CHAPTER 3

EXPERIMENTAL BACKGROUND AND PROCEDURES

Many fabrication methods have been used in the production of metal

oxide and metal nitride semiconductor sensors. Factors that must be considered

when selecting the production technique include; expense (if the films are

expensive, the demand will be low and will have only limited applications),

purity, porosity (if the material is highly porous, the surface area available to the

gas for interaction will be far higher, giving a higher sensitivity), reliability and

reproducibility. Common techniques for making the metal oxide and nitride

films for gas sensors are mainly chemical vapour deposition (CVD), screen-

printing of ceramic powders, sol-gel techniques and physical vapour deposition

(PVD), electron beam and sputtering. In order to fabricate the high quality ITO

thin films, several methods have been reported, such as rf magnetron sputtering

[82-84], and dc magnetron sputtering [85-87]. The effects of the deposition

parameters on properties of aluminum nitride thin films were investigated by dc

[88, 89] and rf sputtering [90-95].

In the present work, ITO and AlN thin films have been prepared by

magnetron sputtering. Sputtering may be a more desirable process for the

deposition of oxide and dielectric materials due to its processing simplicity and

scalability in the semiconductor industry. The advantages of reactive magnetron

sputtering include inexpensive equipment, low temperature growth conditions

and ease of deposition. It is important to be able to control the crystal

orientation and crystalline quality of the film for many applications. Properties

of films are strongly dependent on deposition parameters such as target power,

growth temperature, sputtering pressure and gas compositions [96-102].

3.1 SPUTTERING

Magnetron sputtering is a thin film deposition technique based on the

physical sputtering effects caused by the bombardment of a target material with

accelerated ions produced within glow discharge plasma. A wide variety of thin

film materials, from metals to insulators, may be produced using this technique

[88]. Usually the thin film composition will be determined exclusively by the

elements composing the target, since the only elements occurring in the gas

phase are the noble gas ions used to generate the plasma and other particles

ejected from the target as the result of the sputtering process. In some cases a

reactive element may be introduced in the gas phase that will interact with the

deposited elements; this latter case is called reactive sputtering. In order to

understand the thin film growth processes and the dependence of its

characteristics on the system parameters, it is necessary to know how the

particles are removed from the target and how they interact on the substrate

surface. Therefore in the following sections ion production, sputtering

mechanisms and film growth will be briefly explained, before presenting the

system used to produce the sensor film layers and the correlations found with

the deposition parameters.

3.1.1 Plasma process

Ions used in planar magnetron sputtering systems are generated by glow

discharge processes. A glow discharge may be established when a potential is

applied between two electrodes in a gas. Once established two or three different

regions can be clearly distinguished between the electrodes. Close to the

cathode there is a dark region that exists also near the anode although the latter

is generally too thin to be observed. Next there is negative glow region and if

the separation between the electrodes is higher than a few times the dark space

thickness, a positive column develops between the negative glow and the anode.

The positive column is the region that more closely resembles a plasma. The

plasma does not take a potential between those of the electrodes, but rather

acquires a potential slightly higher than that of the anode. Inside the plasma, as

expected, the electric field is very low. The main potential difference is

therefore observed at the sheaths next to each of the electrodes. The role of the

magnetron is forming electron traps that help to sustain the discharge.

Without the magnetron, the electrons emitted from the cathode by ion

bombardment are accelerated nearly to the full applied potential in the cathode

dark space and enter the negative glow, where they collide with the gas atoms

producing the ions required to sustain the discharge. As the electron mean free

path increases with gas pressure decrease, at low pressures the ions will be

produced far from the cathode where the chances of being captured by the walls

are high. At the same time many primary electrons (the ones emitted from the

cathode) reach the anode with high energy, inducing secondary electron

emission. Therefore, ionization efficiencies are low and self sustained

discharges cannot be maintained in planar diodes at pressures below 1:3Pa

[103]. Currents are also limited since voltage increases the primary electron

energy and consequently their mean free path. At high pressures the sputtered

atom transport is reduced by scattering, which at some point starts to force a

decrease in the deposition rate. On the other hand, with the magnetron system

configuration, the primary electron motion is restricted to the vicinity of the

cathode and thereby ionization efficiency is increased. This effect is easily

obtained imposing a magnetic field parallel to the cathode surface and thus

normal to the electric field.

In a planar configuration, the E x B motion causes the discharge to be

swept to one side. This difficulty can be overcome using cylindrical cathodes,

which allow E x B to close on them. Planar magnetrons can be achieved by

placing the magnets, directly behind the cathode, in a configuration such that at

least one region in front of the cathode surface has a closed path which is

perpendicular to the magnetic field lines that are parallel to the surface.

Although there are many variations in geometry, all have in common a closed

path or region in front of a mostly flat cathode surface, where the magnetic field

is normal to the electric field. Bounding this region the magnetic field lines

enter the cathode surface. Ideally, at the entry points the field lines are

perpendicular to the cathode surface. The ionization region is thus confined to

an area adjacent to the cathode surface by one or more endless toroidal electron

trapping regions, bounded by a tunnel shaped magnetic field.

3.1.2 Magnetron sputtering

When an ion impinges on the cathode surface several processes may take

place: the incident ion will either be implanted or reflected, probably as a

neutral and with a large loss of energy; a target atom may be ejected (sputtered);

the ion impact and the resulting cascade will cause an amount of structural

reordering in the surface layers and secondary electrons may be ejected. Of the

mentioned processes the most important for the thin film formation is the

sputtering process.

The sputtering yield, S, is defined as the number of atoms ejected from

the target per incident particle. Important factors that affect the sputtering yield

include surface structure, mass of the bombarding ion and incident energy. The

sputtering process is rather insensitive to temperature, and in certain cases, there

is even a decrease in S with increasing target temperature [104]. Near the

threshold energy, the sputtering yield rises rapidly with ion energy increase.

From about 100 eV upwards, the sputtering yield increases almost linearly with

ion energy, where the values begin to be large enough to be useful for film

deposition. With further increase in ion energy the sputtering yield reaches a

maximum and starts to decrease, mainly due to the larger penetration depth of

the ions within the target surface. For light ions, such as hydrogen or helium, the

maximum of S is reached at a few thousand eV, since this particle penetrate

rather easily. For heavy ions, such as xenon or mercury, the maximum may not

be attained until values near 50 KeV or higher. The sputtering yield dependence

on the atomic number of the target atoms for various inert gas ions shows some

kind of periodicity.

A sputtering yield increase is observed, for instance, when the target

material changes from group III-B to group I-B transition metals, but a steep

decrease follows from a metal of group I-B to the metal of group III-B of the

next period (row of the periodic table). This can be qualitatively understood

looking at the electronic structure of transition metals. As the d-orbitals are

filled by electrons, collisions among atoms within the solid become more

elastic, this results in more efficient energy transfer. On the other hand the bond

energies between the atoms become weaker and thus the removal of the atoms is

easier. Neutral gas atoms or molecules may also initiate target material

sputtering, although in plasma-solid interactions these processes have lesser

importance due to the extremely small momentum of the neutral particles.

3.1.3 Growth kinetics of thin film

In sputtering deposition as in other standard vacuum deposition

processes, the material arrives at the substrate mostly in an atomic or molecular

form. Using the kinetic theory of gases, it is possible to estimate the frequency

with which gas particles impinge on a surface, υ, when the gas phase pressure is

P

…. (3.1)

In equation 3.1, m is the mass of the gas particles, K the Boltzmann constant

and T the temperature. Considering that for air the mean particle mass is 4.8 x

10-23

g, collision frequency will be approximately 3 x 1024

cm-2

s-1

at 25 0C and

1 atm. Given that a perfectly smooth surface of 1 cm3 has about 10

15 atoms,

when immersed in a gas at 1 atm pressure, each atom on the surface will be hit

about 109 times each second. At 5 x 10

-3 mbar, collision frequency will be

reduced to 1.5 x 1019

cm-2

s-1

which is still a very high frequency. In the case of

sputtering the particles that will interest to the film growth will be the ones

evaporated from the target and these will have a much lower collision

frequency.

The condensed particles may diffuse around the surface, with a motion

determined by their binding energy to the substrate, may be incorporated into

the lattice or evaporate. Given the high collision frequency of the gas particles

inside the deposition chamber these will have a non-negligible influence on the

adsorbed particles diffusion. The diffusion process may lead to adsorption,

particularly at special site edges or other defects, or the diffusing particle may

desorb. During these processes, characteristic activation energies have to be

overcome. The corresponding activation energies for adsorption or diffusion

depend on the atomic details of each process. Besides adsorption and surface

diffusion, nucleation of more than one adsorbed particle might occur.

Interdiffusion of adsorbed particles with the substrate often happen leading to

film-substrate interface smoothing. In thermodynamic equilibrium all processes

take place in two opposite directions at equal rates. Therefore, in equilibrium no

net film growth would be observed, so that layer growth must be a non-

equilibrium kinetic process.

The final macroscopic state of the system may not be the most stable one,

since it is kinetically determined. In general however, certain parts of the overall

process may be kinetically forbidden, whereas others may be in local

thermodynamic equilibrium. In this case equilibrium arguments may be applied

locally even though the whole growth process is a non-equilibrium one. Given

this fact, a global theory of film growth requires a description in terms of rate

equations for each of the processes taking place at the surface. Instead of

following a more theoretical atomistic approach it is possible to consider the

film growth mechanism using a more phenomenological perspective.

Usually, three distinct modes of film growth may be considered: layer by

layer growth mode or Frank-Vander Merve mode; island growth mode or

Vollmer-Weber mode and layer-plus island growth mode, that is also called

Stransky-Krastanov mode; each named after the -investigators associated with

their initial description. In layer by layer growth mode the interaction between

the substrate and the layer atoms is stronger than between neighbouring atoms.

Each new layer starts to grow, only when the last one is completed. If, on the

contrary, the interaction between neighboring atoms is stronger than the

overlayer-substrate interaction, the particles will rather form aggregates over the

surface that grow in size and eventually coalesce during film growth. This

makes up the island growth mode. The layer-plus-island growth mode is an

intermediate case where the film starts to grow layer by layer in a first stage and

only afterwards begins the formation of island agglomerates. In island growth

mode, each island is usually a single crystal or contains just a few crystals.

On a polycrystalline or amorphous substrate, the orientation of each

island will be random and the resultant film will be polycrystalline. On single

crystal substrates, the islands orientations may be constrained to a given

direction by the substrate structure, so that growth and coalescence leads to the

formation of a single crystal film. This case is usually known as epitaxy. If

surface atoms have high mobility, they have greater opportunity of finding low

energy positions consistent with crystal growth. Knowing that mobility is

increased by surface temperature, it is expected that higher substrate

temperature will promote crystal growth. The same effect can be achieved by

reducing the deposition rate, which gives more time to the adsorbed species to

find an energetically favourable lattice position. Epitaxial growth was also

found to be promoted by electron or ion bombardment and increased energy of

deposited atoms [100].

The environmental conditions around the substrate during magnetron

sputtering deposition deserve also to be mentioned, since they will inevitably

affect the film structure obtained. Namely, it was ignored in the above

description the effect of the energy of the impinging sputtered atoms, as well as

the effect of many other particles that may impact on the surface. In first place

there might be contaminants arriving at the substrate. These may result from an

internal source, such as outgassing from a heating substrate, or by an external

source such as the sputtering gas. If the contaminant atoms are chemically

active, the contamination will be particularly effective and can only be

minimized by reducing the contaminant partial pressure. If the contaminant

source is outgassing, the system may have to be evacuated to a higher vacuum

or the sources of outgassing heated in order to reduce the outgassing rate and

guarantee a sufficiently low partial pressure of the contaminant during

deposition. On the other hand, if the contaminant comes with the sputtering gas

there is no way to reduce its partial pressure without affecting simultaneously

the sputtering gas pressure. Thus it is very important to use high purity

sputtering gases.

The sputtering gas atoms might also become part of the deposited film.

Although the sputtering gas used is usually an inert gas, given its high partial

pressure when compared with that of the sputtered atoms it is not surprising that

some of these atoms happen to be trapped in the growing film. Inert gas atoms,

are only expected to be physisorbed, so if substrate temperature is increased

during deposition, they are less likely to be adsorbed and at the same time more

likely to be subsequently desorbed. However, there exists an appreciable

difference between the interaction of fast gas particles and that of thermal

neutrals with the surface. Energetic neutrals may result from the reflection and

neutralization of ions that impinge on the target. These neutrals, arriving at the

substrate, have much higher probability to be embedded in the growing film

than thermal neutrals. In fact, it has been observed that nickel films deposited by

sputtering in an argon atmosphere have higher argon content than nickel films

produced in a similar atmosphere, but using evaporation [105]. Positive ions

may also impact on the substrate due to the sheath voltage drop near its surface.

Negative ions and electrons can only reach the substrate if they have

enough energy to cross the space-charge sheath. Once again it is necessary to

distinguish fast from slow particles. Fast negative ions and electrons are

produced in the target and accelerated in the target sheath. These fast particles

can have a major influence on the structure and properties of the growing film

and also cause substrate heating. Finally there are also photons arriving at the

substrate. Photons can be produced by ion or electron bombardment on any

surface or result from relaxation of excited atoms in the glow. In the former case

the photons may have high energy, at most as high as the energy of the

bombarding particle. Such energies may be, in a sputtering system, of the same

order of the accelerating potential at the target, which is usually higher than 200

eV. The main effect of photon bombardment of the substrate will be electron

emission, which may also affect the film growth processes occurring at the

substrate.

3.1.4 Vacuum coating unit

A vacuum coating unit consists of vacuum chamber, rotary pump,

diffusion pump and penning guage.

(a) Rotary oil pump

This pump is most commonly used to produce moderately low pressures

of the order of 10-3

torr. It is widely used in laboratories in vacuum coating

units. The working is based on the principle of displacement of a gas. A portion

of the gas inside a chamber is isolated by a rotating disc and is then compressed

till it attains a sufficiently high pressure and gets discharged to the atmosphere.

As a gas consists of fast moving molecules, the residual gas comes and occupies

the whole space and in turn, also gets removed. This process continues and the

pressure of the chamber keeps on decreasing. The rotary pump consists of a

solid cylinder R called Rotor rotating eccentrically inside a hollow cylinder S

called Stator (Figure 3.1). The stator has an inlet I connected to the chamber to

be evacuated and an outlet O which always open outward only. The rotor is

rotated by an electric motor. The space inside the stator S is divided into two

air-tight compartments by means of a vane V between the inlet and the outlet.

The vane is always tightly pressed against the rotor by means of a spring P. To

avoid any leakage, the whole pump is kept immersed in a special of oil called

vacuum oil or hydrocarbon oil.

The working of the rotary pump can be explained by analyzing, four

successive positions of the rotor R as shown in Figure 3.1. In the first position

Figure 3.1(a) the air or gas from the chamber just enters into the space in

between the cylinder and the rotor R. As it rotates, the space inside the stator is

divided into two parts as in Figure 3.1(b). As the rotor rotates further the

volume of the outlet portion decreases and hence air inside these is further

compressed. Figure 3.1(c) shows the final stage of compression which opens the

valve at the outlet O and the gas is expelled out. Meanwhile, the inlet volume

increases, and hence air enters from the chamber. Thus during each rotation of

the cylinder the air is drawn from the chamber and is removed from it

continuously. Within a few minutes, the pump can produce a low pressure of 10-

3 torr in the chamber.

Fig 3.1(a) Fig 3.1(b) Fig 3.1(c)

Fig 3.1 Schematic diagram of rotary pump

(b) Diffusion Pump

These pumps can produce very low pressure down to 10-7

torr. It cannot

operate directly from the atmospheric pressure but requires force vacuum or

backing vacuum from 10-1

to 10-2

torr. The basic principle used in these pumps

is that in a mixture of gases, the diffusion of gas occurs from a region where its

partial pressure is higher to that where it is lower, irrespective of the total

pressure in the two regions. The diffusion pump is made up of stainless steel

and it consists of a cylindrical body having a boiler at the base with pumping

fluid (Silicone DC 704). A number of nozzle (combination of nozzles and jet)

assembles are arranged inside the cylinder. Various nozzles originate at

different parts of the boiler and are designed such that the pressure rise across

the various jet streams increases progressively as one goes from the vacuum

chamber side to the backing pump side. The temperature of the walls is usually

maintained few degrees below room temperature by water cooled coils.

Figure 3.2 shows the schematic diagram of a diffusion pump. The vapour

from the boiling fluid passes up the cylindrical chimney. It jets out through the

nozzle with high speed being directed at an angle downwards. Molecules of gas

which wander are diffused from the chamber towards the jet stream and will be

struck by vapour molecules of the heated pumping fluid. The molecules are

given a downward motion into the dense part of the vapour jet. The net effect is

to compress the gas to the point where they can be removed by a backing pump.

The compression is carried out by stages by using several nozzles in line. As

each stage handle the same number of molecules, the pressure decreases from

the top most nozzle in the last, the annular spacing between the nozzle and the

pump casing increases in the same direction. The pump fluid must be removed

to the boiler after it has performed its function and hence the water cooling of

the pump casing.

Figure 3.2 Schematic diagram of a diffusion pump

(c) Penning gauge

The penning gauge is widely used for Ultra High Vacuum (UHV)

pressure measurements. In the penning gauge, the filament has dispensed width.

It consists of three electrodes kept inside a bulb and are connected to air tight

seal. This bulb is placed inside a permanent magnet. The opening of the bulb is

connected to the vessel whose pressure is to be measured. The electric field

between the electrodes is verified due to the short distance of separation

between them. Here the electrons are produced by field emission. The magnetic

field is applied perpendicular to the electric field. So the electrons travel in an

elliptical path. These electrons produce more ionization due to their collision

with the gas molecules. The ion current is registered by the milli ammeter in the

external electric circuit. This ion current is the measure of the pressure.

Figure 3.3 Schematic diagram of penning gauge

Figure 3.3 shows the schematic diagram of the penning gauge. The

electrons are generated at the cathode and are accelerated towards the anode.

They pass through the anode ring and are then repelled by the other cathode and

they oscillate back and forth eventually winding up the anode ring. This process

will lead to ionization by collision. These ions impinge upon the cathode

releasing electrons which will maintain the discharge. The effective path of the

electron increases since they take a helical path due to the magnetic field. The

long paths of the electrons, result in the production of many more ions than if

they were able to go directly from the cathode to anode. The positive ions thus

produced are collected by the cathode and the ion current is measured. The ion

current is proportional to the gas pressure in the chamber. The micro ammeter is

calibrated in terms of pressure.

(d) Vacuum chamber

The schematic of the vacuum coating unit is shown in Figure 3.4. The

chamber is pumped to vacuum using a diffusion pump which is able to reduce

the pressure below 10-3

Pa. If the chamber is kept open during a long time

period it will take longer to reach high vacuum pressures because of outgassing.

Heating the chamber walls often accelerates evacuation, by promoting

outgassing. The diffusion pump is backed by a rotary pump that is also used to

do primary vacuum inside the chamber before connecting the diffusion pump.

In order to change the substrates the chamber has to be open, so that usually the

frequency of experiments is dependent on the time required to reach high

pressures inside the chamber. The sputtering gases are fed into the chamber

using two mass flow meters. One is calibrated for argon, other for oxygen. The

maximum flow reached by each mass flow meter is 200 sccm for the argon

device, and 20 sccm for N2. The control and measurement is performed using a

power supply and readout system that permits to set the flow of the mass flow

controller. The substrates are positioned on a stainless steel holder, with suitable

holes that work as masks for the deposition.

Figure 3.4 Schematic diagram of vacuum coating unit

The substrate holder can be fixed at different distances above the target and is

maintained at the same potential as the anode and the chamber walls. It is

possible to insulate the substrate holder electrically in order to apply a voltage

bias to control the charged particles that impinge on the substrates. The

substrate heating is performed using a metallic base weaved with an electrical

resistance. The temperature is measured with a thermocouple inserted into a

hole made in one side of the metallic base and controlled by a temperature

controller.

The setup used to heat and measure the substrate temperature does not

make possible to know the actual substrate temperature, because there is an

unknown temperature gradient between the substrate surface and the point

where the thermocouple is placed. However, if given enough time for the heat

transfer fluxes to stabilize, the temperature conditions can be reasonably

reproduced in repeated experiments and the effect of temperature upon the

films‘ characteristics may be studied. Nevertheless, if the substrates pressure or

the sputtering power is changed, for instance, it is not possible to maintain a

fixed temperature on the substrate over repeated experiments, since the

temperature gradient is very likely to change.

Figure 3.5 shows the photograph of the dc/rf magnetron sputtering unit.

The dc power source is able to operate at a maximum voltage of 500V and to

deliver a maximum current of 5A. The user may set the voltage, the current and

the power between 5% and 100% of respective range. The push on button

arrangement with an LED display provision on the unit was used both to set the

source parameters and read the voltage and current values during operation. Rf

frequency of 13.56 Hz is the maximum frequency prescribed for the unit. Peak

to peak voltage greater than 1000V, current densities of 1 mA/cm2,

discharge pressure of 0.5 to 10 m torr are the typical value range for rf

magnetron sputtering. Pressure inside the chamber is measured using a Pirani

gauge in the pressure range from 105

Pa to 10-1

Pa and using a Penning gauge

in ranges from 10-1

Pa to 10-5

Pa. The Penning gauge is cleaned periodically to

ensure repeatability. If the electrodes of this device are covered by an oxide

layer the measurements stray toward lower pressures.

Figure 3.5 Photograph of the dc/rf sputtering unit

3.2 ANALATICAL INSTUREMENTS AND TECHNIQUE [106]

3.2.1 Rutherford backscattering spectrometry (RBS)

Rutherford Backscattering Spectrometry (RBS) is a widely used nuclear

method for the near surface layer analysis of solids. A target is bombarded with

ions at an energy in the MeV-range (typically 0.5–4 MeV), and the energy of

the backscattered projectiles is recorded with an energy sensitive detector,

typically a solid state detector. RBS allows the quantitative determination of the

composition of a material and depth profiling of individual elements. RBS is

quantitative without the need for reference samples, nondestructive, has a good

depth resolution of the order of several nm, and a very good sensitivity for

heavy elements of the order of parts-per-million (ppm). The analyzed depth is

typically about 2 μm for incident He-ions and about 20 μm for incident protons.

An RBS instrument generally includes three essential components:

An ion source, usually alpha particles (He2+

ions) or, less commonly,

protons.

A linear particle accelerator capable of accelerating incident ions to high

energies, usually in the range 1-3 MeV.

A detector capable of measuring the energies of backscattered ions over

some range of angles.

Figure 3.6 shows the photograph of the RBS instrument used to take the

RBS spectra. Tandem accelerator starts with a source of He- ions and position

the positive terminal at the center of the acceleration tube. A stripper element

included in the positive terminal removes electrons from ions which pass

through, converting He- ions to He

++ ions. The ions thus start out being attracted

to the terminal, pass through and become positive, and are repelled until they

exit the tube at ground. A typical tandem accelerator with an applied voltage of

750 kV can achieve ion energies of over 2 MeV.

Figure 3.6 Photograph of Rutherford backscattering spectrometer

Ions which reach the detector lose some of their energy to inelastic

scattering from the electrons, and some of these electrons gain enough energy to

overcome the band gap between the semiconductor valence and conduction

bands. This means that each ion incident on the detector will produce some

number of electron-hole pairs which is dependent on the energy of the ion. The

energy loss of a backscattered ion is dependent on two processes: the energy

lost in scattering events with sample nuclei, and the energy lost to small-angle

scattering from the sample electrons. The first process is dependent on the

scattering cross-section of the nucleus and thus on its mass and atomic number.

For a given measurement angle, nuclei of two different elements will therefore

scatter incident ions to different degrees and with different energies, producing

separate peaks on an N(E) plot of measurement count versus energy. These

peaks are characteristic of the elements contained in the material, providing a

means of analyzing the composition of a sample by matching scattered energies

to known scattering cross-sections. Relative concentrations can be determined

by measuring the heights of the peaks.

3.2.2 X-ray Powder Diffraction

X-ray powder diffraction (XRD) is a rapid analytical technique primarily

used for phase identification of a crystalline material and can provide

information on unit cell dimensions. The analyzed material is finely ground,

homogenized, and average bulk composition is determined. Max von Laue, in

1912, discovered that crystalline substances act as three-dimensional diffraction

gratings for X-ray wavelengths similar to the spacing of planes in a crystal

lattice. X-ray diffraction is now a common technique for the study of crystal

structures and atomic spacing. X-ray diffraction is based on constructive

interference of monochromatic X-rays and a crystalline sample. These X-rays

are generated by a cathode ray tube, filtered to produce monochromatic

radiation, collimated to concentrate and directed towards the sample. The

interaction of the incident rays with the sample produces constructive

interference (and a diffracted ray) when conditions satisfy Bragg's Law (nλ= 2d

sinθ) (figure 3.7). This law relates the wavelength of electromagnetic radiation

to the diffraction angle and the lattice spacing in a crystalline sample. These

diffracted X-rays are then detected, processed and counted. By scanning the

sample through a range of 2θ angles, all possible diffraction directions of the

lattice should be attained due to the random orientation of the powdered

material. Conversion of the diffraction peaks to d-spacings allows identification

of the mineral because each mineral has a set of unique d-spacings. Typically,

this is achieved by comparison of d-spacings with standard reference patterns.

Figure 3.7 The geometry of the incident X-rays impinging the

sample satisfies the Bragg Equation

X-ray diffractometer consist of three basic elements: an X-ray tube, a

sample holder, and an X-ray detector. X-ray are generated in a cathode ray tube

by heating a filament to produce electrons, accelerating the electrons toward a

target by applying a voltage, and bombarding the target material with electrons.

When electrons have sufficient energy to dislodge inner shell electrons of the

target material, characteristic X-ray spectra are produced. These spectra consist

of several components, the most common being Kα and Kβ. Kα consists, in part

of Kα1 and Kα2. Kα1 has a slightly shorter wavelength and twice the intensity as

Kα2. The specific wavelengths are characteristic of the target material (Cu, Fe,

Mo, Cr). Filtering, by foils or crystal monochrometers, is required to produce

monochromatic X-rays needed for diffraction. Kα1and Kα2 are sufficiently close

in wavelength such that a weighted average of the two is used. Copper is the

most common target material for single-crystal diffraction, with CuKα radiation

= 1.5418 Å. These X-rays are collimated and directed onto the sample. As the

sample and detector are rotated, the intensity of the reflected X-rays is recorded.

When the geometry of the incident X-rays impinging the sample satisfies the

Bragg equation, constructive interference occurs and a peak in intensity occurs.

A detector records and processes this X-ray signal and converts the signal to a

count rate which is then out putted to a device such as a printer or computer

monitor.

The geometry of an X-ray diffractometer is such that the sample rotates in

the path of the collimated X-ray beam at an angle θ while the X-ray detector is

mounted on an arm to collect the diffracted X-rays and rotates at an angle of 2θ.

The instrument used to maintain the angle and rotate the sample is termed a

goniometer. For typical powder patterns, data is collected at 2θ from ~5° to 70°,

angles that are preset in the X-ray scan. Figure 3.8 shows the schematic of the

detection of diffracted X-rays by diffractometer. The photograph of the XRD

unit used for recording the XRD patterns is shown in Figure 3.9.

Figure 3.8 Schematic of the detection of diffracted X-rays by

diffractometer

Figure 3.9 Photograph of X-ray diffraction unit

3.2.3 Scanning electron microscopy

The scanning electron microscope (SEM) uses a focused beam of high-

energy electrons to generate a variety of signals at the surface of solid

specimens. The signals that derive from electron-sample interactions reveal

information about the sample including external morphology (texture), chemical

composition, and crystalline structure and orientation of materials making up

the sample. In most applications, data are collected over a selected area of the

surface of the sample, and a 2-dimensional image is generated that displays

spatial variations in these properties. Areas ranging from approximately 1 cm to

5 microns in width can be imaged in a scanning mode using conventional SEM

techniques (magnification ranging from 20X to approximately 30,000X, spatial

resolution of 50 to 100 nm).

Accelerated electrons in an SEM carry significant amounts of kinetic

energy, and this energy is dissipated as a variety of signals produced by

electron-sample interactions when the incident electrons are decelerated in the

solid sample. These signals include secondary electrons (that produce SEM

images), backscattered electrons (BSE), diffracted backscattered electrons

(EBSD that are used to determine crystal structures and orientations of

minerals), photons (characteristic X-rays that are used for elemental analysis

and continuum X-rays), visible light (cathode luminescence--CL), and heat.

Secondary electrons and backscattered electrons are commonly used for

imaging samples: secondary electrons are most valuable for showing

morphology and topography on samples and backscattered electrons are most

valuable for illustrating contrasts in composition in multiphase samples (i.e. for

rapid phase discrimination). X-ray generation is produced by inelastic collisions

of the incident electrons with electrons in discrete orbital‘s (shells) of atoms in

the sample. As the excited electrons return to lower energy states, they yield X-

rays that are of a fixed wavelength (that is related to the difference in energy

levels of electrons in different shells for a given element). Thus, characteristic

X-rays are produced for each element in a mineral that is "excited" by the

electron beam. SEM analysis is considered to be "non-destructive"; that is, x-

rays generated by electron interactions do not lead to volume loss of the sample,

so it is possible to analyze the same materials repeatedly.

Essential components of all SEM include the following:

Electron Source ("Gun")

Electron Lenses

Sample Stage

Detectors for all signals of interest

Display / Data output devices

Infrastructure Requirements

Power Supply

Vacuum System

Cooling system

Vibration-free floor

Room free of ambient magnetic and electric fields

In a typical SEM, an electron beam is thermionically emitted from an

electron gun fitted with a tungsten filament cathode. Tungsten is normally used

in thermionic electron guns because it has the highest melting point and lowest

vapour pressure of all metals, thereby allowing it to be heated for electron

emission, and because of its low cost. The electron beam, which typically has an

energy ranging from 0.2 keV to 40 keV, is focused by one or two condenser

lenses to a spot about 0.4 nm to 5 nm in diameter. The beam passes through

pairs of scanning coils or pairs of deflector plates in the electron column,

typically in the final lens, which deflect the beam in the x and y axes so that it

scans in a raster fashion over a rectangular area of the sample surface. When the

primary electron beam interacts with the sample, the electrons lose energy by

repeated random scattering and absorption within a teardrop-shaped volume of

the specimen known as the interaction volume, which extends from less than

100 nm to around 5 µm into the surface.

The size of the interaction volume depends on the electron's landing

energy, the atomic number of the specimen and the specimen's density. The

energy exchange between the electron beam and the sample results in the

reflection of high-energy electrons by elastic scattering, emission of secondary

electrons by inelastic scattering and the emission of electromagnetic radiation,

each of which can be detected by specialized detectors. The beam current

absorbed by the specimen can also be detected and used to create images of the

distribution of specimen current. Electronic amplifiers of various types are used

to amplify the signals, which are displayed as variations in brightness on a

cathode ray tube. The raster scanning of the CRT display is synchronized with

that of the beam on the specimen in the microscope, and the resulting image is

therefore a distribution map of the intensity of the signal being emitted from the

scanned area of the specimen. The image may be captured by photography from

a high-resolution cathode ray tube, but in modern machines is digitally captured

and displayed on a computer monitor and saved to a computer's hard disk.

Figure 3.11 shows the photograph of the Scanning electron microscopy used to

take SEM image.

Figure 3.10 Schematic diagram of the scanning electron microscope.

Figure 3.11 Photograph of scanning electron microscope

3.2.4 Fourier Transform Infrared spectrophotometer

FT-IR stands for Fourier Transform Infrared, the preferred method of

infrared spectroscopy. In infrared spectroscopy, IR radiation is passed through a

sample. Some of the infrared radiation is absorbed by the sample and some of it

is passed through (transmitted). The resulting spectrum represents the molecular

absorption and transmission, creating a molecular fingerprint of the sample.

Like a fingerprint no two unique molecular structures produce the same infrared

spectrum. This makes infrared spectroscopy useful for several types of analysis.

The interferometer produces a unique type of signal which has all of the infrared

frequencies ―encoded‖ into it. The signal can be measured very quickly, usually

on the order of one second or so. Thus, the time element per sample is reduced

to a matter of a few seconds rather than several minutes. Figure 3.12 and Figure

3.13 shows the schematic diagram and photograph of FTIR-interferometer

system used to take the FTIR spectra.

Most interferometers employ a beamsplitter which takes the incoming

infrared beam and divides it into two optical beams. One beam reflects off of a

flat mirror which is fixed in place. The other beam reflects off of a flat mirror

which is on a mechanism which allows this mirror to move a very short distance

(typically a few millimeters) away from the beam splitter.

Figure 3.12 Schematic diagram of FTIR-spectrometer system

Figure 3.13 Photograph of Fourier transform infrared spectrometer

The two beams reflect from their respective mirrors and are recombined

when they meet back at the beamsplitter. Because the path that one beam travels

is a fixed length and the other is constantly changing as its mirror moves, the

signal which exits the interferometer is the result of these two beams

―interfering‖ with each other. The resulting signal is called an interferogram

which has the unique property that every data point (a function of the moving

mirror position) which makes up the signal has information about every infrared

frequency which comes from the source. This means that as the interferogram is

measured; all frequencies are being measured simultaneously. Thus, the use of

the interferometer results in extremely fast measurements.

Because the analyst requires a frequency spectrum (a plot of the intensity

at each individual frequency) in order to make identification, the measured

interferogram signal cannot be interpreted directly. A means of ―decoding‖ the

individual frequencies is required. This can be accomplished via a well-known

mathematical technique called the Fourier transformation. This transformation

is performed by the computer which then presents the user with the desired

spectral information for analysis. The normal instrumental process is as follows:

Source: Infrared energy is emitted from a glowing black-body source.

This beam passes through an aperture which controls the amount of

energy presented to the sample (and, ultimately, to the detector).

Interferometer: The beam enters the interferometer where the ―spectral

encoding‖ takes place. The resulting interferogram signal then exits the

interferometer.

Sample: The beam enters the sample compartment where it is transmitted

through or reflected off from the surface of the sample, depending on the

type of analysis being accomplished. This is where specific frequencies

of energy, which are uniquely characteristic of the sample, are absorbed.

Detector: The beam finally passes to the detector for final measurement.

The detectors used are specially designed to measure the special

interferogram signal.

Computer: The measured signal is digitized and sent to the computer

where the Fourier transformation takes place. The final infrared spectrum

is then presented to the user for interpretation and any further

manipulation.

Because there needs to be a relative scale for the absorption intensity, a

background spectrum must also be measured. This is normally a

measurement with no sample in the beam. This can be compared to the

measurement with the sample in the beam to determine the ―percent

transmittance.‖ This technique results in a spectrum which has all of the

instrumental characteristics removed. Thus, all spectral features which are

present are strictly due to the sample. A single background measurement

can be used for many sample measurements because this spectrum is

characteristic of the instrument itself.

3.2.5 Ultraviolet - Visible spectrophotometer

Molecules containing π-electrons or non-bonding electrons (n-electrons)

can absorb the energy in the form of ultraviolet or visible light to excite these

electrons to higher anti-bonding molecular orbitals [106] . The more easily

excited the electrons (i.e. lower energy gap between the HOMO and the

LUMO) the higher the wavelength of light it can absorb. The instrument used in

ultraviolet-visible spectroscopy is called a UV/Vis spectrophotometer. The UV-

visible spectrophotometer is configured to measure reflectance. In this case, the

spectrophotometer measures the intensity of light reflected from a sample (I),

and compares it to the intensity of light reflected from a reference material (Io).

The ratio I / Io is called the reflectance, and is usually expressed as a percentage

(%R). The basic parts of a spectrophotometer are a light source, a holder for the

sample, a diffraction grating in a monochromator or a prism to separate the

different wavelengths of light, and a detector. The radiation source is often a

Tungsten filament (300-2500 nm), a deuterium arc lamp, which is continuous

over the ultraviolet region (190-400 nm), Xenon arc lamps, which is continuous

from 160-2,000 nm; or more recently, light emitting diodes (LED) [100] for the

visible wavelengths. The detector is typically a photomultiplier tube, a

photodiode, a photodiode array or a charge-coupled device (CCD).

Single photodiode detectors and photomultiplier tubes are used with

scanning monochromators, which filter the light so that only light of a single

wavelength reaches the detector at one time. The scanning monochromator

moves the diffraction grating to ―step-through‖ each wavelength so that its

intensity may be measured as a function of wavelength. Fixed monochromators

are used with CCDs and photodiode arrays. As both of these devices consist of

many detectors grouped into one or two dimensional arrays, they are able to

collect light of different wavelengths on different pixels or groups of pixels

simultaneously. In a double-beam instrument, the light is split into two beams

before it reaches the sample. One beam is used as the reference; the other beam

passes through the sample. The reference beam intensity is taken as 100%

Transmission (or zero Absorbance), and the measurement displayed is the ratio

of the two beam intensities. Some double-beam instruments have two detectors

(photodiodes), and the sample and reference beam are measured at the same

time. In other instruments, the two beams pass through a beam chopper, which

locks one beam at a time. Figure 3.14 shows the schematic diagram of UV-

visible spectrometer.

Figure 3.14 Schematic diagram of UV-VIS spectrophotometer

The detector alternates measuring between the sample beam and the

reference beam in synchronism with the chopper. There may also be one or

more dark intervals in the chopper cycle. In this case, the measured beam

intensities may be corrected by subtracting the intensity measured in the dark

interval before the ratio is taken. Samples are typically placed in a transparent

cell, known as a cuvette. Cuvettes are typically rectangular in shape, commonly

with an internal width of 1 cm. (This width becomes the path length, L, in the

Beer-Lambert law.) Test tubes can also be used as cuvettes in some instruments.

The type of sample container used must allow radiation to pass over the spectral

region of interest. The most widely applicable cuvettes are made of high quality

fused silica or quartz glass because these are transparent throughout the UV,

visible and near infrared regions. Glass and plastic cuvettes are also common,

although glass and most plastics absorb in the UV, which limits their usefulness

to visible wavelengths. Figure 3.15 shows the photograph of UV-Visible

spectrometer used to take the transmission and absorbance spectra.

Figure 3.15 Photograph of UV-Visible spectrophotometer

3.2.6 LCR Meter

Impedance measurements are a basic means of evaluating electronic

components and materials. Every material has a unique set of electrical

characteristics that are dependent on its dielectric or insulation properties.

Accurate measurements of these properties can provide valuable information to

ensure an intended application or maintain a proper manufacturing process.

When making these measurements, connection of the material to the measuring

instrument (an LCR Meter) is one of the major challenges faced, special fixtures

are generally required depending on the material type. The most common piece

of test equipment for holding a variety of solid materials is the LD-3, a liquid

tight, three terminal connection cell with electrode spacing adjustable by a

precision micrometer shown in Figure 3.16.

Figure 3.16 Photograph of Precision LCR Meter

Specimen Air

Cxm and Dxm Ca and Da

(a) (a) (b)

Figure 3.17 Schematic diagram of LCR measurement set up (a) with

specimen (b) without specimen

First the sample is inserted in the cell and the electrodes closed with the

micrometer until they just touch the sample lightly, Fig 3.17 (a). The

micrometer spacing hm is then recorded for this specimen. The capacitance

value, Cxm and dissipation factor, Dxm are then measured with the LCR meter.

The specimen is then removed from the cell, micrometer readjusted to and the

measurements repeated in air as Ca and Da, Figure 3.17(b).