thin film properties depend

on source material purity, substrate, deposition technique and deposition

3

parameters, treatments given to thin film and ambient air exposure effect after

deposition. Out of these, the deposition technique is the most important. The

optical and mechanical properties of deposited thin films depend on it. There are

two types of thin film deposition techniques physical and chemical deposition

technique. The flow chart given below represents the various streams of thin film

deposition techniques.

1.2.1 Physical deposition techniques:

In this technique the deposition process has to take place in vacuum

environment. Vacuum evaporation technique has been used in this work,

Advantages of vacuum evaporation:

Thin Film Deposition

Chemical Process Physical Process

Sol gel CBD CVD Resistive heating

evaporation

Sputter deposition

Arc vapor deposition

Electron beam

Laser ablation

Ion Plating

Spray Pyrolysis

4

• Vacuum in deposition chamber is the key to reduce the contamination

level.

• High-purity films can be deposited from high-purity source material.

• Source of material to be vaporized may be a solid in any form.

• The line-of-sight trajectory and "limited-area sources" allow the use of

masks to define areas of deposition on the substrate.

• Deposition rate monitoring and control are relatively easy.

1.2.1.1 Resistive heating [7-8]:

Vacuum evaporation by using resistive heating is a physical vapour

deposition (PVD) process where material from a thermal vaporization source

reaches the substrate without collision with gas molecules in the space between the

source and substrate. The evaporation takes place in a gas pressure range of 10-5

to

10-9

Torr. Source material vaporizes from a solid to vapour by sublimation process

or by melting the solid material by applying the electric current between two

electrodes under high vacuum. Resistive filament or boat has to be used in

between them. The filament or boat is made of refractory metals such as tungsten,

molybdenum, tantalum, beryllia and zirconia. The material of filament or boat

depends on the melting temperature of source material and resisting to alloying or

chemical reaction with the source material. In present work spiral filament and

boat shaped filament of tungsten have been used.

1.2.1.2 Other deposition techniques:

Various deposition techniques have been used for the deposition of thin

film. The selection of deposition technique depends on source material and

optimisation condition of required thin film. Some of the deposition techniques are

given here.

A) Sputter deposition [9-11]:

Sputter deposition is the deposition of particles vaporized from a surface

(sputter target) by the physical sputtering process. Physical sputtering is a non-

thermal vaporization process where surface atoms are physically ejected by

5

momentum transfer from an energetic bombarding particle that is usually a

gaseous ion accelerated from plasma or an ‘ion gun’. This PVD process is often

called sputtering.

B) Vacuum arc vapour deposition [9]:

In arc vapour deposition, the material deposition source is the vaporisation

of the anode or cathode of a low-voltage, high-current electric arc in a good

vacuum or low-pressure gas. In the anodic arc configuration, the arc is used to

melt the source material that is contained in a crucible. The vaporized material is

ionized as it passes through the arc plasma to form charged ions of the film

material.

C) Ion plating [9]:

Ion plating uses concurrent or periodic energetic particle bombardment for

the depositing thin film to modify and control the composition and to improve

surface coverage and adhesion. The energetic particles used for bombardment are

usually ions of an inert or reactive gas or ions of the depositing material (film

ions). Ion plating can be done in a plasma environment where ions for

bombardment are extracted from the plasma, or it can be done in a vacuum

environment where ions for bombardment are formed in a separate ion gun.

D) Electron Beam Evaporation [12-13]:

In the electron beam evaporation process, material (source) is heated to the

point where it starts to boil and evaporate by using beam of electron. Then the

material is allowed to condense on the substrate. This process takes place inside a

vacuum chamber, enabling the molecules to evaporate freely in the chamber,

where they can condense on all surfaces.

E) Laser Ablation Evaporation [7, 14]:

In laser ablation evaporation the source material is heated by applying the

laser energy and the material used to evaporate or sublimate under high vacuum.

At high laser flux, the material is typically converted to plasma. Usually, laser

6

ablation refers to removing material with a pulsed laser, but it is possible to ablate

material with a continuous wave laser beam if the laser intensity is high enough.

F) Sol-gel thin film deposition technique [15]:

The sol-gel process is used primarily for the fabrication of materials

(typically a metal oxide) starting from a chemical solution which acts as the

precursor for an integrated network (or gel) of either discrete particles or network

polymers. Typical precursors are metal alkoxides and metal chlorides, which

undergo various forms of hydrolysis and polycondensation reactions. The

formation of a metal oxide involves connecting the metal centres with oxo (M-O-

M) or hydroxo (M-OH-M) bridges, therefore generating metal-oxo or metal-

hydroxo polymers in solution. Thus, the sol evolves towards the formation of a

gel-like diphasic system containing both a liquid phase and solid phase whose

morphologies range from discrete particles to continuous polymer networks. Sol-

gel deposition technique is widely used in the fields of materials science and

ceramic engineering.

G) Chemical Bath Deposition (CBD) [16, 17]:

Chemical Bath Deposition is a very simple technique. In CBD technique,

the substrates only have to be dipped into appropriate solutions in beaker. The

optimisation of deposition condition has to be done. No current is necessary to

pass through it. This technique also called as electroless deposition technique. The

principle mechanism of this technique is the reduction of metal salt on a surface

substrate by using reducing agent. Once an initial coating is formed further

deposition can go on automatically.

H) Chemical Vapour Deposition (CVD) [18, 19]:

In this process, the substrate is placed inside a reactor to which a number of

gases are supplied. The fundamental principle of the process is that a chemical

reaction takes place between the source gases. Precursor gases (often diluted in

carrier gases) are delivered into the reaction chamber at approximately ambient

temperatures. As they pass over or come into contact with a heated substrate, they

7

react or decompose forming a solid phase which are deposited onto the substrate.

The substrate temperature is critical and can influence the reaction.

I) Electrodeposition technique [9]:

In the electrodeposition process the substrate is placed in a liquid solution

(electrolyte). When an electrical potential is applied between a conducting area on

the substrate and a counter electrode (usually platinum) in the liquid, a chemical

redox process takes place resulting in the formation of a layer of material on the

substrate and usually some gas generation at the counter electrode. This process is

typically restricted to electrically conductive materials.

J) Spray Pyrolysis:

Spray pyrolysis technique consists of a thermally stimulated chemical

reaction between clusters of liquid or vapour atoms of different chemical species.

It involves spraying of a solution containing soluble salts of the desired compound

on to preheated substrates. Every sprayed droplet reaching the surface of the hot

substrate undergoes pyrolytic decomposition and forms a single crystalline or

cluster of crystallites as a product. The other volatile by-products and solvents

escape in the vapor phase. The substrates provide thermal energy for the thermal

decomposition and subsequent recombination of the constituent species, followed

by sintering and crystallization of the clusters of crystallites and thereby resulting

in coherent film. The atomization of the spray solution into a spray of fine droplets

also depends on the geometry of the spraying nozzle and pressure of a carrier gas.

1.3 Thin film thickness measurement:

Thickness of thin film is the most significant and important parameter in

thin film deposition. It plays a crucial role in structural, morphological, optical,

electrical, mechanical etc. properties variation. The thickness of thin film depends

upon the monitoring and controlling of evaporation rate. To detect the effect of

thickness variation on the properties of optical thin film, accurate thickness

measurement is central aspect. Some techniques have been given for the thin film

thickness measurement.

8

A) Electrical methods:

i) Film resistance [20, 21]:

This is a simple thickness measurement method; it is applicable to metallic

and low-resistivity semiconductor films. Resistance is related to the film thickness

and mean free path of charge carriers. In ultra thin film (thickness <100 Å)

situation is complex, so no reliance can be put on this method. In case of

semiconductor, resistance method is applicable only for comparison of film

thickness rather than for absolute measurement because semiconductor film

resistivity is very sensitive to deposition condition. In case of metals by measuring

resistivity by four probe method and using resistivity-thickness relationship, we

can calculate thickness of thin film. For resistivity measurement four probe

method is a convenient method. In this method collinear four-point probe with

equal separation between the points.

ρ = 4.53 (V/I) t ------------------------ t<<S/2

where, S= spacing between probs.

By using four-probe method resistivity can be measured in range 10-3

to

10+3

ohm-cm.

ii) Capacitance monitors:

The thickness of dielectric films may be determined by directly monitoring

the electrical capacitance. Riddle [22] measured the rate of evaporation by

measuring changes in the capacitance of a parallel-plate condenser due to changes

in the dielectric constant resulting from the presence of the vapor of the source

material. This method is not so sensitive and required careful measurements.

iii) Ionization monitors [7]:

By ionizing the vapour from the source material and measuring the

resultant ion current, the evaporation rate can be monitored and controlled via a

feedback servomechanisum. The ion current is proportional to the total number of

vapour atoms and their ionization. It allows films of thickness from 50 to 1500 Å

9

to be deposited by electron bombardment at selected rate in case of high/ultrahigh

vacuum.

B) Microbalance Monitors [7]:

i) Microbalance:

These monitors are termed the ‘gravimetric’ or ‘momentum’ type

depending on whether they measure the weight or the momentum of impinging

vapour. This is most convenient to use for determining film thickness. The

electrical signal from the microbalance detection system may be used to monitor

and control the deposition rate. In this method total weight (average thickness) and

force measures using the various positions of vane with respect to direction of

vapour stream.

ii) Quartz-crystal monitor [23]:

This is a highly sensitive thickness monitor. Working is based on the

measurement of changes in the resonant frequency of a quartz-crystal oscillator

with mass loading when operational in a particular mode of vibration. A quart-

crystal monitor can be used for monitoring and controlling the deposition and

evaporation rates of material. This measuring technique can be used for metals,

non-metals, multicomponent films. In case of deposition, vibrating uniform

preliminary frequency of quartz-crystal decreases and by measuring the reduction

in frequency deposited mass can be measured. The thickness of thin film can be

calculated from the formula,

f = (1/2a) (C/ ρq)1/2

= N/d = 1.67/d mm kc /sec.

Where, d= crystal thickness, ρq= crystal density, C= elastic constant, N=(C/4 ρ) 1/2

After deposition frequency decreases as ∆f and the thickness of thin film can be

calculated as,

t = (∆f/ Cf ρfilm)

Where, ρfilm is film density. Cf = constant.

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C) Mechanical methods (stylus) [24]:

Variations in movements of a mechanical stylus can be amplified

electronically, so that step heights and surface irregularities of ~10 Å can be

measured. In surface profiler thickness measurement instrument, the stylus is used.

The vertical movement of stylus is detected with transducer, amplified 105 to 10

6

times and then fed into a recorder. The thin films of soft materials can be

deformed by stylus during the thickness measurement.

D] Radiation-absorption and radiation-emission [7]:

The thickness dependence of absorption of light, x-rays, γ-rays, β-rays may

be used to determine the thickness and the thickness distribution of a film. Optical

absorption has exponential dependence on film thickness for continuous film. The

absorption in discontinuous films is strongly influenced by the granular nature of

films, but it may still be used for relative measurement of the average film

thickness. X-ray emission, also known as x-ray fluorescence, is an important and

most frequently used nondestructive method for determining the mass of the

material components of the film and hence its thickness and the chemical

composition. When radiations are incident on a surface, some scattering takes

place. The back scattering of β-rays depends on the atomic number, density, and

thickness of scattering material and its measurement therefore allows a

determination of the film density and thickness. If the evaporant is radioactive, the

activity of deposited film will be proportional to its thickness.

E) Optical-Interference method:

i) Photometric method [25]:

The photometric method can be used for measuring the optical density as

well as thickness of a film. If transparent or slightly absorbing film is deposited on

a transparent substrate of different refractive index, the optical reflectance and

transmittance behavior of the film-substrate combination shows an oscillatory

behavior with increasing film thickness because of interference effects.

Reflectance is reduced or enhanced depending on the relative refractive indices

11

values of the thin film and substrate material. Film thickness is determined from

the maxima and minima of the reflectance which occur at intervals given by,

2m (λ/4) = nf t

where, nf -film refractive index, t- thickness of thin film, λ- wavelength of light,

m- the order of maxima and minima.

ii) Spectrometric method [26]:

In this method light has to be incident at an angel Ө from medium of index

no onto a film of index n1 and thickness t, deposited onto a substrate of index n2

with n1lying between no and n2, the reflected light will show an interference

maximum for a wavelength λ. When the path difference 2n1t cos Ө between the

successive beams reflected at each surface is equal to m λ, where m is integer. If

n1 is greater than no and n2, the reflected intensity will show a minimum (dark band

results) when 2n1t cos Ө = m λ and maximum if 2n1t cos Ө = (m- ½) λ. When

white light is used, the reflected light will show maxima for various wavelengths

for which the interference condition is satisfied. This is the basis of the visual

method of monitoring film thickness. A spectrometer can measure the

transmittance or reflected intensity as a maxima and minima. If mth

order

maximum occurs at λ, and the (m+1)th

order at λ2 we have for normal incidence,

2n1t = 2 λ1 = (m+1) λ2

Therefore,

2n1t = (λ1 λ2 / (λ1- λ2))

iii) Interference fringes (Tolansky method) [27]:

When two same reflecting surfaces are brought into close proximity,

interference fringes are produced. By measuring fringes shift we can calculate

thickness of thin film. The interferometer consists of two slightly inclined optical

flats, one of them supporting the film which forms a step on the substrate. When

the second optical flat is brought in contact with the film surface and the

interferometer is illuminated with a parallel monochromatic beam at normal

incidence and viewed with a low power microscope, dark fringes can be observed

12

which trace out the points of equal air gap thickness. The two successive fringes

are separated by λ/2. The optical flat should be highly reflecting by adjusting the

relative positions of the flats to form a wedge-shaped air gap and the fringes can

be made to run in straight lines perpendicular to the steps.

Figure 1.1: Schematic diagram for Tolansky method set up.

The fringes show a displacement as they pass over the film step edge. This

displacement expressed as a fraction of the λ/2 fringe spacing gives the film

thickness. Instead of using multiple beam interference as in the preceding case

with a beam splitter to produce two-beam, we get broader fringes.

The optical flat does not physically touch the film so that it does not get

scratched and we can measure thickness as low as 100 Å with all accuracy to ~20

Å. In reflecting mode fringes appears dark on bright background. In transmission,

the complementary pattern is seen. If one of the plate is covered by a film, a

displacement ∆λ is seen in mth

order fringe from which film thickness can be

measured as,

t = (m/2) ∆λ

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1.4. Optical properties of thin films:

The use of thin films in various advanced optoelectronic device

applications motivates, to study the light and material interaction behaviour i. e.

optical properties. The light wave and thin film interaction deals with optical

transmission, reflection, and absorption properties and their relation to the optical

constants of thin films. When a single frequency strikes an object, a number of

things can happen. The light wave can be absorbed, reflected or transmitted by the

object. When a light passes through the thin film it give the information of optical

properties of that material [28]. The manner in which visible light interacts with an

object is dependent upon the frequency of the light and the nature of the object.

Frequency of light is a tunable parameter; so the material involvement in visible

light wave interaction is the main part of discussion here. For the light interaction

with deposited material i.e. nature of material, thickness of coating, density and

the defects present within the thin films are important. The optical transmission,

absorption, reflectance, and scattering of optical signal phenomenon may be used

to study the various optical constants. When light passes through the thin film, the

incident beam is assumed to arrive at the surface of the medium from the vacuum

or air medium. When it travels through the medium, some of the incident energy is

absorbed and remaining is transmitted or reflected. If a light wave of a given

frequency strikes a material with electrons having the same vibrational

frequencies, then those electrons will absorb the energy of the light wave and

transform it into vibrational motion. During its vibration, the electrons interact

with neighboring atoms in such a manner to convert its vibrational energy into

thermal energy. Subsequently, the light wave with that given frequency is

absorbed by the object, never again to be released in the form of light. So the

selective absorption of light by a particular material occurs because the selected

frequency of the light wave matches the frequency at which electrons in the atoms

of that material vibrate. Since different atoms and molecules have different natural

frequencies of vibration, they will selectively absorb different frequencies of

14

visible light. The presence of lattice defects or the inclusion of the impurities

inside the medium scatters the incident light and give rise to optical absorption

[29].

Reflection and transmission of light waves occur because the frequencies of

the light waves do not match the natural frequencies of vibration of the objects.

When light waves of these frequencies strike an object, the electrons in the atoms

of the object begin vibrating. But instead of vibrating in resonance with larger

amplitude, the electrons vibrate for brief periods of time with smaller amplitudes

then the energy is reemitted as a light wave. If the object is transparent, then the

vibrations of the electrons are passed on to neighboring atoms through the bulk of

the material and reemitted on the opposite side of the object. Such frequencies of

light waves are said to be transmitted. If the object is opaque, then the vibrations

of the electrons are not passed from atom to atom through the bulk of the material.

Rather the electrons of atoms on the material's surface vibrate for short periods of

time and then reemit the energy as a reflected light wave. Such frequencies of light

are said to be reflected.

Due to that, the optical properties are considered in terms of macroscopic

properties i.e. “optical constants”. The refractive index, optical band gap and

extinction coefficient are considered as a macroscopic optical constant but in

reality they are not costant because they vary with the frequency. The refractive

index of material gives the information about the velocity of light with respect to

air, whereas the material’s band gap provides information about the electronic

properties.

1.4.1 Determination of optical constant:

Optical band gap is an important optical constant. It is a function of

incident photon energy. If the photon energy is below the band gap or well above

the band gap, the sample absorption coefficient is either zero or very small. When

the photon energy is increased from below to above the band gap, the absorption

coefficient increases rapidly and finally sample becomes opaque for the photon

15

energies equal to the band gap value unless its thickness is very small. Therefore

the intensity of the light transmitted through the sample decreases exponentially

with the thickness increment.

Other important optical constants are refractive index ‘n’ and extinction

coefficient ‘k’, which over a imaginary part of the complex refractive index ‘N’

i.e. N= n- ik. Refractive index is defined as the ratio of the velocity of light from

rarer medium to the denser medium. The extinction coefficient carries the role of

attenuation. This attenuation causes due to the absorption and its coefficient called

as absorption coefficient ‘α’.

The refractive index depends on various parameters such as deposition

method, deposition condition i. e. substrate temperature, oxidation temperature,

pressure of gas used during deposition and oxidation. The refractive index of a

dielectric film often differs from that of the bulk material. For example, refractive

index of bulk ZnS is 2.35 whereas that of thin film ZnS of thickness 100 nm is

2.27 at wavelength 633 nm [28].

Refractive index is an important parameter for optical waveguide

application. In waveguide design, the material used as the core must have a

refractive index higher than that of the base and cladding material. The deposited

thin film over glass substrate itself acts as planar waveguide. In this planer

waveguide, air and glass substrate plays the role of cladding and the deposited

materials thin film as a core. Air and glass substrate have 1.0008 and 1.515

refractive indices respectively, which are lesser than the deposited magnesium

oxide (1.62) and Al2O3 (1.72) the materials investigated in this work. The

refractive index can be influenced by the density variation of the deposited

material. The surface and volume imperfection scatters the incident light from the

surface and imperfections. The imperfections in the thin film can be surface

roughness, grain boundaries and density fluctuations, porous microstructure,

pinholes, cracks, splashes, micro dusts etc. The imperfection within the thin film

introduces the scattering of light. This scattering results in optical signal losses.

16

The optical transmission loss increases due to this scattering phenomenon, and

also optical transmission loss depends on the material’s refractive index.

The second parameter responsible for RI variation is polarizability of

deposited thin film. It is the relative tendency of a charge distribution, like the

electronic cloud of an atom or molecule, to be distorted from its normal shape by

an external electric field, which may be caused by the presence of a nearby ion or

dipole. There are three types of the polarizability: electronic, atomic or ionic and

dipole orientation polarization contributes to the total polarization of a material.

• An electrical field will always displace the center of charge of the electrons

with respect to the nucleus and thus induce a dipole moment is called as

the ‘electronic polarizability’.

• In ‘ionic polarizability’, a (solid) material must have some ionic character.

It then automatically has internal dipoles, but these built-in dipoles exactly

cancel each other and are unable to rotate. The external field then induces

net dipoles by slightly displacing the ions from their rest position.

In oriental polarization, the (usually liquid or gaseous) material must have natural

dipoles which can rotate freely. In thermal equilibrium, the dipoles will be

randomly oriented and thus carry no net polarization. The external field aligns

these dipoles to some extent and thus induces a polarization of the material.

1.4.2 Various methods of measuring refractive index:

Various methods have been used for the determination of refractive index

of optical thin film. The commonly used techniques are Abele’s method [29-32],

Spectrophotometry [33-37], Ellipsometry [7, 38-41], critical angle method, Male’s

method [42], Schultz method [43-44] etc.

Out of these method Abele’s method and spectroscopic method has been

used in this work for the determination and confirmation of refractive index of

deposited thin films, therefore these two have been explained in detail.

17

1.4.2.1 Abele’s method [29]:

Abele’s method is sensitive and accurate method for the refractive index

measurements. This method requires the substrate to be grounded on the back side

to avoid the back reflection and the film coated only on half portion of the

substrate to obtain clear demarcation. When unpolarized light reflects off a

nonconducting surface, it is partially polarized parallel at the plane of the

reflective surface. There is a specific angle called Brewster's angle at which the

light is 100% polarized. This occurs when the reflected ray and the refracted ray

are 90 degrees apart. Brewster’s angle obtained from Fresnel equations, the

reflection coefficient ||r (TM radiation) will be equal to zero when the indices of

refraction are equal, so 21 nn = or

η2 Cos θ = η1 Cos θ’ (1)

θ is the angle of incidence and θ’ is the angle of refraction.

Cos2 θ’ = (

1

2

η

θη Cos )2 (2)

Solving for Sin2θ’ in Snell’s law

η1 Sin θ = η2 Sin θ’ (3)

Sin2 θ’ = (

2

1

η

θη Sin )2 (4)

Adding (2) and (4) yields

Sin2θ’ + Cos

2θ’ = 1= Sin

2θ + Cos

2θ

= 2

1

2

2

η

η

Cos2 θ + 2

2

2

1

η

η

Sin2 θ (5)

18

Sin2θ ( 2

2

2

1

2

2

η

ηη − ) = Cos2 θ ( 2

1

2

1

2

2

η

ηη − ) (6)

tan2 θ = 2

1

2

2

η

η

(7)

θB = tan -1 (

1

2

η

η ) (8)

where Bθ is the Brewster’s angle from the normal,

θB = tan-1 (

A

f

n

n)

where, nf is the refractive index of the films.

nA is the refractive index of the medium (air)

The tangent value of the angle of incidence or reflection gives the value of the

refractive index of the films.

Figure 1.2: Polarized Light (%) vs. Angle (degrees) graph for Brewster’s angle

measurement.

Polarized Light (%) vs. Angle

(degrees)

Brewster’s angle

19

The best procedure for θB measurement is to move the spectrophotometer

arm, to measure all angles and then plot the graph between polarized light Vs

angle, shown in figure 1.2. The coincidence of graph with x-axis gives the

Brewster’s angle

1.4.2.2 Spectrophotometric method [45]:

In this method, the optical transmittance and absorption has to be measured

by using UV-VIS spectrophotometer. The reflectance has been calculated from,

reflectance = (1- Transmittance-Absorbance) equation. In all cases substrate has

the finite depth with rear surface that reflects some energy, so that the measured

transmittance T and reflectance R requires some modification

Fig. 1.3 shows a schematic diagram of a thin-film system studied here. The

thickness and the complex refractive index of the film are d and n. The incident

medium is usually air with a refractive index of unity. For simplicity, the substrate

is treated as a one-sided slab of material of infinite depth with a refractive index of

ns that is known.

Figure 1.3 Schematic diagram of a thin-film system.

If the extinction coefficient of the film is much smaller than the refractive

index (k<<n), the transmittance Tf and reflectance Rf of the film at the normal

incidence can be given by [48]

d, n-ik

ns

Tf

Rs

Rf

R

no

T

20

Tf =)2cos(2

16

21

22

2

2

1

2

δαα

α

CCCC

nns

++

……………. (1)

Rf = )2cos(2

)2cos(2

21

22

2

2

1

21

22

2

2

1

δαα

δαα

CCCC

BBBB

++

++……………. (2)

Where α = exp (-4 π kd/λ), δ = 2 π ηd/λ

C1= (1+n) (n + ns), C2 = (1- n) (n - ns), B1= (1- n) (n + ns), B2 = (n - ns) (1+n)

When the optical thickness of the film is an odd number of quarter wavelengths,

the transmittance and the reflectance at these wavelengths become

Tf = )2cos(2

16

21

22

2

2

1

2

δαα

α

CCCC

nns

−+

……………. (3)

Rf = )2cos(2

)2cos(2

21

22

2

2

1

21

22

2

2

1

δαα

δαα

CCCC

BBBB

−+

−+ ………..…. (4)

In almost all practical cases, the substrate has finite depth with a rear surface that

reflects some of the energy. Therefore if measured reflectance R and transmittance

T include the reflection of the rear surface of the substrate, the transmittance Tf

and the reflectance Rf of the film should be modified as

Tf = 222

)1(

ss

ss

RTT

RRTT

−

−…………………….…… (5)

Rf = ss

ss

RTT

RTRT22

22

−

− ………………………….. (6)

Where, Ts and Rs are the transmittance and the reflectance, respectively, of the

single surface of the bare substrate. Equations (5) and (6) are valid only for

nonabsorbing and weakly absorbing thin films.

By solving Eqs. (3) and (4) we can derive the unknown parameters, the refractive

index n, of the film:

21

( )

( )

2/1

2

22

1

1

−+

++

=

fsf

fsfs

RnT

RnTnn

where,

n= refractive index of the films

k= extinction coefficient of the films

ns= refractive index of the substrate

Tf= transmittance of the films

Rf= reflectance of the films

1.5 Mechanical Properties:

During the use of thin film in device level, the durability of thin film have

enormous importance. The films should not be affected due to the external

surrounding and it should remain scratchfree when in use. Thin films exhibit

unusually high tensile strength, which is related to their internal microstructure,

growth and in particular to the high density of defects frozen in thin film during

the deposition process. There are numerous mechanical properties related to the

thin film. Adhesion and intrinsic stress of thin film are the important properties

which have been studied in this work.

1.5.1 Adhesion:

The mechanical stability, durability and strong adhesion to the substrate are

essential qualities of thin films for their device applications. The study of thin film

adhesion is fundamental and of practical interest because it is related to the nature

and the strength of the binding forces at the interface between the two materials in

contact with each other. Adhesion depends on the concentration of contact point

between substrate and thin film. The higher adhesion is obtained from higher

contact point concentration. The behaviour of thin film adhesion depends upon the

various theories as given below in brief,

22

1. Adsorption theory [49]: The adsorption theory relates adhesion to the

interatomic or intermolecular attractive forces between the films and substrate

at the interface.

2. Diffusion theory[49]: This theory state that adhesion occurs due to the

interdiffusion of the adatoms inside the substrate. Adhesion is considered a

three dimensional volume process rather than two dimensional surface

processes.

3. Electrostatic theory [49]: This theory states that the electrostatic force

developed across the films and surface interface is responsible for adhesion.

The force is due to the formation of electrical double layer of opposite charges

at the interface between dissimilar materials.

4. Mechanical theory [49]: This theory states that the mechanical interlocking

developed due to the surface roughness of the substrate promotes adhesion.

This provides a greater number of interlocking sites and maximum surface area

for bonding. Such interlocking takes place when adatoms flow insides the

pores and voids causing mechanical bonding.

Surface morphology play an important role in the selection of particular theory,

each theory does not apply to all phenomena.

1.5.1.1 Different techniques for adhesion measurement:

Durability of film is of prime importance in its application. Adhesion is

related to the strength of the binding forces and contact point concentration at

interface between the two materials, so that a study of thin film adhesion is of both

fundamental and practical interest.

The adhesion measurements consists of two types of measurement

techniques mechanical and non-mechanical techniques. They can be divided into

qualitative and quantitative. The different methods for the adhesion measurement

shown in table 1.1

Mechanical methods are basically destructive methods i.e. we can not reuse

the same thin film for another characterization or for adhesion measurement again.

23

In these methods thin film has to detach from substrates by applying external

force. During the measurement, applied external force has to increase, at a certain

force thin film separates from substrate; this force is measured and by using the

formula the adhesion of thin film is calculated.

Table 1.1: Qualitative and quantitative techniques for adhesion measurements

1.5.1.1.1 Mechanical technique:

Some of the adhesion measurement methods are shown in figure 1.4. In

scratch test, the smoothly rounded chrome-steel diamond point which is drawn

across the film surface. A vertical load applied to the point which is drawn across

the film surface. It is deformed to the shape of indentation, causing the scratching

of the film. The applied force has maximum value at the tip of intender. The atoms

interlocking with substrate material gives more adhesion due to the formation of

adhesive force at film-substrate interface. Direct pull of method is very suitable

Qualitative Quantitative

Mechanical methods

Scotch tape test [60-62] Direct pull off method [50-51]

Abrasion test [62- 63] Moment or topple test [52]

Bend and stretch test [64, 65] Ultracentrifuge test [53]

Shearing stress test [66] Ultrasonic test [54.]

Peeling test [55]

Tangential shear test [9]

Scratch test [56-58]

Blister test [59-61]

Non-mechanical methods

X ray diffraction [2, 67] Thermal method [7]

Capacity test [62]

Nucleation test [9]

24

and accurate method. The accuracy of this method is ±9.81 kgf/m2. In tangential

sheared force test, there is a formation of sheared force in between deposited thin

films and substrate. This shear force is assumed to move an atom of one layer

from one equilibrium position to the next and is a direct measure of adhesion.

Figure 1.4: Schematic diagrams of various methods for adhesion measurements.

1.5.1.1.2 Non-mechanical methods:

These methods are different than the mechanical methods. These methods

are non-destructive methods, where these sample thin films can be used for

another characterization. Thin films are not detached from substrate. Non-

25

mechanical methods are not very well developed and field of application is very

limited. These methods can be used only for basic investigations.

X-ray diffraction method is qualitative and non-destructive method used to

measure adhesion of epitaxial films on noncrystalline substrate. The measurement

of the adhesion depends on the mechanical stress inside the films and deformation

at the boundary films, causes to produce change in the diffraction pattern. By

using the diffraction pattern stress can be calculated which helps to calculate the

adhesion of the films. This technique have very limited field of applications.

1.5.2 Intrinsic stress:

During the thin film deposition using various techniques, stresses

(force/cross-sectional area) develop in the thin films. It depends on the thin film

preparation parameters and deposition conditions. Large stresses may result in

peeling off or cracking of the film. Stresses also affect the physical properties such

as mechanical properties, optical device performance, secondary electron emission

coefficient, hardness etc. It is of paramount importance to control the stress

formation and sign of the stress i.e. tensile or compressive stress of the film,

during deposition and understand the relevant stress-relaxation processes to

minimize the stress. Adhesion and stress properties are interrelated with each

other, and to maximise the adhesion the absolute level of thin film stress should be

minimum.

Stress in thin film originates mainly due to dislocations, cracks and voids

formation, impurities present in thin film or due to lattice mismatch and

differences in thermal expansion of film-substrate interface. During the thin film

deposition the film experiences a volume change through the introduction of

defects, a phase transformation or a change in temperature. These are the root

causes of defects and voids formation, which initiate the stress in thin film.

The grain boundaries present in thin film might also be a certain reason for

developing the stress within thin film. Some polycrystalline films contain a large

amount of low-angle grain boundaries separating the grains [63]. On an average

26

these grain boundaries produce a tensile stress in the films. The atomic forces

acting between the boundary atoms of neighboring grains due to the distances

between these atoms being larger than the equilibrium atomic distances and the

force between these grains are attractive, i.e. neighboring grains are strained in

tension. Recrystallisation may occur, during or after deposition, when the self-

diffusion is sufficiently high. During recrystallisation, defects are annealed out and

the average grain size of polycrystalline films increases. This leads to a

densification of the film and therefore a tensile contribution to the stress is

expected [64].

The measured stress in thin film involves two types of stresses such as

thermal stress and intrinsic stress [65-67].

• Thermal stress – It arises due to the difference in the thermal

expansion coefficients of the film and the substrate.

• Intrinsic stress – It results due to the structure and growth of the

films resulting in microstructure change.

The sum of these stresses are called as total stress.

Due to heating during oxidation and cooling after oxidation from the

oxidation temperature (Td) to the stress measurement temperature (Tm), thermal

stress will develop because of the difference between the thermal expansion of the

film and the substrate.

The thermal stress is calculated by equation [68]

)()( mdfsfth TTYS −−= αα

Where, αf – thermal coefficient of film varies with temperature in range, αS –

thermal coefficient of substrate, Yf - Young's modulus for the film, Td –

Deposition Temperature, Tm - Temperature at the time of stress measurement.

(Room temperature ~ 300 K).

27

The intrinsic stress occurs due to defects, microstructural variation, material

phase transform, voids formation during deposition or lattice mismatch between

thin film-substrate interfaces. The intrinsic stress is obtained from equation [68]

thin SSS −=

Where Sin= intrinsic stress, Sth= thermal stress and S= measured total stress

obtained within thin film. There are various methods for the total stress

measurement as follows:

1.5.2.1 Methods for intrinsic stress measurement:

The study of internal stress of thin film has attracted much attention. If

deposited thin film is strongly adherent to the substrate, the film-substrate

composite will bend because of stress in thin film. If the thin film tends to contract

and is restrained from doing so by substrate, the thin film is in a state of tension.

On the other hand, if the film tends to expand it will be placed under a

compressive stress by the substrate.

1.5.2.1.1 Bending-plate or beam method [7]:

In bending plate method the stress has to be measured by measuring the

bending of a film deposited substrate caused by the deposition of a stressed film.

The glass or mica substrate of ~0.1mm thick is used, in which the bending or

deflection can be easily measured. One end of substrate is clamped at one side for

an observation of the deflection of another free end, or held on knife-edges for

measuring the deflection. This deflection can be measured by various ways as

optically, through a capacitance change, using the film as one plate of the

condenser, mechanically by using a stylus-type probe or electromechanically.

1.5.2.1.2 Disk method [8]:

In this method, stress is measured by observing deflection of the centre of

circular plate when the film is deposited on one side. Disk, strained by the

presence of a film, will bend. The stress can be related to the deflection by the disk

at a distance ‘r’ from the centre of the disk

28

S = (1/6) * (Esr (1- υυυυ)) * (ts2/tf)

where

Es – Young modulus of the substrate

ts – substrate thickness

tf – film thickness

r – radius of the curvature

υ - Poission’s ratio

1.5.2.1.3 Interferometric method [69]:

Radius of Newton’s rings between disk and optical flat, measures the

deformation of a substrate. The detailed explanation of interferometric method is

given in chapter 2.

1.5.2.1.4 Pressure measurement [70]:

The bowing of the disk is observed hydrostatically by having a fluid in

contact with the side of the disk and observing the movement of capillary.

1.6 Introduction to optical waveguide:

The scientist Anderson first proposed the concept of integrated optics in

1966. He constructed a planer waveguide and its component for applications in

infrared range. During that period, there were limited applications of waveguide.

Few years later, Miller in 1969 [71] introduced the term “Integrated optics” and

discussed the long term outlook in the area, that research began to gain momentum

in optoelectronics devices.

In optoelectronics devices interconnections plays a vital role during the

signal transmission between two components. To make possible various optical

interconnections, architectures need optical waveguides which are integrable with

various electronic and optoelectronic devices and system. The optical waveguides

having the following advantageous:

• Small size and weight

• Signal security

29

• Low transmission loss

• High speed of operation around 104 times higher than electronic circuit.

• Due to lack of electromagnetic interference in optical signals, eliminates

cross talk and stray capacitances in computers.

• Ruggedness and flexibility

• Potential low cost

Due to high optical signal transmission speed, high fan-out capability, high

positional accuracy and low transmission loss, optical waveguide attract great deal

of attraction.

In case of optical waveguide, deposited thin films can act as planer

waveguide. These two-dimensional thin films are analog of optical fiber. The

planar waveguide has great contribution in waveguide developments, so it is a

basic need to give the details of optical planar waveguide.

1.6.1 Optical planar waveguide:

Basically, a planar optical waveguide constitutes a slab of optical material

sandwiched between two layers of lower refractive index. This three layer

assembly is called as planar optical waveguide. The film is deposited on the

substrate of lower refractive index and it should possess good optical waveguiding

properties. Waveguiding medium is core and surrounding medium is cladding

medium.

Figure 1.5: Basic diagram of thin film planner optical waveguide structure.

30

The basic configuration consists of optical media with refractive indices n1,

n2 and n3 for the substrate, film and cover medium, where n2 > n1, n3. In planar

optical waveguide, if both upper and lower cladding material have same refractive

index then it is called as symmetrical optical waveguide and for different

refractive index called asymmetric optical waveguide. The important feature of

the planar optical waveguide is the possibility of light being guided in the film

layer of the structure, as illustrated in fig. 1.5.

1.6.2 Light guiding in a planar optical waveguide:

In a planar optical waveguide, light is guided in the waveguide film by

principle of total internal reflection (TIR) at the film/cover- and the film/substrate-

boundary, respectively, as shown in the Fig. 1.6 for illustration of reflection at the

film/cover-boundary.

Figure 1.6: Light reflected by TIR at the film/cover-boundary when the

propagation angle of the light, θ is above the critical angle, θCritical.

Total internal reflection can be achieved only when,

• The refractive index of the core medium (n2) should be higher than the

surrounded cladding medium (n1, n3).

• The angle of incident on the boundary of core cladding interface should be

higher than the critical angle (θ Critical). At critical angle the angle of

refraction is equal to 90 o and the ray travels parallel to interface along the

direction of the propagation. This is the final limit of the refraction and if

31

the angle of incident increases above that angle then the ray of light

completely reflected back in the medium and undergoes total internal

reflection.

1.6.3 Fundamental Theory of Optical Waveguide:

(i) The Wave Equation and Plane Waves:

While studying the propagation of electromagnetic wave from medium, the

electromagnetic wave theory must be considered. Maxwell provides basic

equations for electromagnetic wave theory [72]

ρ=⋅∇ D ---------------------- (1)

0=⋅∇ B ---------------------- (2)

t

BE

∂

∂−=×∇

----------------------- (3)

t

DJH

∂

∂+=×∇

---------------------- (4)

where,

D = electric flux density

B = magnetic flux density

E = electric field

H = magnetic field

ρ = charge density

J = current density

Above equations can be solved only if we know the constitutive relations

between D to E, B to H & J to E for linear, isotropic & homogenous medium.

D = εE ----------------------- (5)

B = µH ----------------------- (6)

J = σE ----------------------- (7)

where,

ε = dielectric permittivity

32

µ = magnetic permeability

σ = conductivity of the medium

The optical waveguide consists of a slab of dielectric material. So that to

obtain propagation of light from optical waveguide, Maxwell’s equations for a

non-conducting medium is taken into consideration.

Therefore for dielectric medium

ρ = 0 ------------------------ (8)

J = 0 ------------------------ (9)

Therefore equation (1) & (4) becomes,

0=⋅∇ D ----------------------- (10)

t

DH

∂

∂=×∇

----------------------- (11)

By substituting the constitutive equation (5) & (6) in equation (4) & (3)

respectively then equation becomes,

t

HE

∂

∂−=×∇ µ

----------------------- (12)

t

HH

∂

∂=×∇ ε

----------------------- (13)

The wave equation can be derived by using above equation. Taking a curl

of equation (12) we get,

)()( H

tE ×∇

∂

∂−=×∇×∇ µ

∂

∂

∂

∂=

t

E

tεµ

2

2

)(t

EE

∂

∂=×∇×∇ µε

------------- (14)

Now we use a vector identity,

33

EEE 2)()( ∇−⋅∇∇=×∇×∇

Therefore equation (14) becomes,

2

2

2)(

t

E

EE

∂

∂=

∇−⋅∇⋅∇=

µε

2

22

t

EE

∂

∂=∇∴ εµ

--------------- (15)

Similarly we can derive the wave equation for ‘H’. Then, equation (13) becomes,

2

22

t

HH

∂

∂=∇ µε

----------------- (16)

where, 2

∇ is the Laplace operator. For rectangular cartesian & cylindrical

polar co-ordinates, the above wave equation hold for each component of the field

vector, every component satisfy the scalar wave equation,

2

22

t∂

Ψ∂=Ψ∇ εµ

-------------------- (17)

where, Ψ may represent a component of E or H then the velocity of

propagation (v) is given by,

00

11

µµεεεµ rr

pv ==

-------------------- (18)

pvis the phase velocity of dielectric medium, rµ & rε

are the permeability

and permittivity of the dielectric medium and 0µ& 0ε

are the permeability and

permittivity of the free space.

00

1

εµ

=C

---------------------- (19)

34

If the planar waveguide described by a rectangular co-ordinates (x, y, z)

then equation (17) becomes,

2

2

2

2

2

22

zyx ∂

Ψ∂+

∂

Ψ∂+

∂

Ψ∂=Ψ∇

----------------- (20)

The basic solution of the wave equation is sinusoidal wave & the most

important form of which is a uniform plane wave given by,

)](exp[0 tKrj ω−Ψ=Ψ

--------------- (21)

But we know that Ψ is a function of E or H. If the plane wave propagation

along the +Z direction the wave equation for E & H are,

)](exp[0 tKziEE ω−=

---------------- (22)

)](exp[0 tKziHH ω−=

-------------- (23)

These are the equations of the plane wave propagating through dielectric

waveguide for both electric & magnetic wave components.

(ii) Modes in planar waveguide [72]:

Electromagnetic wave consists of a periodically varying electric field ‘E’

and magnetic field ‘H’, which are oriented perpendicular to each other. The most

basic form of an optical waveguide for electromagnetic radiation in the slab

waveguide is shown in the Figure 1.7. We assume propagation in the +z direction.

There is no variation in the y direction, but in the x direction.

In the case of transverse electric field, ‘E’ components are perpendicular to

the direction of propagation and hence Ez = 0 but that time magnetic field vector is

along the direction of propagation i.e. along the +Z direction. In this case mode of

propagation is called TE (transverse electric mode).

In the case of transverse magnetic field, components are along the direction

of propagation i.e. along +Z direction. The components of the magnetic field is

35

perpendicular to the direction of propagation hence Hz = 0. This mode of

propagation is called as transverse magnetic mode (TM).

Figure 1.7: Basic form of an optical waveguide for electromagnetic radiation in

the slab waveguide

Transverse electromagnetic mode (TEM) only observed in metallic

conductors (coaxial cables). They are seldom found in optical waveguide.

Hence interest is to solve the plane wave equation in case of

Ez = 0 and Hz = 0 --------------- (24)

(a) TE mode:

In transverse electric mode, the component of electric field vector may

choose the x-axis

Ez = Ey = 0 ------------------ (25)

And component of magnetic field vector transverse along the +Z direction,

Hx = Hy = 0 ----------------- (26)

The solution of the component of electric field vector and magnetic filed vector

is calculated from equation (22), (23), (25) and (26)

)(

0

tKxjeExE

ω−

=)

------------------ (27)

)(

0

tKxieHzH

ω−

=)

------------------ (28)

(b) TM mode:

For TM mode the electric and magnetic field components are

Ex = Ey = 0 ------------------ (29)

36

Hz = Hy = 0 ------------------ (30)

Therefore the solution of transverse wave equation for TM mode is from

equation (22), (23), (29) and (30) is given by,

)(

0

tKzieEzE

ω−

=)

------------------- (31)

)(

0

tKxieExH

ω−

=)

------------------- (32)

1.7 Introduction to plasma display panel (PDP):

Plasma display panels (PDPs) [1-5] are now established as large-scale flat-

panel display system and have good characteristics, such as high luminance, large

angle of visibility and high resolution. A plasma display panel (PDP) is essentially

a matrix of submillimetre fluorescent lamps (PDP’s pixel) which are controlled in a

complex way by electronic drivers. Each pixel of a PDP is composed of three

elementary visible light emitting discharge cells. The UV light is converted into

visible light by phosphors in the three primary colors (red, green & blue).

1.7.1 Basic plasma display panel operation:

Plasma Display Panel consists of two parallel sealed glass plates. On the

front glass plate, two conducting ITO electrodes are deposited parallel to each other

as shown in fig. 1.8 On top of these electrodes; glass-like dielectric layer is coated.

To protect this dielectric a thin layer is deposited.

The back plate is sectioned into individual cells by barrier rib. Each cell is

coated with red, green or blue luminescent phosphor. The two glass plates (front

and back) are separated by a gap of about 100µm and enclosed volume is filled

with a rare gas mixture (generally Xe–Ne or Xe–Ne–He) capable of emitting UV

photons. Arrays of electrodes are deposited on each glass plate and covered by a

20–40µm thick dielectric layer. There are three electrodes: two parallel on front

glass plate and one orthogonal on the back plate. The coplanar parallel electrodes

called display electrodes as well are made of transparent conductive material e.g.,

indium-tin oxide (ITO). Their width is about 200–300µm in a 42″ panel.

37

Figure 1.8 Basic Plasma Display Panel structure.

Since the resistivity of ITO is finite and the length of the electrodes can be

as large as 1m, a metal bus electrode of smaller width is attached to each to

maintain a constant potential. The data (or address) electrodes are metallic and their

width is of the order of 80µm in standard PDPs. Successive pairs of coplanar

electrodes are separated by dielectric barrier ‘rib’ structures formed on the inner

surfaces of the glass plates. A dielectric layer of thickness ~ 20 - 40µm covers the

address and coplanar electrodes. The protective layer is deposited on the dielectric

surface above the coplanar electrodes to protect the dielectric from sputtering and

to provide large secondary electron emission under ion impact. Phosphors in the

three colours are deposited above the data electrodes and on the dielectric ribs. An

AC dielectric barrier discharge that is sustained between the two adjacent

electrodes generates UV photons that are converted into visible light by the

phosphor in order to produce the picture [73-76]. Visible light emission from

plasma display panels follow the sequence given in flow diagram given below (fig

1.9).

38

1.7.2 Protective layer and Secondary electron emission coefficient (γγγγ):

The emission of electrons from the surface of a solid into vacuum caused by

bombardment with charged particles (in particular with electrons). The number of

secondary electrons emitted per incident particle is called secondary emission yield.

The bombarding electrons and the emitted electrons are referred to, as primaries

and secondary electron respectively. Its pictorial representation is given in fig.1.10.

The Ion induced secondary electron emission is defined as the number of electrons

emitted per incident ion and it is denoted by γi.

Figure 1.9: The flow diagram of PDP operation.

Movement of electrons

Multiplication of electrons by gas ionization: Plasma

Xe excitation

UV radiation

Phosphor excitation

Visible light emission

39

Secondary emission has important practical applications because the

secondary yield, that is, the number of secondary electron emitted per incident

primary electron, may exceed unity. Thus, secondary emitters are used in electron

multipliers, especially in photomultipliers, and in other electronic devices such as

television pickup tubes, storage tubes for electronic computers, plasma display

panel and so on.

Figure 1.10: Pictorial representation of secondary electron emission process.

The emission of secondary electrons can be described as the result of three

processes:

1) Excitation of electrons in the solid into high-energy states by the impact

of high-energy primary electrons

2) Transport of these secondary electrons to the solid-vacuum interface

3) Escape of the electrons over the surface barrier into the vacuum.

The efficiency of each of these three processes, and hence the magnitude of

the secondary emission yield δ, varies greatly for different materials.

Most of the materials used in practical devices are semiconductors or

insulators whose band-gap energies are much larger than their electron affinities.

Examples are magnesium oxide (MgO), beryllium oxide (BeO), cesium antimonide

(Cs3Sb), gallium phosphide (GaP) and potassium chloride (KCl). Maximum δ

40

values in the 8–15 range are typically obtained at primary energies of several

hundred volts.

The secondary electron emission (SEE) property has to be measured to

analyze the PDPs performance. Secondary electron emission coefficient (γ) plays

an important role in lowering the firing voltage, low sputtering yield provides long

life time. The MgO thin coating layer has been used in this work as protective

layer. MgO is a leading candidate for protective layer in PDPs. It has excellent

properties, such as high secondary electron emission coefficient (γ), low sputtering

yield (i.e., large sputtering resistance) and high optical transparency over visible

range (400-700 nm).

Inside PDP, protective layer (MgO) is bombarded by ions and other plasma

species like electron, photon and metastable ions. The obtained SEE coefficient (γ)

is not only obtained by electron bombardment but also by photon and metastable

ion bombardment. The measure the total secondary electron emission called as an

effective parameter (γeff) ions. In others words, gamma effective (γeff) contains the

total contribution of ion-induced emission and other processes. The SEE coefficient

depends on layer properties, ionization energy of gas, band gap and electron

affinity of the material, i.e. higher the ionization energy of the gas, more will be

the magnitude of γ coefficient [73, 74]. Ionization energy of Ne (21.58 eV) is larger

than Ar (15.76 eV) and Xe (12.1eV). The secondary electron emission coefficient

of MgO under xenon ions bombardment is therefore lower than that under neon

ions. Therefore, use of Ne lowers the breakdown voltage and is preferred as a

buffer gas in PDPs [75, 76].

1.8 Brief review of optical materials:

Glass is used extensively in various applications as a result of its

outstanding properties. It is an extremely stable material and is relatively scratch

resistant. Importantly for optical applications it also has high transparency for the

visible electromagnetic spectrum. These properties have led to the widespread use

of glass in architecture and modern information and communication technologies

41

such as TV and data displays screens. However glass is not a perfect material

particularly for architectural use. It has low reflectance for the far infrared (room

temperature radiation) resulting in the loss of thermal energy from buildings and

its high transmission of the near infrared (solar radiation) increases the energy

necessary for cooling buildings. So the deposition of thin layer of various optical

coatings on glass has been used to solve all these problems

Basically optical coatings are of two types’ metallic coating and dielectric

coating. The use of the appropriate coating depends on the use of coating thin film

for expected purpose or applications. The metallic coating like aluminium thin film

has been used due to its’ excellent reflecting properties. It gives reflectivity of

around 88%-92% over the visible spectrum and 95%-99% even into the far

infrared.

The magnesium fluoride, calcium fluoride, and various metal oxides can be

used as optical coatings. In case of dielectric coating, the materials of different

refractive index have been used. By careful choice of the exact composition,

thickness and number of coating layers, it is possible to modify the reflectivity and

transmittivity of the coating to produce almost any desired characteristic. The

versatility of dielectric coatings leads to their use in many scientific optical

instruments such as lasers, optical microscopes, refracting telescopes and

interferometers as well as consumer devices. The properties of optical coating

depends on the various parameter tabulated in table 1.2 [77]. It shows that

refractive index, transmittance, absorbance, adhesion, stress, temperature stability,

defect formation within thin film and ageing of thin film properties are dependent

on the substrate cleaning, evaporation method, evaporation rate, maintained

vacuum pressure and substrate temperature.

Many reports are available on the various materials. Some of the reports are

given in table 1.3. Due to the increase in interest in the optical coating, it is a

present need for a wider range of material with different refractive indices. To

overcome the material limiting problem of useable material for optical coating, a

42

simple way is to utilize the atom-by-atom condensation features of the growth of

vapor deposited films. This process allows homogeneous mixing of different

materials irrespective of their solubility restrictions. The most direct method is to

prepare a mixture of the material to be co-deposited and then to deposit from a

single source. Due to the problem of fractionation of the different materials, this

method has several limitations. The most flexible method is simultaneous

evaporation from several sources [78]. Theoretical calculation studied by Jacobsson

[79-81] shows that, the values of refractive index and optical constants of

homogeneously mixed films can be predicted by assuming that the material obeys

the Lorentz-Lorenz or Drude dispersion theory.

1.9 Literature review of materials used in this work:

Due to the enormous applications of versatile material magnesium oxide,

aluminium oxide was used in this present work. These materials are suitable for

optoelectronics applications. Co-deposited mixed thin films of MgO-Al2O3 was

also studied.

1.9.1 Magnesium oxide thin films:

Magnesium oxide (MgO) has generated appreciable interest among the

scientists due to its higher secondary electron emission (SEE) yield (γ) properties

[3, 4], transparency [1], refractive index nearer to glass [2] and high density [5].

MgO layer plays an important role as protecting layer in ac plasma display panel

(PDP). It prevents the energetic plasma particles from bombarding the dielectric

layer over the electrodes on the front glass panel of a PDP.

Number of methods has been reported for deposition of MgO thin films

such as vacuum arc deposition [1], magnetron sputtering [2,3], electron beam

evaporation [4,5], spray pyrolysis [6] and ion beam assisted deposition [7].The

interesting properties of magnesium oxide thin film has attracted a number of

application [1-4,8]. The refractive index of these films is in the range suitable for

optical waveguide purpose [8-10].

43

Table 1.2: Various properties of optical coating depend on various parameters.

Coating

properties

Substrate

cleaning

Evaporation

method Evaporation Rate

Vacuum

pressure

Substrate

temperature

Refractive index - - Strongly

Dependant

Weakly

Dependant

Strongly

Dependant

Transmittance-

Absorbance - Dependant Dependant

Weakly

Dependant

Strongly

Dependant

Stress Weakly Dependant Weakly

Dependant Dependant

Weakly

Dependant

Strongly

Dependant

Adhesion Strongly

Dependant

Strongly

Dependant Dependant

Weakly

Dependant Dependant

Temperature

stability Dependant Dependant Dependant Dependant

Strongly

Dependant

Ageing Weakly Dependant Dependant - - Dependant

Defect formation - - Dependant Strongly

Dependant

Strongly

Dependant

44

Materials Refractive

index

Region of

transparency Remarks References

Zinc oxide (ZnO) 1.9 - 2.1 400 nm - 10 µm Soft 82-84

Titanium dioxide

(TiO2)

2.2 – 2.7 350 nm – 12 µm Can be produced by oxidation of Ti films 85-87

Bismuth oxide (Bi2O3) 2.45 at 550 nm - Can be produced by oxidation of Bi films 88-89

Tin oxide (SnO2) 1.9 - Can be produced by oxidation of Sn films 90-94

Aluminium oxide

(Al2O3)

1.59 at 600 nm - Can be produced by anodic oxidation of

Al

65, 95-97

Cerium oxide (CeO2) 2.18 - Tends to inhomogeneous layer 98

Hafnium oxide (HfO2) 2.08 at 600 nm 220 nm – 12 µm Fairly hard 99

Magnesium oxide

(MgO)

1.7 – 1.74 210 nm – 10 µm Hard & resistant 100

Silicon dioxide (SiO2) 1.46 200 nm – 8 µm - 101

Yttrium oxide (Y2O3) 1.93 250 nm – 2 µm Hard 9102, 103

Zirconium oxide

(ZrO2)

1.97 – 2.1 340 nm – 12 µm Hard 104-106

Magnesium fluoride 1.36 21o nm – 10 µm Films on hot substrate are much more 107-109

45

Table 1.3: Various coatings and their properties.

(MgF2) rugged

Lanthanum fluoride

(LaF3)

1.58 220 nm – 72 µm - 109

Neodymium fluoride

(NdF3)

1.59 220 nm – 1 µm - 108

Zinc sulphide (ZnS) 2.2 – 2.35 - - 104

46

Out of the numerous research work reported only those relevant to the

thesis work is mentioned here.

• A. M. E. Raj et al. [3] reported the optical properties of highly (100) oriented

magnesium oxide thin films deposited by pyrolysis technique. The deposited MgO

thin films were heated to the substrate temp 400-600 0C. The band gap, resistivity

and refractive index have been reported as 4.5 to 5.25 eV, 2.06 x 107Ωcm and 1.79

respectively, these parameters vary with substrate temperature.

• K. Itatani et al [109] studied the bulk MgO properties of sintered powder

using vapor-phase oxidation process. Using this method the 11 to 26 nm sized

particle was prepared with high density and compact transparent pellet. It showed

that, the line transmission of MgO specimen is reduced by scattering of light at

pores, grain boundaries and rough surfaces, these defects gives the transmission

loss in optical signal.

• As per S. Baba et al [110], the MgO thin films has been prepared by RF-

sputtering and optical constants were determined. It was observed that surface

roughness of thin film can affect on the optical constants and other properties of

thin film. The optical and microstructural properties of MgO thin film deposited

by unbalanced magnetron sputtering technique. By using this technique dense,

transparent, and well crystallined film can be obtained.

• A comparative study of the evaporated and ion beam assisted MgO thin film

has been investigated by S. J. Rho et al [2]. The crystallinity, density and

refractive index was investigated. The relation between refractive index and

packing density has been given, both are linear. It was found that, the porous free

dense thin film gives higher transmission.

• K. Vedam et al [111] has reported the variation of refractive index of MgO

with pressure upto 7 kbar. In this research paper the effect of oxygen pressure

variation during deposition of thin film on refractive index was investigated. It

was observed that, the refractive index decreases linearly with pressure.

47

• The initial oxidation of magnesium at oxygen pressure and temperature just

above room temperature has been investigated in situ with x-ray photoelectron

spectroscopy (XPS) and ellipsometry [112]. Ellipsometric measurements indicates

that the band gap values of the oxide layers are considerable smaller than then the

bulk (~2.5 eV Vs 7.8 eV).

• The mechanical properties of magnesium oxide thin films were studied by

various workers. P. Vuoristo et al [113] deposited the MgO thin films by Rf-diode

sputtering unit, the effect of substrate variation on structure and adhesion of MgO

thin films were studied.

• The stress was investigated [114] of the thin film deposited by e-beam

evaporation. Annealing from room temperature to 300 0C. The effect of stress

properties on the performance of plasma display panel was studied. It was

observed that, the stress of MgO film was increased with increase in substrate

temperature. The lower compressive stressed and highly dense MgO thin film

reduces the firing and sustained voltage.

• MgO thin films were deposited by Rf-sputtering and the effects of annealing

temperature on glow discharge properties were investigated [115]. It was found

that, the firing voltage decreases with substrate temperature.

• K. H. Nam has reported that, MgO thin film can be used as protective layer in

ac-PDP [10]. Magnesium oxide thin film was deposited by novel magnetron

sputtering. The dense film is needed to reduce the firing voltage and to enhance

life time of panel.

• The valence band and electronic structure of thin magnesium and magnesium

oxide thin film were measured by using electron momentum spectroscopy by

Cauney et al [116]. The band structures have been calculated within the linear

muffin-tin orbital approximation. The free electron like parabola characteristic of

metallic solid was observed for Mg whereas valence structure split into two

distinct, less dispersive band typical of an ionic solid observed for MgO.

48

• X. Zhang has succeeded for the preparation of single crystal of magnesium

oxide with an average size of 6 cm, grown by an arc-fusion method [117].

• P. Ghekiere et al have reported the deposition of MgO thin films by

unbalanced magnetron sputtering. (111) preferred orientation was observed [118].

• The use of magnesium oxide thin film as substrate for various purposes also

has been reported [119, 120].

1.9.2 Aluminium Oxide Thin films:

Aluminium oxide has enormous potential to be of use in optoelectronics

and microelectronics devices. Al2O3 thin film combines many properties such as

high dielectric constant, high thermal conductivity, wear resistance and protective

coating, mechanical strength, chemical inertness, good adhesion to glass substrate

and transparency over wide wavelength range [1-5]. The refractive index (1.76) of

these films is in the range suitable for optical waveguide purpose [4, 5]. Due to

these versatile properties aluminium oxide thin film is on of the suitable material

for integrated optics. It has been studied by various workers for different purposes,

some articles has been referred in this research work.

• Z. W. Zhao et al [121] studied the optical properties of aluminium oxide thin

films prepared by cathodic vacuum arc system at room temperature. It was found

that, deposition rate of thin film decrease with increase in oxygen pressure

whereas the transmittance of thin film goes on increasing. The refractive index

and packing density also varies with applied oxygen pressure. They found the

refractive index in range1.60-1.82.

• The influence of oxygen pressure on structural and optical properties of Al2O3

thin films deposited by pulsed laser deposition method has been reported by A.

Pillonnet et al [122]. The application of Al2O3 thin films for optical waveguide

has been suggested. The reported maximum optical transmission loss for Al2O3

was 9dB/cm.

49

• M. A. Frutis et al [123] have studied the aluminium oxide thin films

deposited by spray pyrolysis technique using different molarities and substrate

temperatures from 450-650 oC. It was observed that, overall resistivity of film

decreases when both the molarity of the solution and deposition temperature

increase.

• The optical losses of evaporation-deposited dielectric waveguide were studied

by R. T. Kersten et al [124], and the different oxide materials and the

attenuation was measured. The reported attenuation is 10 dB /cm for Al2O3.

• F. Jahan et al was determined the band gap and the refractive index by using

several deposition techniques. The observed band gap range is 1.92-1.62 eV

[125].

• Aluminium oxide thin films were deposited by reactive magnetron sputtering

by K. koski et al [126]. It was found that, the nanohardness, elastic modulus,

film roughness, structure, intrinsic stress density of thin film and surface defects

is caused by arcing. The nanohardness was in between 177-219 GPa, residual

stress was 249-242 MPa, the density was in between 2.32-3.77 g/cm3. The

surface roughness was in between 0.72-2.64 nm.

• The microstuctrural, surface morphology and their annealing behaviour of

Al2O3 thin films deposited by ion beam assisted deposition was studied in Q. Y.

Zhang et al [127]. The surface morphology is related with the phase transition of

films during annealing. It was found that, thin film phases were changing with

increasing annealing temperature.

• The electrical properties of aluminium oxide thin films deposited by Rf

sputtering was studied by M. Voigt et al [128]. Significant scattering of the I–V

curves and burn-in effects are observed. It was found that an admixture of 1% of

O2 in the sputter gas, improves the electrical properties and revealed a dielectric

constant of ∼∼∼∼7 for all Al2O3 films was investigated.

50

• Z.W. Zhao et al [129] has observed the optical properties of Al2O3 thin films

for higher temperature 200-600oC. The refractive index was 1.73 and extinction

coefficient was ~10-4

at 550nm was observed.

• In B.K. Tay et al [130] studied the effects of substrate bias and growth

temperature on properties of aluminium oxide thin films was studied. The

refractive index and residual stress was investigated. It was found that, RI and

stress was in 1.73-1.68 range and 425 -550 MPa range respectively. Hardness

and stress of deposited thin film was increases with increase in substrate

temperature.

• The structural variation correlation with substrates was studied by Atul

Khanna et al [131]. The Al2O3 thin films were deposited on various substrates

glass, silicon and WC-Co by reactive AC magnetron sputtering. The phase

variation was observed with respect to substrate and temperatures. While the Al

pre-coating improves thin film adhesion and leads to less severe failure, it

significantly decreased the thin film hardness as measured by nanoindentation

technique.

• The optical properties of anodic aluminum oxide films were studied by T. S.

Shih et al [132]. It was observed that, porosity of deposited thin films decreases

with applied current density whereas it increases with increase in bath

temperature.

• Ellipsometric analysis of porous anodized aluminum oxide films has been

studied Y.W. Jung et al [133]. It was observed that, honeycomb shape

approximately overlaps the pore structure of Al2O3 thin films. In this it is clear

that, optical influence is caused by the disordered pores, the void fraction and

thickness, whereas refractive index and the thickness of the film are in general

correlated.

• S. W. Whangbo et al [134] has reported the epitaxial growth of Al2O3 thin

films on Si (100) using ionized beam deposition. It was observed that, as the

51

substrate temperatures increased, the crystalline quality and the surface flatness

were improved.

• Y. F. Mei et al has reported the formation mechanism of alumina nanotubes

and nanowires from highly ordered porous anodic alumina template [135].

• The mechanism and effect of UV laser light interaction with wide band gap

materials has been reported [136]. The interaction mechanisms responsible for

the production of free carriers and defects in UV laser irradiated ceramics, which

affects on various properties of Al2O3 thin films have been discussed.

• The large area deposition of Al2O3 thin films with molecular beams in high

vacuum has also been studied [137].

• Comparative study of refractive index variation of electron beam evaporated

vapor chopped and nonchopped Al2O3 thin films were investigated by P. V.

Patil et al. [50].

1.9.3 MgO- Al2O3 co-deposited mixed thin films:

In this thesis the mixed Mg-Al metal thin films were prepared and oxidised.

The vapor chopping technique was used during the metal thin films deposition. To

the authors’ knowledge no reports are present for MgO-Al2O3 mixed thin films.

The various mixed thin films studied by various workers are given here

• Basset et al [138] studied the ZnS-Na3AlFl6 mixed films obtained by

evaporation using two sources. The properties were not far away from the

theoretically predicted one.

• Jacobsson et al [78] have studied the CeO2-MgF2 and ZnS-Na2AlFl6 mixed

thin films.

• Composition dependence structural Si-YF3, ZnSe-SrF2 mixed thin films

have been investigated by Sankur et al [139].

• Co-sputtering of CeO2-SiO2, TiO2-SiO2 produced both homogeneous and

inhomogeneous optical films. [140].

52

• Electron beam co-deposition was used for depositing ZrO2-MgF2 mixed

thin films [141].

• Multicomponent mixture especially of CeO2-SiO2, CeF3-ZnS, CeO2-CeF3

have been studied in detail [142-144].

• R. F. reactive sputtering has been used by Ganner [145] to deposit Al2O3-

Ta2O5 mixed films.

• The vacuum evaporated and e-beam evaporated co-deposited mixed CeO2-

MgF2 thin films were studied by P. V. Patil [146]. The MgF2 and ZnS thin

films also prepared by using double material depositing sources.

• The enhanced optical waveguiding properties were reported by using co-

deposition technique. The effect on optical transmission loss of mixing of

CeO2 and MgF2 for CeO2- MgF2 mixed thin films were studied by P. V.

Patil et al. [147]

• Adhesion enhancement is reported by using co-deposition of Cu-Ag, Cu-

Al, MgF2_ZnS and MgF2-cryolite by Vijaya Puri and R. K. Puri [148]

1.10 Aim and scope of this work:

The basic aims of nanotechnology are to achieve high speed, high clarity,

fast response, low cost, less energy utilization, enhancement in device

performance with longer lifetime and so on, which make human life comfortable.

Optoelectronic integration is a new technological revolution in the field of

electronics and optics. It has wide scope for development. There is a need for

optical coatings which do not change their properties with time. Our laboratory

has been investigating the optical and mechanical properties of various types of

thin films and have devised the vapor chopping technique for improvement of the

properties [50, 106, 149-152]. In this work magnesium oxide, aluminium oxide

and MgO-Al2O3 co-deposited mixed has been investigated for their structural,

optical and mechanical properties. The widely preferable vacuum evaporation

technique was used for this work. The major advantage of this technique is that

53

required the thin films are deposited under vacuum up to 10-5

mbar. It leads to

minimizing the defect level as compared to conventional non-vacuum deposition

methods. The minimum defect level is most important aspect in optical material.

Magnesium oxide (MgO) has high transparency, large band gap, higher

secondary electron emission coefficient, refractive index nearer to glass and higher

adhesion to glass substrate, which make it very suitable for optical waveguide and

protective layer for PDPs. But MgO has ambient air exposure effect problem; it

changes its various properties with air exposure time. On air exposure, the

performance of optical waveguide as well as PDP deteriorates. Due to air

exposure, the optical transmission loss increases, the transmittance of thin film

decreases, there is increase of stress as well as defect level, it also causes the

reduction in the secondary electron emission coefficient and enhance the firing

voltage of PDP which give rise in operational cost of display panel [5, 153, 154].

To avoid this aging phenomenon of MgO thin films, aluminium oxide has been

used as supportive material by co-mixed deposition.

Aluminium oxide has been used as a material for co-deposition because it

has high transparency, higher secondary electron emission coefficient, greater

adhesion, refractive index nearer to glass and it has more environmental stability

so these results suggest it as most preferable material. Due to co-mixed deposition

of magnesium oxide and aluminium oxide a new MgO-Al2O3 co-deposited mixed

film is obtained. In order to understand the properties of the mixed films the

aluminium oxide thin film has also been studied in this work. During the material

evaporation process, the evaporated atoms (adatoms) acquires the kinetic energy,

these adatoms may or may not be completely thermally equilibrated. When

adatoms reaches nearer to the substrate, it moves horizontally over the substrate

surface by jumping from one potential well to the other because of thermal

activation from the surface and /or its own kinetic energy parallel to the surface.

The adatoms has residence time on the surface during which it may interact with

other adatoms to form a stable cluster. This residence time depends on the

54

evaporation rate (vapor flow) of material [7]. Vapor chopper interrupts the vapor

flow during the deposition process before the condensation on the substrate with

constant rate which gives more residence time to the previously evaporated

adatoms to settle completely at the minimum energy potential well. This helps to

create new nucleation centers and helps to minimize the columnar growth and

form a uniform dense thin film. This is the main theme of vapor chopping

technique.

The aim of the work in this thesis is to obtain MgO and Al2O3 and mixed

MgO-Al2O3 thin films for optical coatings, optical waveguide and also for PDP.

Towards this aim the work has been done and reported in the thesis. The effects of

vapor chopping on the various properties of these thin films have also been

investigated.

The thesis has been divided into six chapters. Chapter I contain the

introduction of material used in the present work, different properties, techniques

for properties measurement of properties. The experimental details are given in

chapter II. Chapter III, IV and V contains results of the various properties of

magnesium oxide, aluminium oxide and MgO-Al2O3 co-deposited mixed thin

films studied respectively. The discussion of the results along with the summary

and conclusions are given in chapter VI.

55

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