20

Chapter 2

Theory

Thin Film deposition and

Characterization techniques

21

2.1 Thin film deposition Techniques

Introduction to Thin Films

This film is a layer of material under consideration ranging

from nano layers (fraction of a nanometer) to several nanometers in

thickness. Electronic semiconductor devices and optical coatings are

the main applications of thin films. Some of the direct applications of

thin films are microelectronics, magnetic sensors, gas sensors, anti-

reflection coatings, corrosion protection, wear resistance and so on.

Thin films possess some special properties which are different from

the bulk materials. Thin films can be under stress, the defect structures

of thin films may vary from thin bulk counterpart, thin films may be

of a two-dimensional structure and they are strongly influenced by

surface and interface effects. Due to these variations, the electrical,

magnetic, optical, thermal and mechanical properties of thin films

change from that of their bulk material [1]

.

Thin films are deposited on to bulk materials to achieve

properties which are not easily attainable by the bulk material.

Additional functionality in thin films can be harnessed by depositing

multiple layers different desired materials [2]

.

Thin films are fabricated by deposition of individual atoms on a

substrate. There are various methods to deposit the desired material as

a thin film on a desired substrate. The following diagram summarizes

the different methods available. Basically there are three categories of

22

thin film deposition methods: (i) Physical vapor deposition (PVD); (ii)

Chemical vapor deposition (CVD); and (iii) Chemical Methods [3]

.

2.1.1 Chemical methods

2.1.1.1 Electroplating Process:

Metallic thin films are deposited with electroplating and electro

less processes by electrolysis methods. An electroplating reactor

always consists of two electrodes, an aqueous solution of metallic ions

and an external circuit to provide the necessary current. In the electro

less process, an oxidation process takes place at the positive

electrode–anode, and the reduction and deposition process takes place

at the negative electrode-cathode. The metal film is formed at the

cathode by the reduction of the metal compound. The ions thus

23

formed are essential in the solution for the conductivity and for the

electro-chemical process. Faraday’s formula specifies the current-time

product of the substance, on the substrate. The layer thickness is given

by

where M – molar mass, Z – number of electrodes, ρ –density of

the metal, I – current, t – time, A – surface area of the cathode, Eff –

efficiency of the process which is determined by the process

parameters, F – Faraday’s number, a constant and is the charge of 1

mole electrons[4]

.

The film thickness and the uniformity of thickness of the film

over the substrate cathode is very important and is mainly determined

by the current density (I/A) distribution in a reactor.

Figure 2.1: Electroplating cell

24

In a typical electroplating cell, the anodes and cathodes are

placed as shown in figure 2.1. When a current is passed through the

cell, a potential field is built up inside the cell. The potential field will

be such that the electrical resistance of the solution is minimal. The

metal deposition process involves electron transfer, the absorption of

the neutralized atom onto the conductive substrate, surface diffusion

and incorporation into the crystal lattice. Small amounts of certain

substances can influence the structure of the metal deposit. These

substances are called the additives and are often used in electro-

deposition process. The conductivity of the electrolyte is influenced

by increase in temperature or by adding a supporting electrolyte to the

solution.

2.1.1.2 Electro less Process:

In the electro less process, the reduction of the metal ions is

similar to the cathodic electro deposition reaction. But here, the

reduction of metal ions and oxidation of the reduction agent occur at

the same electrode. The electrons required for the reduction of the

metal ions are supplied by the oxidation reaction of the reducing

agent. The overall reaction can be decomposed into partial anodic

reaction and partial cathodic reactions. During decomposition, the

partial anodic current is equal to the partial cathodic current. The

external current is zero and the process occurs at a specific potential

called the mixed potential EMP. The electro less process is often called

25

autocatalytic process as the deposited metal catalyzes the oxidation

reaction. This catalytic reaction is necessary for a stable solution and

the selective deposition process on the substrate. Non-conductive

surfaces such as ceramics, glass and plastics can be can be electro less

plated after a sensitizing procedure that creates a catalytic active

surface. The properties of the films deposited by this process depend

on the metal content in the alloy. The most important parameter that

determines the metal content is the pH of the solution.

Main advantage of this process is the uniform layer thickness

over the whole substrate, including protruding parts and holes. The

key advantage of this process is that the process does not require

electrical contact and hence non-conductors and isolated patterns can

be electro less metallized.

Figure 2.2: Electro less plating

26

2.1.1.3 Chemical Vapor Deposition (CVD)

Chemical Vapor Deposition is a chemical process used to

produce high-purity, high-performance (solid materials) thin films. In

a typical CVD process, the substrate is exposed to volatile precursors

which react or decompose on the substrate surface to produce the

desired deposit. Volatile by products produced during the reaction are

removed by gas flow through the reaction chambers.

Types of CVD

There are several types of CVD procedures. The important

among them are;

(i) APCVD – Atmospheric Pressure CVD

(ii) LPCVD – Low Pressure CVD

(iii) PECVD – Plasma-Enhanced CVD

(iv) MDCVD – Metal-organic CVD

(v) RTCVD – Rapid Thermal CVD

Principles of CVD

Chemical Vapor Deposition may be defined as the deposition of

a solid on a heated surface from a chemical reaction in the vapor

phase. It belongs to the class of vapor-transfer processes which is

atomistic in nature.

27

Figure 2.3: CVD Reactor Schematic

The fundamental principle behind any CVD process is

described below:

The reactants from the gas inlets are convectively and

diffusively transported to the reaction zone [5]

. The gas phase

undergoes chemical reactions to produce new reactive species and

byproducts. The initial reactants and their byproducts are transported

to the surface. These species are absorbed and diffused on the

substrate surface. On the surface, heterogeneous reactions are

catalyzed which leads to film formation. Due to surface reaction, the

volatile byproducts are desorbed. These byproducts are convectively

and diffusively transported away from the reaction zone.

28

2.1.1.4 Sol Gel Processing

Sol Gel process is a wet chemical technique widely used in the

field of Material Science and Ceramic Engineering. These methods

are primarily used for the fabrication of materials starting from a

colloidal solution called the ‘sol’ that acts as the precursor. Typical

precursors are metal alkoxides and metal salts which undergo various

forms of hydrolysis and poly condensation reactions. The sol consists

of a liquid with colloidal particles which are suspended in it, and do

not agglomerate or sediment [6]

. The solvent used in the colloidal

particles is either pure water or a solution composed mostly of water

and alcohol.

A gel is a porous, three dimensional interconnected solid

network that expands in a liquid medium. If the solid network is made

of colloidal sol particles, then the gel is said to be colloidal.

The sol-gel process is hence a colloidal route used to synthesize

ceramics with an intermediate stage including a sol and/or a gel state.

This sol can be coated on the substrate by either spin coating or

dip coating to form a xerogel film. The solvent from the sol can be

evaporated to precipitate particles of uniform size and then it can be

screen-printed. The sol can be allowed to gel completely to obtain

either xerogel or an aerogel [7]

.

The sol-gel method can be employed to form nonstructural thin

films for sensing applications and the grain size of these films can be

engineered for the right application. Unlike many other fabrication

methods which require starting materials to have the same

composition as that of the final product, sol-gel offers an

29

economically feasible route, to explore different ratios and

combinations of the compound [8]

.

2.1.2 Physical Processes:

2.1.2.1 Evaporation

Evaporation of a material and its subsequent condensation on a

substrate is the simplest process for thin film deposition. Here, the

material to be deposited is in the form of solid or a liquid and requires

thermal energy for transformation into the vapor phase. This vapor

expands into an evacuated chamber that contains the substrates also.

The vapor condenses on the substrate that is at a lower temperature

than the evaporation source [9]

.

Figure 2.4: Thin film evaporation schematic

30

2.1.2.2 Sputtering:

In this process, a target and a substrate are placed opposite to

each other in a chamber which is filled with argon gas. The target and

the substrate are separated by a few centimeters. When the target is

bombarded with energetic particles such as accelerated ions, surface

due to of the target are scattered backwards due to collisions between

the surface atoms and the particles. This phenomenon is called ‘back-

sputtering’ or ‘sputtering’.

DC Sputtering: This is composed of a pair of electrodes, one

being a cold cathode and the other, an anode. The front surface of the

anode is covered with the target materials to be deposited. The

substrates are placed on the anode. The sputtering chamber is usually

filled with argon. The glow discharge is maintained under the

application of a DC voltage between the electrodes. Here, the target is

a metal since the glow discharge is maintained between the metallic

electrodes.

RF Sputtering: For insulating targets, the glow discharge

cannot be maintained by the DC source and hence, to sustain the glow

discharge, RF voltage is supplied to the target [10]

.

In Magnetron Sputtering, a magnetic field is superimposed on

the cathode and glow discharge. The electrons in the glow discharge

show cycloid motion and the center of their orbits drifts. The magnetic

31

field is oriented such that these drift paths for electrons form a closed

loop. This electron trapping effect increases the collision rate between

the electrons and sputtering gas molecules. This enables one to reduce

the sputtering gas pressure. The magnetic field increases the plasma

density which in turn increases the current density at the cathode

target, effectively increasing the sputtering rate.

Reactive Sputtering: When a reactive gas species is introduced

into the chamber such as oxygen or nitrogen, thin films of oxides or

nitrides are deposited by the sputtering of the metal targets. This

technique is called reactive sputtering and this may be used with

either DC or RF modes. This technique is used in practice for high-

rate deposition of insulting metal oxide films.

2.2 Characterization of Thin films

Material Characterization: Characterization refers to the use

of external techniques to study the internal surface and properties of a

material [11]

. Characterization of a film necessarily requires application

of methods that determine properties of the film and not the substrate.

The properties of thin films often differ from properties of bulk

materials of the same composition. The substrate on which the thin

film is grown also influences these properties. Hence characterization

of thin films involve the study of the properties of a three dimensional

substrate and a two dimensional thin film on top of it [12]

.

Characterization of a thin film becomes very important as (i)

properties of thin films are different from the bulk material, (ii)

32

properties depend on the deposition techniques used, (iii) and

properties depend on the deposition parameters.

Deposition Parameters such as substrate temperature,

atmosphere and technique specific parameters, such as voltages,

influence the structural properties which in turn influence the

macroscopic properties such as electrical conductivity, oxidation, and

hardness. The density of defects in the film increases the internal

stress and in turn reduces adhesion, reducing the tool lifetime.

2.2.1 Optical Characterization:

The optical methods of measurement of thin film structure give

the necessary information about the structural and physical properties

which are used in optics and microelectronics. The problem of

estimating the thickness and optical constants can be done by

analyzing experimental spectra of the reflection and or transmission of

the thin film structure.

Visible and ultraviolet light with many frequencies, when

incident towards the surface of a material (thin film), the material has

a tendency to selectively absorb, reflect or transmit certain light

frequencies [13]

. The manner with which the incident light interacts

with the material depends on the frequency of light and the nature of

atoms of the object.

33

The electrons in the atom of the material have natural frequency

at which they tend to vibrate. If light with which a given frequency

strikes a material with electrons having the same vibrational

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

transform it into vibrational motion. Different natural frequencies of

vibration and hence selectively absorb different frequencies of light.

Reflection and transmission of light waves occur as the light

frequencies do not match the natural frequencies of vibration of the

electrons of the desired material. When the incident frequencies do

not match, the electrons do not vibrate in resonance but vibrate with

small amplitudes and the energy is reemitted as alight wave. If the

object is transparent, the reemitted light passes through and comes out

of the opposite side of the object. Such frequencies of light are said to

be transmitted.

If the object is opaque, the electrons on the material’s surface

vibrate for a short period and the energy is reemitted as a rejected

light wave. Such frequencies of light are said to be reflected [14]

.

Spectroscopy deals with the effect of the medium and matter

on the transfer of electromagnetic radiation. The basic processes

involved in are reflection, absorption and transmission of radiation.

Observation of these phenomena offers the possibility of examining

the transfer of energy and the analysis of matter.

When light strikes a sample, all the three phenomena -

reflection, absorption and transmission occur. Studying either the

34

absorbed or transmitted radiation yields a large number of data on the

optical properties of the material of the thin film.

UV-visible Spectroscopy (UV-VIS Spectroscopy) surveys the

electronic transitions of molecules, as they absorb light in the UV and

visible regions of the electromagnetic spectrum. The UV-VIS

absorption spectrum of a sample is normally represented as a graph of

radiation absorbed as a function of wavelength. The height of the

absorption peaks is proportional to the concentration of the species in

the sample.

The transmission spectra, similarly is a graph of the transmitted

light as a function of wavelength. Using these data a sample is

characterized by reflection coefficients R1, R2, absorption coefficient

α, complex refractive index [N = (n0-jk1)] and thickness,‘d’ of the thin

film. From the derived data, the optical band gap of the material can

also be estimated.

35

2.2.1.1 UV-visible Spectrometer:

Components of a typical spectrometer are shown in Figure 2.5

Figure 2.5: A Schematic diagram showing the components of a UV-VIS

Spectrophotometer

A beam of light from a visible and/or UV light source is

separated into its component wavelengths by a prism or diffraction

grating. Each monochromatic beam in turn is split into two equal

intensity beams by a half-mirrored device. One beam, the sample

beam, passes through a cuvette containing a solution of the compound

being studied in a transparent solvent. The other beam, the reference,

passes through an identical cuvette containing only the solvent. The

intensities of these light beams are then measured by electronic

detectors and compared. The intensity of the reference beam, which

should have suffered little or no light absorption, is defined as I0. The

intensity of the sample beam is defined as I. Over a short period of

time, the spectrometer automatically scans all the component

36

wavelengths. The ultraviolet (UV) region scanned is normally from

200 to 400 nm, and the visible portion is from 400 to 800 nm.

If the sample compound does not absorb light of a given

wavelength, I = I0. However, if the sample compound absorbs light

then I will be less than I0, and this difference may be plotted on a

graph versus wavelength.

Absorption may be presented as transmittance (T = I/I0)

or absorbance (A= log I0/I). If no absorption has occurred, T = 1.0

and A= 0. The wavelength of maximum absorbance is a characteristic

value, designated as λmax. Different compounds may have very

different absorption maxima and absorbance. The most commonly

used solvents are water, ethanol, hexane and cyclohexane.

Figure 2.6: UV-VIS Spectrophotometer (UV-1700 Pharma Spec,

SHIMADZU)

37

The peak of the transmittance spectrum is unique for a given

material. The range of the wavelength can typically vary between 200

nm to 1100 nm. The spectrum shows maxima and minima with

varying wavelength. The data at these points helps to calculate the

thickness of the given thin film [15]

.

The thickness of the film can be calculated by the formula

Where ‘λ1’ is the wavelength at which maxima occurs and ‘λ2’

is the wavelength at which the consecutive minima occur and ‘n’ is

the refractive index of the material.

The optical band gap of the film can be estimated from the

graph of

(α h ν) 2 versus E = (h ν).

Close to fundamental absorption, transmission characteristics

can be converted to the absorption spectra using the relation

Where ‘α’ is the absorption coefficient, ‘t’ is the thickness and

‘T’ is the percentage of transmission.

The frequency dependent complex refractive index N is given

by N= (n-jk) where ‘k’ is the extinction coefficient which is related to

38

the decay or damping of the oscillation amplitude of the incident field.

From the absorption coefficient, its relation with ‘k’ can be written as

or

The refractive index ‘n’ of the film can also be calculated from

the transmittance spectrum, from the relation

n = [N + (N2 – S

2)1/2]1/2

,

Where,

Where, TM and Tm are the maximum and minimum

transmittance envelope functions, and ‘S’ is the refractive index of the

substrate.

Absorbance is the logarithm of the ratio of the intensities of the

incident (I0) and the transmitted light (I). It is related to the molar

absorptivity ε, thickness of the film and molar concentration (C) of the

film according to the Beer-Lambert law as,

A = log (I0 / I) = εdC

It is also related to the transmittance as A = - log 10 (T)

2.2.2 Structural Characterization:

2.2.2.1 X-ray Diffraction

X-ray diffraction (XRD) spectroscopy is one of the most

important techniques for material identification. The advantages of X-

ray Diffraction are that it can differentiate reliably between thousands

39

of crystalline material types. From the data available from XRD,

additional information on the disorder within the lattice, substitution

of one element for another, the distance between the atoms, grain size

of the material etc. can be obtained.

Principle of Operation:

X-rays are electromagnetic radiation with typical photon

energies. As the wavelength of x-rays is comparable to the size of

atoms, they are ideally suited for probing the structural arrangement

of atoms in a wide range of materials. The energetic x-rays can

penetrate deep into the materials and provide information about the

bulk structure.

X-rays primarily interact with electrons in atoms. When x-ray

photons collide with electrons, some photons of the incident beam

will be deflected or scattered away from the direction where they

originally travel. If this scattering is elastic, they carry information

about the electron distribution in the materials. The diffracted waves

from different atoms can interfere with each other and the resultant

intensity distribution is strongly modulated by this interaction. If the

atoms in the sample are arranged in a periodic fashion, the diffracted

waves will consist of sharp interference maxima (peaks) with the

same symmetry as in the distribution of atoms. Measuring the

diffraction pattern hence allows us to estimate the distribution of

atoms in material. The peaks in an x-ray diffraction pattern are

directly related to the atomic distances [16]

.

40

Figure 2.7: Schematic showing the atomic distance

The relation 2d sin θ = n λ is known as Bragg’s law [16]

. In this

relation, λ in the wavelength of x-ray, θ is the scattering angle and ‘n’

is an integer representing the order of the diffraction peak.

The basic geometry of an X-ray diffraction meter involves a

source of monochromatic radiation and an X-ray detector situated on

the circumference of a graduated circle centered on the specimen.

Divergent slits, located between the X-ray source and the specimen,

and divergent slits, located between the specimen and the detector,

limit scattered (non-diffracted) radiation, reduce background noise,

and collimate the radiation. The detector and specimen holder are

mechanically coupled with a goniometer so that a rotation of the

detector through 2x0 occurs in conjunction with the rotation of the

specimen through x0 in a fixed 2:1 ratio.

A curved-crystal monochromator containing a graphite crystal is

normally used to ensure that the detected radiation is monochromatic.

When positioned properly just in front of the detector, only the K-

alpha radiation is directed into the detector, and the K-beta radiation,

because it is diffracted at a slightly different angle, is directed away.

The signals from the detector are filtered by pulse-height analysis,

41

scaled to measurable proportions, and sent to a linear rate meter for

conversion into a continuous current. Common output devices include

strip-chart recorders, printers, and computer monitors.

Figure 2.8: Schematic diagram of a part of XRD instrumentation

Figure 2.9: X- Ray Diffractometer

42

2.2.2.2 Atomic force microscopy

Atomic force microscopy (AFM) is a very useful

characterization technique which allows a variety of surfaces to be

imaged and characterized at the atomic level. AFM scans the sample

and take measurements in three dimensions, (normal to the sample

surface), thus presents the three-dimensional images of a sample

surface. AFMs require no special sample preparation, and they can be

used in either an ambient or liquid environment.

Figure 2.10: Schematic diagram showing the principle of AFM

The basic technique involved in AFM is the usage of a sharp

probe which scans across the surface of the desired sample. The

interactions between the surface and the probe are used to produce a

high resolution image of the sample. The probe is the sharp tip of a

cantilever, which scans the sample surface [17]

. The probe is brought

43

into and out of contact with the sample surface with the help of a

piezo crystal, on which, the cantilever is mounted. The deflection of

the cantilever is monitored by the change in the path of a beam of

Laser light, detected from the upper side of the end of the cantilever

by a photo detector. The photo detector converts this signal into an

electric signal, which is then converted into an image with the aid of a

computer and imaging software.

2.2.2.3 Scanning electron microscope and Energy dispersive x-ray spectroscopy

Scanning electron microscope (SEM) is a type of microscope

that images a sample by scanning it with a high-energy beam of

electrons. The electrons interact with the atoms of the sample

producing signals which contain information about the samples

surface topography, composition and other properties. The types of

signals produced by SEM are secondary electrons, back-scattered

electrons, characteristic x-rays, specimen current and transmitted

electrons.

Energy dispersive x-ray spectroscopy (EDS, EDX or EDAX) is

an analytical technique used for chemical characterization of a

sample. This technique is based on the principle that each element has

a unique atomic structure allowing x-rays that are characteristic of an

element’s atomic structure to be identified uniquely from one another.

The scanning EM instrument forms images with electrons and

the EDAX detector on this instrument allows identification of the

elements present in the sample.

44

Principle:

A SEM uses electrons from a filament, typically a heated

tungsten wire accelerated down a column through a voltage potential.

The electron beam is scanned over the surface of the sample, the

secondary or the back-scattered electrons from the sample surface are

detected and an image is formed.

The accelerated electrons forming the beam may have sufficient

energy to displace the inner shell electrons in the target material. the

leads to the production of characteristic x-rays that can be used to

produce a quantitative chemical analysis. In energy dispensive x-ray

spectroscopy, the energy of each individual x-ray arrival is determined

and counted toward the development of an energy distribution

histogram. With sufficient counts, this histogram becomes an EDAX

spectrum consisting of background and characteristic x-ray peaks [18]

.

2.2.3 Electrical Characterization

2.2.3.1 MOS capacitor

MOS stands for Metal Oxide Semiconductor. The structure of a MOS

capacitor can be shown as in Figure 2.11.

45

Figure 2.11: Si – SiO2 System

The Si-SiO2 MOS system consists of a conducting gate

electrode (metal or heavily doped polysilicon) on top of a thin layer of

silicon dioxide grown on a silicon substrate. The band structure of all

the three materials can be related to the vacuum level of convenience.

The vacuum level is defined as the energy level at which the electron

is free. For silicon, the vacuum level is 4.05ev above the conduction

band. This implies that the electrons at the conduction band edge must

gain a kinetic energy of 4.05ev called the electron affinity in order to

break loose from the crystal field of silicon. 9n silicon-dioxide, the

vacuum level is 0.95ev above its conduction band, which means that

the potential barrier is (4.05-0.95) electron volt = 3.1 electron volt

between the conduction bands of silicon and silicon dioxide [19]

.

46

Figure 2.12: Energy level diagram of Si – SiO2 MOS System

For metals, the energy difference between the vacuum level and

Fermi level is called the work function of the metal. Different metals

have different work functions.

MOS behavior under different bias conditions

When there is no voltage applied between the metal and silicon,

their Fermi levels line up, since the work functions are equal, their

vacuum levels also line up and the bands in both silicon and oxide are

flat. This is called flat band condition. There is no charge or field and

the carrier concentration is at the equilibrium value throughout silicon.

When a negative voltage is applied to the gate, it raises the

metal Fermi level with respect to the silicon Fermi level and creates

an electric field in the oxide that would accelerate a negative charge

towards the silicon substrate. Because of the low carrier concentration

47

in silicon, the bands bend upwards toward the oxide interface. The

potential at the silicon surface is called the surface potential. Due to

the band bending, the Fermi level at the surface is much closer to the

valence band than the Fermi level in the bulk silicon. This results in a

hole concentration much higher at the surface than the bulk silicon.

Since excess holes are accumulated at the surface, this is referred to as

the accumulation condition.

When a positive voltage is applied to the gate, it lowers the

metal Fermi level with respect to the silicon Fermi level and creates

an electric field in the oxide that would deplete charges in the oxide –

semiconductor interface, which results in a negative charge built up in

the semiconductor. Initially, this charge is due to the depletion of the

majority charge carriers in the oxide-semiconductor interface. This is

referred to as depletion. The depletion layer width increases with the

increase in the gate voltage.

As the potential across the semiconductor increases beyond

twice the bulk potential, the minority negative charges accumulate at

the oxide-semiconductor surface which forms the inversion layer. As

the gate voltage is further increased, the inversion layer increases

exponentially.

Capacitances voltage curves in a MOS structure:

The applied gate voltage of a MOS Structure divides itself as an

oxide voltage Vox and a band bending voltage Vbb, such that

48

Where Cox is the oxide capacitance per unit area given by Cox = εox /

tox.

Where tox is the oxide thickness and εox is the permittivity of the

oxide material.

The capacitance of a MOS structure can be easily measured by

applying a DC bias voltage above the structure and superimposing a

small AC voltage on top of the DC bias.

The total capacitance of the structure can be visualized as a

combination of the oxide capacitance and silicon capacitance. The

oxide capacitance is constant which depends on the permittivity of the

oxide material and its thickness. The silicon capacitance is a variable

quantity and depends on the depletion and invention conditions of the

MOS structure. Hence the equivalent circuit of the MOS structure can

be drawn as follows.

Figure 2.13: Equivalent circuit of a MOS System

49

The silicon capacitance can be thought of as a parallel

combination of the depletion capacitance and invention capacitance.

The depletion capacitance is due to the majority carriers which

respond to both high and low frequencies. The invention capacitance

however is due to the minority carriers and responds to low

frequencies alone. Hence, when a high frequency AC signal is applied

across the MOS structure, the invention capacitance is low and hence

the effective capacitance is also low and it saturates at a low value

with the increase in gate voltage. On the other hand, when the AC

signal frequency is high invention capacitance is high and hence the

effective capacitance saturates at the higher value with the increase in

gate voltage [20]

.

Figure 2.14: Capacitance – Voltage variation of a MOS System

2.2.3.2 Tunable dielectric materials

Electronic materials exhibit a change in their dielectric constant

with the applied electric field. This effect is too small to be useful or

50

easily measured in most of the materials. But this effect can be quite

pronounced in some special class of high-permittivity materials. Such

materials are called tunable dielectric materials, tunability is the field

dependent permittivity exhibited by high-permittivity materials.

Usually the thin film materials will be polycrystalline, with defects

and vacancies that depend on the choice of growth temperature

substrates, electrodes and impunity concentration for capacitive

applications of such films, the oxygen vaccines give rise to underiable

effects like leakage, reduced lifetime, and bias induced performance

degradation. Such problems can be overcome by the addition of a

small amount of dopants to these materials [21, 22]

.

Capacitance-voltage curves of tunable dielectrics

Figure 2.15: Capacitance – Voltage variation of a tunable capacitor

For a tunable dielectric material, the capacitance peaks at a

certain voltage (usually zero) and decreases on both sides. Usually at

zero the material exhibits maximum capacitance Cmax which depends

on the electrode area, film thickness, frequency, temperature, substrate

51

conductivity etc…. at some desired voltage Vτ, the capacitance is

reduced to Cmin. With these values, the tunability can be defined as τ=

Cmax / Cmin. This tunability is dependent on the choice of the voltage

Vτ.

The tunability is dependent on the type of the material and the

deposition conditions of the samples and the thickness, as dielectric

constant of any material is thickness dependent.

The symmetric center of the capacitance-voltage curve may not

be at zero always. It can shift away from zero due to the work

function difference of the electrodes of the MOS structure [23]

.

The maximum value of the capacitance also shifts away from

zero with varying conductivity of the substrates on which the MOS

capacitor has been fabricated. This can be attributed to different strain

in the thin-films, tuning mainly occurs from the vibrations of Ti and O

ions in the opposite directions. If there is a strain that change in the

ionic displacement, then the tunability also changes to a large extent,

this strain is similar to the applied bias voltage [24]

.

2.2.3.3 Experimental details for Electrical characterization

Electrical characterization of semiconductors covers a number

of different experimental methods giving information on the change

carrier distribution and transport mechanism.

Electrical measurement techniques are the most important

characterization methods as they measure parameters that are of direct

intrest to semiconductor devices – capacitances, current, and dielectric

52

constants and so on. The most important electrical characterization

techniques are the C-V and the I-V measurements.

C-V characterization is very widely used and provides a large

amount of information about dielectric films and their interfaces that

such films make with the underlying semiconductors.

C-V measurements are made applying a DC voltage to the gate

and superimposing a small, high frequency ac voltage on it with

respect to the substrate. The schematic diagram of the experimental

arrangement is a shown in Figure 2.16.

Figure 2.16: Experimental set up for electrical measurements

Another simple measurement of insulator properties is the I-V

characterization. For thin oxides, this measurement can directly

measure tunneling currents which are used in some types of memory

devices.

I-V measurements can be made applying a voltage at the gate

and monitoring the corresponding current.

53

References:

[1] The material science of Thin Films – Milton Ohring,

Academic press limited, California, 1992, xix

[2] Thin Film Deposition – Principles and Practice – Donald

L. Smith, McGraw-Hill International Edition 1995.

[3] Thin Film Technology Handbook – Aicha Elshabini-

Riad and Fred D Barlow III, McGraw-Hill, 1998

[4] Chemical Physics of Thin Film Deposition – edited by

Yves Pauleav Nano Science Series, Kluwer Academic

Publication, 2002.

[5] Material Science of Thin Films - Deposition and

Structure – Milton Ohring – Academic Press

[6] Introduction to Sol Gel Processing - Alain C. Pierre,

Kluwer, Academic Publishers, Second Edition, Pp. 3-4

[7] Science and Technology of Chemiresistor Gas Sensors,

Dinesh K. Aswal, Shiv K. Gupta, Nova Science

Publishers, Inc., 2007, Pp. 156-158

[8] Nanotechnology Enabled Sensors, Kourosh Kalntra-

Zaden and Benjamin Fry: Springer Science and Business

Media, 2008, Pp. 186

[9] Principles of Physical Vapor Deposition of Thin Films,

K.S. Shree Harsha, Elsevier Ltd. First Edition, 2006, Pp.

11-12

[10] Thin Film Materials Technology – Sputtering of

Compound Materials, Kiyotaka Wasa, Makoto

54

Kitabotake, Hdeaki Adachi, William Andrew Inc., 2004,

Pp. 39-41

[11] Characterization of Optical Materials, Material

Characterization Series, Gregory t. Exarhos.

[12] Surface Properties and Engineering of Complex

Intermatallics, Esther Berlin Ferre: Book Series on

Complex Metallic Alloys, Vol. 3, World Scientific

Publishing Co. Pvt. Ltd, 2010, Pp. 1

[13] Substrate Effect on the Optical Reflectance and

Transmittance of thin film structures - Anatoly Barybin

and Victor Shapavlov: International Journal of Optics,

Vol. 2010, Article ID B7572, 18 pages

[14] UV – Visible Reflection Spectroscopy of Liquids, Jukka

Raty, Kai-Erik Peipon, T. Asakura: Springer-Verlag

Berlin Heidelberg, 2004, Pp. 92-94

[15] Optical Properties of Thin Solid Films, O.S. Heavens,

Butterworth Scientific Publications, 1991

[16] ar.wikipedia.org

[17] Atomic Force Microscopy in process Engineering – An

introduction to AFM for improved processes and

products – edited by W. Richard Bowen, Nidal Hilal –

IchemE – 2009, Elsevier Ltd. Pp. 2-4

[18] A web source for the study of Alcali Feldspars and

Perthitic textures using light microscopy, scanning

Electromagnetic and energy dispersive x-ray

Spectroscopy - Anne Argast and Clarence F. Tennis III

55

[19] Fundamentals of Modern VLSI Devices – Yuan Taur and

Tak H. Ning, Cambridge University Press, 1st South

Asian edition 2003

[20] Robert A. York, “Tunable dielectrics for RF circuits”,

multifunctional adaptive microwave circuits and systems,

sciteh publishing 2009

[21] Multifunctional Adaptive Microwave Circuits and

Systems, M. Steer and W.D. Palmer, eds., Scitech

Publishing 2009, 4

[22] Tunable Dielectrics for RF Circuits, Robert A. York,

University of California at Santa Barbara

[23] S. Hyun and K. Char, “Effects of strain on the dielectric

properties of tunable dielectric Sr Tio3 thin films”

Applied physics letters, volume: 79, no.2, (2001) Pp.

254-256

[24] Ph.D. thesis: Thin film barium strontium Titanate

capacitors for tunable RF front-end applications,

University of Twente, The Netherlands, Author: MPJ

Tiggleman. ISBN: 978-90-365-2937-2, 2009