INDEX

1.Introduction

1.1Thinfilm

1.2La2O3

2.Experimental Techniques

2.1Vacuum Techniques

2.2Deposition Techniques

2.3Film Growth

2.4Uniformity and Film Depostion Rate

2.5Thickness Measurement Techniques

3.Preparation and Characterisation Of La2O3 Thinfilm

3.1Preparation

3.1.1 Vacuum Coating Unit

3.1.2 High Pressure Pirani Guage

3.1.3 Thinfilm Deposition

3.2Characterisation

3.2.1 Measurement Of Thickness (Tolansky's)

3.2.2 Study of Optical Properties

3.2.3 X-ray Diffraction

4.Summary

1 . INTRODUCTION

1 .1THIN FILMS :

A thin film as the name implies, is a layer with a high surface to volume ratio.

All the basic researches on thin films are generally confined to a limited range of thickness, say

between few Angstroms to about 5000 Å depending on the properties to be investigated, where

as for technological applications where stability of performance is the most important criterion,

the thickness limit at the lower range is generally higher than 1000 Å and can be as high as 5-

10 μm or even more. In order to cover this wide range of film thickness both researches and

applications , a “thin film” may be arbitrarily defined as a solid layer having a thickness

varying form a few Å to about 10 μm or so. Since the thickness limitation is rather arbitrary,

even somewhat thicker films may also come within the scope of the above definition.

Thin films are normally subdivided as

Ultra thin

Thin

Comparatively thicker

a) Ultra thin films: These films having thickness ranging from a few to about 50-100 Å.

b) Thin films: These films have range from 100 Å to 1000 Å

c) Comparatively thick films: these films having thickness range greater than 1000 Å. They

have a great practical importance.

Thin films as a two dimensional system are of great importance to many real

world problems. Their material costs are very low as compared to the corresponding bulk

materials and they perform the function when it comes to surface processes. Thus, knowledge

and determination of the nature, function and new properties of thin films can be used for the

development of new technologies like solar cell, sensors, optical applications, electronic

engineering, ferroelectric etc.

Thin films are extensively used in wafer fabrication and can be a resistor, a

conductor, an insulator, or even a semiconductor. Electronic semiconductor and optical devices

are the main applications benefiting from the thin film construction. A familiar application of

thin films is the household mirrors which typically has a thin metal coating on the back of a

sheet of glass to form a refractive interface. Thin films behave differently from bulk materials

of the same chemical composition in several ways for instance; thin films are sensitive to

surface properties while bulk materials generally aren’t. Thin films are also relatively more

sensitive to thermo-mechanical stresses.

A transition from the bulk to the thin film state may even cause a drastic change

in its properties as illustrated by the behavior of alkali metals and also noble metals . Thus

highly conducting sodium potassium, rubidium and also gold platinum etc having positive

temperature coefficient of resistance (TCR) in the bulk form show negative TCR when in thin

film states thus behaving as semiconducting films. Bulk bismuth and antimony which are

metallic in nature behave as semiconductors in the thin film state. Buckle and Hilsh observed

that thin film unlike bulk showed super conducting properties at low temperatures. Highly

disordered or amorphous films have electrical or magnetic properties which may differ by

several orders from that of the bulk single crystals.

All films whether prepared by vacuum deposition or by other techniques are

invariably associated with some growth defects or imperfections such as lattice defects,

stacking faults , twinning , disorders in atomic arrangement , dislocation grain boundaries

foreign atom inclusion etc. Surface states of a film also play a dominant role in modifying

electrical and other properties. In addition because of a high surface to volume ratio in a film, a

freshly formed film surface becomes highly reactive. Further because of the unbalancing of

forces near the surface region, new phenomenon such as thermionic emission, absorption of

gases, catalysis, solid state reactions etc characteristic of a surface are more often observed in

thin films rather than in bulk.

Thin film coatings provide enhanced optical performance on items ranging from

camera lenses to sunglasses. Architectural glass is often coated to reduce the heat load in large

office buildings and provide significant cost savings by reducing air conditioning requirements.

Thin films can be prepared from a variety materials such as metals,

semiconductors, insulators or dielectrics etc and for this purpose these are two deposition

techniques

Physical vapor deposition

Chemical vapor deposition.

1.2 INTRODUCTION TO LANTHANUM OXIDE :

The rare earth elements consist of Group 3 metals (21Sc, 39Y, and 57La) and

lanthanides (58Ce→71Lu). The rare earths (RE) and particularly the lanthanides (Ln) form,

chemically and physically, a more or less homogeneous group, the largest in the periodic table.

This is a consequence of the lanthanide contraction, which refers to the gradual

decrease in the ionic radius of the elements from lanthanum to lutetium by 17%. Owing to the

lanthanide contraction, trivalent yttrium is near in size to the heavier lanthanides and the

chemical behavior is closely similar. Many physical and chemical properties of the rare earth

oxides vary only slightly from element to element.

The use of rare earths is extensive. Examples of their classical and most recent

applications include automotive catalytic converters, glass polishing, petroleum refining

catalysts, phosphors, and permanent magnets.

RE oxide thin films have already been utilized or they are being considered as

potential materials in a variety of applications, particularly in optics and electronics. By virtue

of their high relative permittivity values (k=12.3−14.8)

RE oxides have been considered as candidates for new gate dielectrics in microelectronics.

Er2O3 is being used as a doping material in amplifying optical waveguides, while RE2O3-doped

CeO2 films could be employed as oxide ion conducting electrolytes in solid oxide fuel cells

(SOFC).

Lanthanum oxide:

Lanthanum (III) oxide is La2O3, a chemical compound containing the lanthanum and

oxygen. It is used to develop ferroelectric materials, and in optical materials. Production is on

laboratory scale, mostly.

Properties:

La2O3 has largest band gap of the rare earth oxides at 4.3 eV, while also having

the lowest lattice energy, with very high dielectric constant, ε = 27. La2O3 is widely used in

industry as well as in the research laboratory. Lanthanum oxide is an odorless, white solid that

is insoluble in water, but soluble in dilute acid. Depending on the pH of the compound,

different crystal structures can be obtained. La2O3 is hygroscopic; under atmosphere,

lanthanum oxide absorbs moisture over time and converts to lanthanum hydroxide. Lanthanum

oxide has p-type semi-conducting properties because its resistivity decreases with an increase

in temperature, average room temperature resistivity is 10 kΩ·cm.

Structure of La2O3:

At low temperatures, La2O3 has an A-M2O3 hexagonal crystal structure. The

La3+ metal atoms are surrounded by a 7 coordinate group of O2-atoms, the oxygen ions are in

an octahedral shape around the metal atom and there is one oxygen ion above one of the

octahedral faces. On the other hand, at high temperatures the Lanthanum oxide converts to a

C-M2O3 cubic crystal structure. The La3+ ion is surrounded by a 6 coordinate group of O2-

ions.

Applications of La2O3 thin films in Metal-oxide-semiconductor field effect transistors:

The advancement of technology calls for complex applications, which requires

the support of faster and more powerful electronic devices. The need for better performance

devices has been met through the scaling of SiO2 based devices. The use of SiO2 as gate

dielectric is fast reaching its fundamental limit to be an effective gate dielectric when the

physical thickness is scaled down to below 1.5 nm due to the exponential increase of gate

leakage current caused by quantum mechanical tunneling effect. This effect, if left unchecked,

will cause a detrimental effect to the reliability of the device.

The scaling down of metal-oxide-semiconductor field effect transistors has been

a continuous trend in microelectronics, the number of transistors per chip being doubled every

one and a half or two years. This trend was predicted as long ago as 1965 by Gordon Moore

and is known as Moore’s Law.

The motivation for downscaling is manufacturing of faster transistors with

lower power consumption. The dimensions of MOSFETs have now reached the level where the

current silicon-based gate materials, namely SiO2 gate dielectric and poly-Si gate electrode,

will have to be replaced with other materials if miniaturization is to continue.

The rare-earth oxides appear to be promising candidates as suitable

replacements due to their advantageous properties such as

fairly large band gap,

high dielectric constant, and

low leakage current

Which are required in order to be considered as suitable candidates to replace SiO2 as high-

kgate dielectric. Furthermore, some lanthanide oxides show good interfacial characteristics

with little or no preformed interfacial layer.

Lu2O3 is chosen as it has the highest lattice energy −13.871 kJ/mol and the largest band gap

5.5eV, from which we would expect Lu2O3 to exhibit better thermal stability, good insulating

property, and hygroscopic immunity as compared to other rare-earth oxide thin film.

Lu2O3 has been reported recently to exhibit good insulating properties with k value of around

11 as well as low gate leakage currents.

Thermal stability of rare-earth based ultrathin Lu2O3 for high-k dielectrics is also another

reason for considering the same.

RE oxides in Optoelectronics: in this field, RE oxides are considered because of their

superior thermo mechanical properties, their strong Stark-splitting, and their low phonon

energies. The former property makes them promising for high-power lasers, the second one

enables efficient laser action, and the last one ensures large energy storage times.

RE oxide coatings for metals: The use of RE oxide coatings has the advantage of not

affecting adversely the mechanical properties of the alloy and it has also the potential of being

used on surfaces of metallic components exposed to high temperature oxidizing

environments.

RE oxide as anti reflection coatings: Lanthanum oxide film is used as an anti reflection

coating in the fabrication of solar cells to improve the efficiency of the cell.

2. EXPERIMENTAL TECHNIQUES:

2.1 Vacuum Techniques:

In everyday usage, vacuum is a volume of space that is essentially empty of

matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes

from the Latin term for "empty” . The classical notion of a perfect vacuum with gaseous

pressure of exactly zero is only a philosophical concept and is never observed in practice.

A vacuum pump is a device that removes gas molecules from a sealed volume in order to

leave behind a partial vacuum. The vacuum pump was invented in 1650 by Otto von Guericke.

Pumps can be broadly categorized according to three techniques:

Positive displacement pumps use a mechanism to repeatedly expand a cavity, allow

gases to flow in from the chamber, seal off the cavity, and exhaust it to the atmosphere.

Momentum transfer pumps, also called molecular pumps, use high speed jets of

dense fluid or high speed rotating blades to knock gas molecules out of the chamber.

Entrapment pumps capture gases in a solid or adsorbed state. This includes

cryopumps, getters, and ion pumps.

Positive displacement pumps are the most effective for low vacuums.

Momentum transfer pumps in conjunction with one or two positive displacement pumps are

the most common configuration used to achieve high vacuums. In this configuration the

positive displacement pump serves two purposes. First it obtains a rough vacuum in the vessel

being evacuated before the momentum transfer pump can be used to obtain the high vacuum,

as momentum transfer pumps cannot start pumping at atmospheric pressures. Second the

positive displacement pump backs up the momentum transfer pump by evacuating to low

vacuum the accumulation of displaced molecules in the high vacuum pump. Entrapment

pumps can be added to reach ultrahigh vacuums, but they require periodic regeneration of the

surfaces that trap air molecules or ions. Due to this requirement their available operational time

can be unacceptably short in low and high vacuums, thus limiting their use to ultrahigh

vacuums. Pumps also differ in details like manufacturing tolerances, sealing material, pressure,

flow, admission or no admission of oil vapor, service intervals, reliability, tolerance to dust,

tolerance to chemicals, tolerance to liquids and vibration.

Positive displacement pumps :

By definition, positive-displacement (PD) pumps displace a known quantity of

liquid with each revolution of the pumping elements. This is done by trapping liquid between

the pumping elements and a stationary casing. Pumping element designs include gears, lobes,

rotary pistons, vanes, and screws.

PD pumps are found in a wide range of applications -- chemical-processing;

liquid delivery; marine; biotechnology; pharmaceutical; as well as food, dairy, and beverage

processing.  Their versatility and popularity is due in part to their relatively compact design,

high-viscosity performance, continuous flow regardless of differential pressure, and ability to

handle high differential pressure.

Different Positive displacement pumps:

Rotary vane pump, the most common

Diaphragm pump, zero oil contamination

Liquid ring pump

Piston pump, cheapest

Scroll pump, highest speed dry pump

Screw pump (10 Pa)

Wankel pump

External vane pump

Roots blower, also called a booster pump, has highest pumping speeds but low compression

ratio

Multistage Roots pump that combine several stages providing high pumping speed with

better compression ratio

Rotary vane pump :

The modern pumps have an area contact between rotor and stator (and not a line contact).

While vane pumps can handle moderate viscosity liquids, they excel at handling low viscosity

liquids such as LP gas (propane), ammonia, solvents, alcohol, fuel oils, gasoline, and

refrigerants.  Vane pumps have no internal metal-to-metal contact and self-compensate for

wear, enabling them to maintain peak performance on these non-lubricating liquids.

Despite the different configuration, most vane pumps operate under the same general principle

described below.

1.  A slotted rotor is eccentrically supported in a cycloidal cam.  The rotor is located close to

the wall of the cam so a crescent-shaped cavity is formed.  The rotor is sealed into the cam by

two side plates.  Vanes or blades fit within the slots of the impeller.  As the rotor rotates (yellow

arrow) and fluid enters the pump, centrifugal force, hydraulic pressure, and/or pushrods push

the vanes to the walls of the housing.  The tight seal among the vanes, rotor, cam, and side

plates is the key to the good suction characteristics common to the vane pumping principle.

1. pump housing

2. rotor

3. vanes

4. spring

2.  The housing and cam force fluid into the pumping chamber through holes in the cam (small

red arrow on the bottom of the pump).  Fluid enters the pockets created by the vanes, rotor,

cam, and sideplate.

3.  As the rotor continues around, the vanes sweep the fluid to the opposite side of the crescent

where it is squeezed through discharge holes of the cam as the vane approaches the point of the

crescent (small red arrow on the side of the pump).  Fluid then exits the discharge port.

Roots type super charger:

A Roots blower with two-lobed rotors. Most real Roots blowers' rotors have three or four

lobes.

The Roots type supercharger or Roots blower is a positive displacement lobe pump which

operates by pumping fluids with a pair of meshing lobes not unlike a set of stretched gears.

Fluid is trapped in pockets surrounding the lobes and carried from the intake side to the

exhaust. It is frequently used as supercharger in engines, where it is driven directly from the

engine's crankshaft via a belt or, in a two-stroke diesel engine, by spur gears.

Momentum transfer:

In a momentum transfer pump, gas molecules are accelerated from the

vacuum side to the exhaust side (which is usually maintained at a reduced pressure by a

positive displacement pump). Momentum transfer pumping is only possible below pressures of

about 0.1 kPa. Matter flows differently at different pressures based on the laws of fluid

dynamics. At atmospheric pressure and mild vacuums, molecules interact with each other and

1 Rotary vane 1

2. Pump body

3. Rotary vane 2

a. Intake

b. Pumping

c. Forced air or air-fuel mixture into intake manifold

push on their neighboring molecules in what is known as viscous flow. When the distance

between the molecules increases, the molecules interact with the walls of the chamber more

often than the other molecules, and molecular pumping becomes more effective than positive

displacement pumping. This regime is generally called high vacuum.

Molecular pumps sweep out a larger area than mechanical pumps, and do so

more frequently, making them capable of much higher pumping speeds. They do this at the

expense of the seal between the vacuum and their exhaust. Since there is no seal, a small

pressure at the exhaust can easily cause back streaming through the pump; this is called stall.

In high vacuum, however, pressure gradients have little effect on fluid flows, and molecular

pumps can attain their full potential.

The two main types of molecular pumps are the diffusion pump and the

turbomolecular pump. Both types of pumps blow out gas molecules that diffuse into the pump

by imparting momentum to the gas molecules. Diffusion pumps blow out gas molecules with

jets of oil or mercury, while turbo molecular pumps use high speed fans to push the gas. Both

of these pumps will stall and fail to pump if exhausted directly to atmospheric pressure, so they

must be exhausted to a lower grade vacuum created by a mechanical pump.

As with positive displacement pumps, the base pressure will be reached when

leakage, out gassing, and back streaming equal the pump speed, but now minimizing leakage

and outgassing to a level comparable to back streaming becomes much more difficult.

Diffusion pump:

Diffusion pumps use a high speed jet of vapor to direct gas molecules in the

pump throat down into the bottom of the pump and out the exhaust. Presented in 1915 by

Wolfgang Gaede and using mercury vapor, they were the first type of high vacuum pumps

operating in the regime of free molecular flow, where the movement of the gas molecules can

be better understood as diffusion than by conventional fluid dynamics. Gaede used the name

diffusion pump since his design was based on the finding that gas cannot diffuse against the

vapor stream, but will be carried with it to the exhaust. However, the principle of operation

might be more precisely described as gas-jet pump, since diffusion plays a role also in other

high vacuum pumps. In modern text books, the diffusion pump is categorized as a momentum

transfer pump. The diffusion pump is widely used in both industrial and research applications.

Most modern diffusion pumps use silicone oil as the working fluid. Cecil Reginald Burch

discovered the possibility of using silicone oil in 1928.

Turbomolecular pump:

A turbo molecular pump is a type of vacuum pump, superficially similar to a

turbopump, used to obtain and maintain high vacuum. These pumps work on the principle that

gas molecules can be given momentum in a desired direction by repeated collision with a

moving solid surface. In a turbo molecular pump, a rapidly spinning turbine rotor 'hits' gas

molecules from the inlet of the pump towards the exhaust in order to create or maintain a

vacuum.

Entrapment pumps:

Diffusion Pump

Entrapment pumps may be cryopumps, which use cold temperatures to

condense gases to a solid or adsorbed state, chemical pumps, which react with gases to

produce a solid residue, or ionization pumps, which use strong electrical fields to ionize gases

and propel the ions into a solid substrate. A cryomodule uses cry pumping.

Ion pump

Cryopump

Sorption pump

Non-evaporative getter

Ion pump:

An ion pump (also referred to as a sputter ion pump) is a type of vacuum pump

capable of reaching up to 10−11 mbar under ideal conditions.An ion pump ionizes gases and

employs a strong electrical potential, typically 3kV to 7kV, to accelerate them into a solid

electrode. A swirling cloud of electrons produced in hollow Penning cells ionizes incoming gas

atoms and molecules while they are trapped in a strong magnetic field. The swirling ions strike

the chemically active cathode inducing sputter and are then pumped by chemisorption which

effectively removes them from the vacuum chamber, resulting a net pumping action.

Cryopump:

A cryopump is a vacuum pump that traps gases and vapours by condensing

them on a cold surface. They are only effective on some gases, depending on the freezing and

boiling points of the gas relative to the cryopump's temperature. They are sometimes used to

block particular contaminents, for example in front of a diffusion pump to trap back streaming

oil, or in front of a McLeod gauge to keep out water. In this function, they are called a

cryotrap or cold trap, even though the physical mechanism is the same as for a cryopump.

Cryotrapping can also refer to a somewhat different effect, where molecules will increase

their residence time on a cold surface without actually freezing. There is a delay between the

molecule impinging on the surface and rebounding from it. Kinetic energy will have been lost,

the molecules slow down. For example, hydrogen will not condense at 8 kelvins, but it can be

cryotrapped. This effectively traps molecules for an extended period and thereby removes them

from the vacuum environment just like cryopumping.

Sorption pump:

The sorption pump is a vacuum pump that creates a vacuum by adsorbing

molecules on a very porous material like molecular sieve which is cooled by a cryogen,

typically liquid nitrogen. The ultimate pressure is about 10-2 mbar. With special techniques this

can be lowered till 10-7 mbar. The main advantages are the absence of oil or other

contaminants, low cost and vibration free operation because there are no moving parts. The

main disadvantages are that it cannot operate continuously and cannot effectively pump

hydrogen, helium and neon, all gases with lower condensation temperature than liquid

nitrogen. The main application is as a roughing pump for a sputter-ion pump in ultra-high

vacuum experiments, for example in surface physics.

Getter:

A getter is a reactive material used for removing traces of gas from vacuum

systems, such as vacuum tubes. Residual gas can be left in vacuums by inadequate vacuum

pumps, or adsorbed gasses can be released after evacuation by the inner surfaces of the

container. The getter is usually a coating applied to a surface within the evacuated chamber.

When molecules of residual gas strike the getter surface they chemically combine with the

material, removing them from the evacuated space.

2.2Deposition techniques:

There are numerous methods available for the preparation of thin films.

Broadly, these methods may be divided into two classes, namely (a) chemical methods and

(b) physical methods. Details of different methods are given as follows :

a) Chemical methods :

1. Electro plating

2. Ion plating

3. Chemical reduction plating

4. Vapour phase deposition

5. Hydrophilic Method

6. Anodization

7. Gaseous anodization

8. Thermal growth

b) Physical Methods :

1. Sputtering

2. Electron bombardment

a) Chemical Methods : -

In all chemical methods a definite chemical reaction is required to obtain the film.

1) Electro Plating : -

Electro plating has been well established method and many text books exist on

the subject . In this method, metal is deposited on the cathode and the relationship between the

weight of material deposited and other related parameters can be expressed by the first and

second law of electrolysis. (a) The weight of the deposit is proportional to the amount of

electricity passed (b) the weight of material deposited by the same quantity of electricity is

proportional to the electro –chemical equivalent, expressed as an equation.

W = JtEα

Where w - weight of deposited per unit area

E - Electro chemical equivalent

J - Current density

t - Time and

Α-current efficiency

The term currently efficiency α which is the ratio of the experimental to theoretical weight

deposited and it can be expected to be between unity and 0.5

The structure that is obtained can vary from single crystal or crystalline aggregate right

through fiber growth deposits to unoriented deposited of very fine grain site and disordered

structure.

ii) Ion plating:

This method has a certain similarity to electro plating. The metal is in the form

of positive ions are attracted to the cathode substrate. The material to be deposited is

evaporated from a suitable filament. Glow discharge is maintained at a pressure 10-1 torr to 10 -2

torr, between the filament as anode and the substrate as cathode, so that the evaporated atoms

are ionized in the plasma. The discharge potential is maintained at as high a value as

possible so that the ionized atoms are accelerated to the substrate. Adhesion of the deposit is

found to be very good because of the high energy of arrival of the deposit ions.

iii) Chemical reduction plating:-

Films of metals may be deposited directly without any electrode potential being

involved by the chemical reduction of a suitable compound in solution. Such deposition is

known as chemical reduction plating or electro less deposition. This method uses very simple

apparatus. The rate of deposition depends on solution pH and temperature. Structurally the

films grow as nucleated islands. The degree of crystallinity depends on the material being

deposited and the bath temperature. The films prepared at room temperature can be amorphous

but will crystallite on heating.

iv) Vapour phase deposition:-

The deposition of a film on a surface composed of the same or of a different

substrate by means of a chemical reaction occurring from a gaseous phase at the surface is

known as vapour phase growth (or) vapour planting . Usually the surface is hotter than the

surroundings so that heterogeneous reaction occurs at the surface, otherwise the reaction

may occur in a homogeneous manner in the gas phase.

v). Anodization:

The technique is used for preparing thin films of oxides, nitrides on substrates of

the parent metal. The metal that can be anodized are like aluminum, tantalum , niobium , fir

conium, silicon and Titanium . The electrochemical method of doing this is by anodization .

As the name implies , the film grows on the anode in an electrolytic cell. The basic equations

that govern the process can be written as

m + nH2O -------------> mO2 + 2 nH+ + 2ne at anode

2ne + 2 nH2O -------------> nH2 + 2nOH – at cathode

Where ‘M’ is the metal, ‘n’ is an integer, ‘e’ is the charge of election and OH - is the hydroxide

ion. It is clear from the above equation, that an oxide grows on the metal anode surface and

hydrogen is evolved at the anode.

The main feature that is common to anodic oxide films is that they grow in a non-

nucleated manner as continuous layers of amorphous material. Generally the oxide surface

is smooth and featureless, although the metal surface is insufficiently clean or smooth.

vi) Thermal growth:-

Metals like tantalum or Aluminum form only very thin amorphous oxides of

thickness 50-100 A0 at temperatures up to 5000 C. At higher temperature, mixed amorphous

crystalline oxides are formed possessing interior dielectric properties. Satisfactory dielectric

possessing high breakdown strengths have been prepared at temperatures 900-12000 C on

silicon. Although the mechanism of film formation has been explained in terms at thermal

oxidation, it is possible that the process is essentially one form of gaseous anodization.

The methods mentioned above have very limited and specified usage and the

films prepared by these methods usually contain large impurities. In most of the thin film

research and application; the techniques mentioned in the second category namely physical

methods are preferred and discussed below. Further all the present investigations were made

on thin films prepared by using thermal vaccum evaporation.

Physical methods:-

Physical deposition uses mechanical or thermodynamic means to produce a thin

film of solid. Since most engineering materials are held together by relatively high energies,

and chemical reactions are not used to store these energies, commercial physical deposition

systems tend to require a low-pressure vapor environment to function properly; most can be

classified as physical vapor deposition. The material to be deposited is placed in an energetic,

entropic environment, so that particles of material escape its surface. The whole system is kept

in a vacuum deposition chamber, to allow the particles to travel as freely as possible. Since

particles tend to follow a straight path, films deposited by physical means are commonly

directional, rather than conformal.

(a)Sputtering:-

Sputtering is a process used to deposit a very thin film onto a substrate whilst in

a vacuum. A high voltage is passed across low pressure gas to create plasma of electrons and

ions in a high energy state. The ions hit a target of the desired coating material and cause atoms

from that material to be ejected and bond with the substrate. Sputtering is largely driven by

momentum exchange between the ions and atoms in the material, due to collisions. The

process can be thought of as atomic billiards, with the ion (cue ball) striking a large cluster of

close-packed atoms (billiard balls). Although the first collision pushes atoms deeper into the

cluster, subsequent collisions between the atoms can result in some of the atoms near the

surface being ejected away from the cluster. The number of atoms ejected from the surface per

incident ion is called the sputter yield and is an important measure of the efficiency of the

sputtering process.

Other things the sputter yield depends on are the energy of the incident ions, the

masses of the ions and target atoms, and the binding energy of atoms in the solid.

The target can be kept at a relatively low temperature, since the process is

Not one of evaporation, making this one of the most flexible deposition techniques. It is

especially useful for compounds or mixtures, where different components would otherwise

tend to evaporate at different rates. The schematic of sputter deposition are shown in ure .

The impact of an atom or ion on a surface produces sputtering from the surface as a result of

the momentum transfer from the in-coming particle. Unlike many other vapour phase

techniques there is no melting of the material.

(b) Pulsed laser deposition:

Pulsed laser deposition is a thin film deposition technique. It uses a pulsed laser

beam to carry out a process of ablation in order to deposit materials as thin films. Generally, a

high vacuum is necessary for their operation. Pulses of focused laser light transform the target

material directly from solid to plasma; the resulting plume of plasma is thrown perpendicularly

away from the surface by thermal expansion. As expansion cools the plume, it will revert to a

gas, but sufficiently high vacuum will allow momentum to carry this gas to the substrate,

where it condenses to a solid state. Pulsed laser deposition systems work by an ablation

process. Pulses of focused laser light to transform the target material directly from solid to

plasma; this plasma usually reverts to a gas before it reaches the substrate

(c) Electron beam evaporation:

Fires a high-energy beam from an electron gun to boil a small spot of material;

since the heating is not uniform, lower vapor pressure materials can be deposited.

Refractory carbides like titanium carbide and borides like titanium boride and zirconium boride

can evaporate without undergoing decomposition in the vapor phase. These compounds are

deposited by direct evaporation. In this process these compounds, compacted in the form of an

ingot, are evaporated in vacuum by the focused high energy electron beam and the vapors are

directly condensed over the substrate.

Certain refractory oxides and carbides undergo fragmentation during their

evaporation by the electron beam, resulting in a stoichiometry that is different from the initial

material. For example, alumina, when evaporated by electron beam, dissociates into aluminum,

AlO3 and Al2O. Some refractory carbide like silicon carbide and tungsten carbide decompose

upon heating and the dissociated elements have different volatilities. These compounds can be

deposited on the substrate either by reactive evaporation or by co-evaporation. In the reactive

evaporation process, the metal is evaporated from the ingot by the electron beam. The vapors

are carried by the reactive gas, which is oxygen in case of metal oxides or acetylene in case of

metal carbides. When the thermodynamic conditions are met, the vapors react with the gas in

the vicinity of the substrate to form films. Metal carbide films can also be deposited by co-

evaporation. In this process, two ingots are used, one for metal and the other for carbon. Each

ingot is heated with a different beam energy so that their evaporation rate can be controlled. As

the vapors arrive at the surface, they chemically combine under proper thermodynamic

conditions to form a metal carbide film.

The substrate:

The substrate on which the film deposition takes place is ultrasonically cleaned

and fastened to the substrate holder. The substrate holder is attached to the manipulator shaft.

The manipulator shaft moves translationally to adjust the distance between the ingot source

and the substrate. The shaft also rotates the substrate at a particular speed so that the film is

uniformly deposited on the substrate. A negative bias D.C. voltage of 200 V – 400 V can be

applied to the substrate. Often, focused high energy electrons from one of the electron guns or

infrared light from heater lamps is used to preheat the substrate.

Ion beam assisted deposition:

EBPVD systems are equipped with ion sources. These ion sources are used for

substrate etching and cleaning, sputtering the target and controlling the microstructure of the

substrate. The ion beams bombard the surface and alter the microstructure of the film. When

the deposition reaction takes place on the hot substrate surface, the films can develop an

internal tensile stress due to the mismatch in the coefficient of thermal expansion between the

substrate and the film. High energy ions can be used to bombard these ceramic thermal barrier

coatings and change the tensile stress into compressive stress. Ion bombardment also increases

the density of the film, changes the grain size and modifies amorphous films to polycrystalline

films. Low energy ions are used for the surfaces of semiconductor films.

(d) Thermal deposition by resistance heating method:-

Refractory metals like tungsten, molybdenum or tantalum are generally used in

the form of wire or strip having different shapes. The choice of a particular refractory metal as

a heating source depends on the materials to be evaporated, so that the evaporant material does

not react with the refractory metal at the high temperature of evaporation. However the

formation of alloy with the source cannot always be avoided. Hence a coating of refractory

oxides such as of Al2O3, BeO or other suitable materials is often given over the filament or

strip so as to prevent a direct contact between the molten charge and the refractory metal.

In any case, when a direct heating of the charge is made the filament or the strip

is precleaned by passing a heavy current through it so as to make it white hot or incandescent

for a very short period so that all the surface impurities of the filament or the strip are removed

by evaporation and this process is called flash cleaning. After the above cleaning of the

filament, strip or boat whatever may be the form of the evaporating source a little amount of

the charge is then put into it and a current is slowly passed through the source and gradually

increased so that the melt forms a bead or a layer over the heating source. Usually a shutter is

placed in between the heating source and the substrate so that no vapour stream of the charge

can reach the substrate. When appropriate deposition conditions are established ie Vacuo

filament and substrate temperature source to substrate distance and inclination etc, the shutter

is removed out of the line of vapour stream of the charge in vacuo and the deposition on the

substrate starts. When the required film thickness is obtained, this is again brought into the

original position in vacuo so as to cut off further deposition on the substrate and the heating of

the filament is then gradually stopped.

If the charge consists of two or more constituents which have different vapour

pressure, then one having higher vapour pressure will tend to vaporize at a lower temperature

of evaporation the proportion of the higher vapour pressure constituent will be more than the

other. This happens in many alloy systems. The means that the composition of vapour steam

of the charge will be different from that of the charge itself and when condensed the deposits

will have different composition even assuming that the condensation rate is the same for all the

constituent species.

For a binary alloy partial pressures of the constituent, components may be

assumed to follow Raoult’s law of dilute solutions i.e. the vapour pressure of each component

is depressed compared to that of the pure state by an amount proportional to its concentration.

The rate of evaporation of pure element may also be extended to the binary alloys.

n=(2πmkt)-1/2 *p=3.513 x 1022 p(MT)-1/2 molecules/cm2-sec --- (1)

Where

m is the molecular mass

M is gram molecular weight of gaseous species.

G = 5.833 x 10-1p (M/T) ½ (g/ cm2 – sec)

p = 17.14 G(T/M)-1/2

Thus for two pure elements A and B the rates of evaporation at a temperature T and with

particular vapour pressures PA and PB may be denoted as EA and EB respectively. If MA and MB

are the molecular weights of A and B, then the ratio of the rates deposition of two will be

EA /EB = ( PA / PB ) (MA / MB) 1/2- (2)

Assuming that Raoult’s law of deposition of vapour pressure is valid for the binary alloy

system A B then according to Holland, the ratio of evaporation of two components A and B

from the alloy, will be

EA / EB=(WA/WB)(PA/PB)(MB/MA)1/2 - (3)

Where WA and WB are the weight concentration of the two components A and B in the binary

alloy AB if it is assumed that the sticking coefficients of the two components are the same,

then EA/EB ratio will also represent the deposit composition of the alloy on the substrate. It is

thus seen that the composition depends not only on the initial alloy composition but also on the

factor under parenthesis. Unless the latter is unity which is not usually so the deposit

composition will not follow the bulk alloy composition i.e. WA: WB. Further as the evaporation

of the alloy continues the composition of the molten charge also changes becoming richer in

one of the two components as observed in many cases. This leads to a further variation of the

EA/EB ratio and hence of the deposit composition. It is therefore, not easy to deposit films of

the same composition as that of the alloy charge by the usual thermal deposition process. If the

deposition parameters are carefully adjusted by varying the rate of deposition, the substrate

temperature etc.

It has so far been tacitly assumed that in vacuum deposition process the solid

bulk charge changes to the gaseous state by thermal energy and on condensation on a substrate

the deposit mass retains the same composition as that of the charge. This is no doubt, an ideal

situation and is generally valid for elements only but for alloys and compounds it is not so. In

most cases the gaseous phase contains not only the molecules of the original charge but also

the dissociated or excess associated products of them leading to mixtures of several species the

properties of different species depend on the stability of the original material under the thermo

dynamical conditions of evaporation.

The deposit layers are hence likely to be inhomogeneous and the distribution of

these species in the films will be also be non-uniform. Homogeneity of films and uniform

distribution of these species can partially be achieved by raising the substance temperature or

by post deposition annealing treatment invacuo at an appropriate temperature (or) by both

methods. However by a proper combination of different variants it is sometime possible to

obtain films of composition close to that of the charge.

Some of the requirements for a good filament material are

High melting point

Low solubility for the charge materials

Filament should be wettable by the charge materials.

Filament should with stand thermal shocks well.

There are some inherent disadvantages of resistance heated thermal

evaporation that should be kept in mind when selecting a deposition technique.

i. The source may generate impurities which may co-deposit in the

condensing thin film.

ii. Accurate control of the deposition rate is difficult.

iii. The composition of alloy this films deposited by differ from that of

the charge materials ( especially if the elements in the alloy have

markedly different vapour pressures)

iv. The amount of material which may be evaporated per run is limited.

v. The substance will experience heating due to radiant energy from the

source.

Of increased pressure in the lower part of the pump from which the

accumulated gas loads must be removed, by a backing pump. To prevent back diffusion of gas

from the densified into the rarefied zone, the vapour jet should retain as much of its density as

possible. To reconcile this requirement with wide throat area for maximum gas in-take, the

cross – section of the lower zone is narrowed through aerodynamically shaped tapering stacks.

The other walls are water – cooled to recover the work fluid and to produce a denser boundary

layer by removing vapour molecules which travel laterally without contributing to the jet

action. To enhance the directionality and speed of the vapour, most pumps employ multi-stage

stacks, typically with three jets working in series. To minimize contamination in the high

vacuum, the fluid used in the diffusion pump must satisfy two requirements. One is stability in

regard to thermal decomposition and oxidation at operating temperatures and second is a lower

vapour pressure near room temperature. Originally, the only suitable fluids known were

mercury, silicon oil but later a number or organic fluids were developed. For example, diethyl

hexyl phthalate, 5-ring poly phenyl ether, tetra phenyl siloxane and pentaphenyl trimenthyl

siloxane etc.

2.3 FILM GROWTH:

There are several stages in the growth process from the initial nucleation of the

deposits to the final continuous three dimensional film formation state. The prenucleation

stages have already been observed by many workers from their electron microscopic and other

studies. These are valid not only for deposits condensing from the vapour phase but also for

others, i.e. for solutions, by electro deposition, chemical reactions, anodic oxidation, etc. Even

though the controlling factors may differ slightly or widely in the individual cases these stages

have been clearly distinguished by Pashley et al as under

(i) Nucleation (including condensation of vapours, adsorptions of atoms, migration of ad-

atoms, formation of critical nuclei, stable, clusters etc. and

(ii) Growth of larger ones leading to island structure,

(Iii) Coalescence of islands with gaps in between which are interspersed with secondary

nuclei,

(iv) Joining of larger islands with the formation of channels in between and

(v) Finally bridging up the channels with the secondary nuclei to form a continuous film may

be with pinholes. These stages are discussed in brief in the following

(1) Nucleation:

Nucleation or small cluster formation is the basic process for all deposition

modes whether from the vapor phase or otherwise. Even though the method of deposition and

the nature of the impinging species (i.e. atomic or ionic) may differ in different cases the film

growth process is more or less the same. However, the formation of a thin film from the vapor

phase has been studied in details and the thermodynamics of nucleation is briefly described

below. Nucleations and cluster formation from the vapor phase whether undissociated or not or

evenly formed one involve the condensation of vapor stream directly to the solid phase or via

its liquid phase on the surface of a substrate. In an ideal case the surface should be defect free

and atomically smooth. this process can be treated from the Gibb’s free energy conditions of

the vapour pressure of gaseous and solid phases, free surface energies, adsorption of atoms on

the substrate surface and also their desorption, dissociation and association of atoms/molecules

during deposition. other factors which also participate in the process are the possible migration

of the adsorbed species over the substrate surface and joining others to form two dimensional

nuclei or clusters, temperature of substrate, sticking coefficients, etc. The last factor means the

fraction of the impinging atoms staying on the substrate surface after re-evaporation back to

the vapour phase. The general process of additions, adsorption, desorption, migration etc of

atoms envisaged in the above is called nucleation and is schematically shown in ure.

Schematic diagram of growth process from the vapour phase by

(a) Addition of atoms to form cluster.

(b) Addition, migration and re evaporation.

(c) Addition, adsorption, migration and re evaporation.

(d) Addition and migration.

(e) Migration and addition and also migration and re-evaporation and

(f) Migration and re-evaporation and nucleation etc.

The condensation of vapour atoms /molecules on neutral, inert or active solid

generally takes place from a super saturated condition of vapours. The same principal can

be applied after some modifications to cases such as electrodepositing, crystallization from

solids, ion plating, sputtering , plasma deposition and others also. The theory involves film

formation often over a foreign substrate surface which plays a dominant role in nucleation.

This is known as heterogeneous nucleation. If the substrate is the same material as the

vapour atoms then the nucleation is homogeneous. In both cases at the pre nucleation stage

several phenomenons occur either simultaneously or step by step and these can be

envisaged in the following way:

(i) The physical condensation of vapour atoms from a relatively high level of super

saturation involves some loss of Kinetic energy of impinging atoms on the substrate

surface.

(ii) These are then adsorbed on the substrate surface and may or may not be in thermal

equilibrium.

(iii) Adsorbed species (monomer) may then move over the substrate surface from one

potential well to another due to inherent kinetic energy or that induced from the substrate.

(iv) These monomers have certain time of residence over the substrate

(v) Adsorbed species or monomers after migration collide to form sub critical or critical

nuclei or clusters; formation of critical nucleus involves the release of heat of condensation

of vapour (vi)atomsImpinging atoms can reflected back to vapour state without

condensation.

(vii) Evaporation can take place at all the above stages.

The same cycle repeats after each impingement of atoms.

(2) Island Structure Stage (fig a and b)

These islands consist of comparative larger nuclei or embryos say greater than

10 Å and generally of three dimensional natures with their height, however, much less than

their lateral dimensions. The formation of these islands and their growth take place either by

direct addition of atoms from the vapour phase or from other environment or by the diffusion

controlled process of ad-atoms or both as envisaged before. The diffusion controlled process is

more commonly observed except at low substrate temperature. The travel distance of ad-atoms

is dependent exponentially both on and 1/T. According to Walton critical nuclei formed from

the vapour phase will consist of about six or fewer atoms. Electron microscopic observations

by various workers show that the smallest stable nuclei are of radii of about 5 Å and also that

most embryos prior to the formation of island structure are of sizes about 15-30 Å. Since a

nucleus of about 5 Å size is made up of about 20 atoms or so, an island will be made up about

50-100 atoms or more. As these islands grow in size these often have tendencies to develop

some crystallographic facets during the early stage of their growth and such faceted particles of

sizes about 30-50 Å could be observed. This facet formation is generally pronounced for

orientated nuclei of some metal deposits from vapour phase especially on high vacuum cleaved

substrate at high vacuum at higher T, The surface migration distance of ad-atoms of silver or

gold deposited from the vapour phase on MoS2 substrate at T2=4000C has been estimated to be

about 500 Å. The formation of stable clusters and the subsequent capture of more ad-atoms by

diffusion of them to form stable islands and also sometime faceted ones are depicted in (fig. 4a

and b).

(3) Coalescence Stage (fig. 4c and d)

These islands as they grow as mentioned before develop some characteristic

shapes and then with further growth coalesce with the neighboring ones by rounding off their

edges near the joining region (neck) where these deposits assume a liquid like structure. The

coalescence involves considerable transfer of mass between islands by diffusion. Small islands

disappear rapidly. The process resembles the sintering of bulk powder where the individual

particles assume spherical shapes due to the lowering of their necks recrystallisation as well as

annealing takes place leading to some definite shapes of larger islands (fig. 4c and d). The

time of coalescence is very short say about 0.6 second. This process can take place amongst

islands which are appropriately positioned and the coalesced islands generally become

triangular or hexagonal shaped as a result of the rapid decrease of the uncovered substrate

surface area followed by a slow rise of it. In the early stages of the growth of films it is

envisaged as mentioned earlier that there will be a continuous formation of nuclei ( say about

1 to 8 x 1011 nuclei/cm2 for both gold and silver) and as evaporation progresses the

nucleation density may increase to about three times the initial ones. After the coalescence of

islands to larger masses some nuclei can still be observed in between the large coalesced

masses. Often coalescence by bridging of two particles is pronounced.

(4) Channel Stage (fig .4e) and Holes (fig.4f)

As the coalescence continues with deposition there will be a resultant network

of the film with channels in between (fig. 4e). These channels need not remain void and soon

some secondary nuclei start to grow within this void space in the channel. With further

deposition these nuclei will increase in size along with the film thickness in addition to

formation of new islands via stage (i) and eventually join the main islands or aggregate thus

bridging the gaps. It is quite likely that the joining of secondary islands to the parent body may

not be completed or these may not be in perfect matching arrangement with the main

aggregate. As a result some strain may develop due to the stress in between them caused by an

insufficient surface or volume mobility or even because of the non-coalescence at the

peripheries. The resultant effect of mismatching of these in the formation of grain boundaries.

Sometimes these channels may not be completely filled up even with increasing film

thickness thus leaving some holes or gaps in the aggregate mass (fig. 4f) With increasing

film thickness, these holes or gaps will decrease in size.

(5) Continuous Film State

When these gaps are completely bridged by the secondary nuclei…., films will

be continuous. However, it often happens that some void space may still remain unabridged. In

an ideal continuous film there should not be any gap in the aggregate mass. Such a stage in a

film can be attained only when the film has attained certain average film thickness. The

minimum film thickness for the continuous stage is also dependent on the nature of the

deposits, modes of deposition, deposition parameters etc. For a non-metallic deposit such a

stage is generally achieved when the average film thickness is say between 500 to 1000 A0 and

for metallic films less. It depends upon the nature of the film material, substrate temperature

(Ts) rate of deposition etc; the growth of the films as mentioned earlier also involves

recrystallisation and often annealing processes. If the deposits do not have sufficient time for

recrystallisation or annealing before the subsequent uni- or multi coalescence takes place the

film will be in a metastable (or unstable) state. The subsequent layers formed over them will

also be in such a state. Thermal annealing treatment (in vacuum) for a sufficiently long

period of time will cause migration or diffusion of some atoms leading to a stable phase.

This is known as ageing of films. Different stages in the growth of a film are schematically

shown in

(fig a-f).

In figure above different growth stages with deposition time for gold deposits from vapour

phase on MoS2 at 400oC. It may be mentioned here that continuous films are of great

importance from application points of view and these may vary widely in thickness

dependingon the particular use of them.

2.4 Uniformity and film deposition rate :

Semicircular symmetry allows multiple wafers to be evaporated simultaneously.

For liftoff applications, a planar configuration is preferred. Rotating planetary can help with

uniformity.

Maintaining a lower deposition rate will yield greater uniformity.

Placing samples far from the source will help uniformity, but will also lower deposition

rate.

The deposition rate depends on the position and orientation of the wafer in the chamber.

An evaporation rate is the rate at which a material will vaporize (evaporate, change

from liquid to vapor) compared to the rate of vaporization of a specific known material.

This quantity is a ratio, therefore it is unit less.

The vapor pressure of a liquid is the pressure exerted by its vapor when the liquid and

vapor are in dynamic equilibrium. A substance in an evacuated, closed container will

vaporize a finite amount. The pressure in the space above the substance will increase

from zero and eventually stabilize at a constant value, the vapor pressure. Vapor

pressures increase with temperature. The boiling point is the temperature at which the

vapor pressure of a liquid equals the external pressure. In general, the higher the vapor

pressures of a material at a given temperature, the lower the boiling point. In other

words, compounds with high vapor pressures form a high concentration of vapor above

the liquid. When the vapor source is heated, the vapor pressure of the metal to be

evaporated becomes substantial, hence, atoms are sent out into the vacuum chamber,

some of which reach the substrate to form a metal film

Mean Free Path, for purposes of evaporation, is the distance a molecule travels in a

straight line (in vacuum) before its velocity vector is randomized by a collision.

2.5 Thickness measurement techniques:

Thickness plays an important role in the film properties unlike a bulk material

and almost all film properties are thickness dependent at least for thin films as will be evident

from different chapters of this book. Reproducible properties are achieved only when the film

thickness and the deposition parameters are kept constant. In many application, particularly so

in the case of optical devices such as interference filters, anti-reflection coatings, etc., the

success of the fabrication depends only on the deposition of specific thickness of the dielectric

layers. In other cases even though a specific thickness may not be strictly necessary, a good

control of it will still be essential.

In all thickness measurements it is generally assumed that these films are

homogenous and more or less uniformly on the substrate so that these will have a mean

thickness (d or t). However, when films are thin, i.e. their thickness are low say less than about

500 Å and specially in the cases of ultra-thin films, the deposit layers will be discontinuous and

non-uniform and may be interspersed with voids or pinholes. In such cases a mean film

thickness may not have any significance at all since 'd' at any specific area may vary from 0 to

any value much higher than the so called mean film thickness. It only indicates the likely

magnitude of the film thickness if all deposits would have been uniformly deposited or

distributed over the total substrate surface. If the deposition is carried out in controlled

conditions then mean 'd' or 't' will be close to the film thickness at any place over the

substrate surface.

Film thickness measurement techniques are based on different principles such as

the mass difference, light absorption, interference effect, conductivity, capacitance, etc. of the

films with increasing thickness. These measurements can be either in a dynamic or static

condition. In the former 'd' or 't' is measured during the deposition process whilst in the

latter, after its completion. In the following, these are described in brief.

There are several techniques to measure the thinckness of the film . They are

Microbalance Technique

Crystal Oscillator

Photometric

Ellipsometry

Tolanskys Fizeau fringes method

3) PREPARATION AND CHARACTERISATION OF La2O3 THIN

FILM:

3.1 Preparation:

3.1.1 Vacuum Coating unit :

It is a versatile laboratory model coating unit for thin film application with facilities for

evaporation, Optional accessories like Substrate Heating, Rotary Drive, Flash Evaporation, EB

Gun evaporation etc. It is an ideal unit for thin film coatings in Research, Educational

institutions, semiconductor, optics the Vacuum coating unit & chamber gadgetries are

manufactured using high vacuum compatible materials.

Salient Features

• Versatile coating unit.

• Compact and elegant.

• Can accommodate wide range of accessories.

• Highly reliable and proven.

• Suitable for mounting a 3 KW Electron Beam Gun.

Optional Accessories:

Rotary Drive, Radiant Heaters, Cold Fingers, Thickness Monitor, Flash Evaporations, Multi

Filament Turrets, Liquid Nitrogen Trap(LNT), substrate heater, Electron Beam Gun 3KW Single

Source/4 Source.

Technical Specification :

1. Vacuum Chamber ……………………….Beljar Type

2. Chamber Size ……………………………300 mm Dia x 350 mm height (nominal)

3. Material …………………………………Stainless Steel - SS 304.

4. Base Plate ………………………………SS304, 330mm dia, with 11 Nos. ports for

Various feed through

5. Chamber Lifting…………………………Manual lifting with spring & ball cage assembly

6. CHAMBER GADGETORIES: (STD.)

a. Work holder Size …………… 225 mm Dia.

b. LT Evaporation ……………… 10 V A.C(Sequential)

c. Ion Bombardment …………… 1 set, 3.5 KV, 50 MA D.C

d. Source Shutter (manual)……… 1 set

7. VACUUM PUMPING SYSTEM

a. Diffusion pump type & speed…… ………………OD-114D, 280 Lit/Sec

b. Rotary Vacuum Pump type & speed……………… ED-15, 250 Lit/Min

c. High Vacuum Valve……………………………….100mm Butterfly Valve

d. Roughing, Backing Valve…………………………25mm size(CV-25)

e. Vacuum Gauges……………………………………Analog, Pirani, Penning Gauge

with sensors to measure Vacuum in the range of 0.5 mbar to 1 x 10-3 mbar

and 10-3 to 10-6 mbar)

8. Ultimate Vacuum with DC-704 oil……………………… 6.5 x 10-6 mbar. without LN2 &

1 x 10-6 m.bar. With LNT

9. Utilities Required

a. Power…………………………………………… 230V AC, 50Hz, Single Phase, -

15 Amps.

b. Water at 25 C ………………………………… .. 2 Lit/min at a pressure of

1.5 - 2 Kg/cm2

3.1.2 High Pressure Pirani Gauge - Analog

Change of pressure in vacuum systems brings about a rise or fall in number of

gas molecules present and hence a rise (or) fall in the thermal conductivity of the gas. Thus the

heat loss of constant voltage The pirani gauge head filament has high temperature co-efficient

of resistance. So a slight change in System pressure brings about useful change in filament

resistance resulting in an out of balance of the This electrically heated filament is an arm of a

self balancing wheat stone bridge circuit. An automatic control amplifier corrects bridge

voltage automatically. Thus the required bridge voltage (which varies depending upon the

pressure at the sensor head filament) is a measure of the pressure which afterElectrically heated

filament in the system varies with the pressure.

Salient Features:

• Compact solid state electronics

• Two gauge head capability

• Factory calibrated

• Reliable and repeatable

Pressure indications

• Excellent Zero stability

3.1.3 Thin Film deposition:-

The Thin Film have been deposited by using thermal evaporating Vacuum

Coating. Thin film of Al and Fe was deposited on glass substrate. The fabrication process is

explained here in following steps.

Opening of the chamber:

First of all thermal evaporator unit was installed in vacuum chamber the chamber

should be cleaned to avoid contamination the required material to be deposited was kept in

filament or boat with stand assembly

Closing of bell jar and vacuum pumping :

1.Close the chamber. We can use the Vacuum grease to tightly attach the bell jar with chamber.

2.Now, turn on power supply and Water flow. Start the rotary pump by using puss button

switch.

3.Open back pump and turn on pirani gauge power. Wait until pressure drops to 0. 5 torr. Open

the chamber in the backing mode.

4.When pirani gauge shows pressure drops above 0.05 torr. Initiate the roughing pump and

start the heater of the diffusion pump. Keep the chamber in this mode until the pressure drops

to more than 0.05 torr .It will take approx. 30 min.

Evaporation of material :

1.Turn on heater power.

2.Rotate current dial slowly increasing current.

3.The amount of current required varies for different metals.

4.When you begin to get a deposition, press open shutter on deposition meter and open shutter

inside bell jar by flipping shutter switch down.

5.Adjust current up or down to maintain ideal deposition rate for specific metal.

6.At desired thickness, close shutter.

7.Slowly ramp down current when finished.

Opening of chamber :

Turn off high vacuum switch.

Wait 10 minute for everything to cool down.

Open the Chamber.

Take out the glass substrate.

Find the V-I characteristics of the film.

Thermal evaporation in vacuum :

In the evaporation deposition technique, the material is heated until fusion by means of

an electrical current passing through a filament or metal plate where the material is deposited

(Joule effect). The evaporated material is then condensed on the substrate. Other ways of

heating are used, such as a RF coil surrounding a graphite or BN crucible, where the material

to be evaporated is fused. The assembly of the technique is simple and results appropriate for

depositing metals and some compounds with low fusion temperature (Al, Ag, Au, SiO, etc.).

Typical metals used as heating resistance are tantalum (Ta), molybdenum (Mo) and wolfram

(W), with vapor pressure practically zero at the evaporation temperature (Tevap = 1000-2000

°C). When a helical filament surrounds the material it is convenient that the evaporant material

wets the metal. A scheme of the deposition equipment used in the laboratory is showed in the

Fig 2.1 The thermal evaporator uses resistive energy to evaporate thin films onto a given

substrate. The thickness is controlled by the use of a quartz crystal monitor

fig.Thermal evaporation

3.2 Characterisation:

3.2.1Measurement Of Thickness (Tolansky's method):

When two reflecting surfaces are brought into close proximity, interference

fringes are produced, the measurement of which possible a direct determination of the film

thickness and surface topography with high accuracy. Weiner (1887) was the first to use

interference fringes to measure the thickness of films. Later on the interference Fringe

methods have been developed to a remarkable degree by Tolansky(1948, 1955, 1970) and are

now accepted as the standard methods.

The interferometer consists of two slightly inclined optical flats ( ure 2.6(a)) 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, fine dark

fringes can be observed which trace out the points of equal air-gap thickness. The two adjacent

fringes are separated by λ /2 where ‘ λ ’ is the wavelength of the monochromatic light used. If

the surfaces of the optical flats are highly reflecting ( the upper flat however possesses an

observable transmission) and very close to each other, the reflected fringe system consists of

very fine

The Schematic diagram of the experimental set up for the measurement of film thickness.

dark fringes against a white background with a fringe width which can be made as small as

λ/100 Å . By adjusting relative positions of the flats to form a wedge shaped air gap, the

fringes can be made to run in straight lines perpendicular to the steps on the opaque film. 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. It is very essential to coat the

film as well as the exposed glass surface with same reflecting layer in order that the phase

changes on reflection from the two sides of this step will be the same (Tolansky 1948). The

schematic diagram of the experimental technique is shown in figure 2.6(b). In this way, very

contrasting interference fringes were obtained. The thickness‘d’ of the film from the

interference fringes, has been calculated using the formula,

d = (S / W) ( λ / 2)where ‘S’ is the fringe shift at the step, ‘W’ is the fringe spacing and ‘ λ ' is the wavelength of

monochromatic light used. In our investigations sodium vapour lamp of λ = 5893 A0 is

employed.

Fig.Equal displacement fringes

Observations:

From the above method the values were observed to be

shift (s) = 0.0016 cm

fringe width (w) = 0.01 cm

λ for Na vapour lamp = 5893 A0

By substituting all the above values in the formula

d = (S / W) ( λ / 2) d = 500 A0 (app)

3.2.2 Optical Properties :

Optical absorption measurement constitutes one of the most important means of

determining the band structure of semi conductors . Photon induced electronic transitions can

occur between different bnds , which lead to the determination of the energy band gap , or

electron ( or hole) transitions within a band from one single particle state to another such as the

free carrier absorption . Optical absorption measurements can also be used to study lattice

vibrations .

The Transmission coefficient ( T) and the Reflection coefficient ( R) are the two important

quantities that are generally measured . For normal incidence they are given by

T = {(1-R)2 exp(- αt )} / { 1- R2 exp (-2αt)} and

R = {(1-n)2 + k2}/{( 4n)2 + k2}

λ-wavelength

n- refractive index

k- extinction coefficient

t- film thickness of the sample

The Absorption coefficient per unit length is giben by

α = 4πk/λ

By analysing the T- λ or R- λ data at normal incidence or by making observation of R or T at

different angles of incidence , both n and k can be determined and related to transition energy

between bands.

In the present study , the optical transmittance of the film substrate system was recorded at

room temperature (303 K) with unpolarised light at normal incidence in the photon energy

range hʋ =1.0-2.1 ev using Hitachi US400 UV-visible-near IR double beam spectrophotometer.

The extinction coefficient (k) and hence the optical absorption coefficient (α ) of the

experimental films was obtained from the Transmittance ( T) data using the relation employed

by Neuman et al. for single absorbing layer on a transparent substrate,

T= A [ B1 exp ( β) + B2 exp (- β) + cos Ψ + D sin Ψ ] -1

where ,

A= 32 n a 2 n s

2 ( n2+ k2) ,

B 1,2 = [ (n a 2 ± n ) 2 + k2 ] [ (n s

2 + n a 2 ) ( n s

2 + n2 + k2 ) ± 4 n a n n s 2 ] ,

C = 2[ ( n a 2 - n2 - k2 ) ( n2 - n s

2 + k2 ) ( n s 2 + n a

2 ) + 8 n a 2 n s

2 k2 ] ,

D = 4n a k [ ( n a 2 + k2 - n s

2 ) ( n s 2 + n a

2 ) - 2 n s 2 (n a

2 – n2 – k2) ] ,

β = 4πkt/λ

Ψ= 4πnt/λ

n, n a , n s - refractive indices of the experimental film , surrounding medium ( air)

and substrate respectively.

λ – wavelength

t – film thickness

The optical band gap of the experimental films was obtained form their absorption edges. Near

the bsorption edge , the absorption coefficient ( α ) can be expressed as

α = [ hʋ – E g ] γ

hʋ – photon energy

E g – band gap

γ - a constant

A plot of α 1/γ as a function of hʋ yields a straight line with an intercept on the

photon energy axis equal to the band gap of the experimental film. In the one – electron

approximation γ equals (½) and (3/2 ) for allowed direct transitons and forbitten direct

transition , respectively ; the constant γ equals 2 for indirect transitions where phonons are

involved and equals 3 for forbidden indirect transitions . In addition , γ equals (½) for all

allowed indirect transitions to exciton states , where an exciton is bound electron- hole pair

with energy levels in the band gap and moves through the crystal lattice as a unit.

Near the optical band gap edge , where the values of hʋ -E g become comparable

with the binding energy of an exciton , the culoumb interaction between the free hole and

electron must be taken into account . For hʋ < E g , the absorption merges continuously into

the absorption caused by the higher excited states of the exciton . When hʋ > E g , higher

energy bands participate in the transition processes , and complicated band structures are

reflected in the optical absorption coefficient .

The refractive index of the film (n) and the film thickness (t) were obtained from the relations

n2 = { [ (n s 2 + n a

2 ) /2 ] + 2 n a n s ( T max – T min / T max T min )} +

{ [(n s 2 + n a

2 ) /2 ] + 2 n a n s ( T max – T min / T max T min ) ] 2 – n a

2 n s 2 } ½

and t = M/2 [ λ 1 λ2 / n1 λ1 – n2 λ2 ] ,

where n a and n s are the refractive indices of the air and substrate respectively ,

n1 and n2 represent refracrtive indices at wavelenght λ 1 and λ2 respectively ,

T max & T min are the transmission maxima and minima and M is the number of oscillations

between two extremes of T - λ curve.

Observations:

From the experimental values of Absorption and Transmittance from the UV visible method

the graphs were plotted

1) Wavelength vs Absorption coefficient

2) Energy vs absorption coefficient

290 340 390 440 490 540 590 640 690 740

0

50000

100000

150000

200000

250000

300000

350000

400000

λ vs α

α=A/(t=500*10^-8) (/cm)

wavelength (λ-nm)

ab

sorp

tion

co

effi

cie

nt (

α /c

m)

3)Energy (ev) vs (αhʋ)^1/2 (ev/cm)1/2

In order to determine the optical band gap parameter Eg , the inset curve of the above figure is

extra polated on its linear region from where Eg = 4.08 ev.

Obviously this band gap indicates that it has an energy gap 4.08 ev so it can be used as a anti

reflecting coating in solar cells.

Transmittance :

1.5 2 2.5 3 3.5 4

0

50000

100000

150000

200000

250000

300000

350000

400000

α vs hʋ

α=A/(t=500*10^-8) (/cm)

hʋ

α

1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3

0.00

200.00

400.00

600.00

800.00

1000.00

1200.00

1400.00

1600.00

1800.00

2000.00

hʋ vs (αhʋ)^1/2

(αhʋ)^1/2

hʋ

(

αh

ʋ)^

1/2

From the experimental values of trasmittance we can plot a graph between Transmittance vs

Wavelength (nm)

3.2.3 X-Ray 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.

The incoming beam (coming from upper left) causes each scatterer to re-radiate a small portion

of its intensity as a spherical wave. If scatterers are arranged symmetrically with a separation d,

these spherical waves will be in sync (add constructively) only in directions where their path-

290 390 490 590 690 790

0

10

20

30

40

50

60

70

80

90

100

Column B

wavelength

tra

nsm

itta

nce

length difference 2d sin θ equals an integer multiple of the wavelength λ. In that case, part of

the incoming beam is deflected by an angle 2θ, producing a reflection spot in the diffraction

pattern.

Crystals are regular arrays of atoms, and X-rays can be considered waves of

electromagnetic radiation. Atoms scatter X-ray waves, primarily through the atoms' electrons.

Just as an ocean wave striking a lighthouse produces secondary circular waves emanating from

the lighthouse, so an X-ray striking an electron produces secondary spherical waves emanating

from the electron. This phenomenon is known as elastic scattering, and the electron (or

lighthouse) is known as the scatterer. A regular array of scatterers produces a regular array of

spherical waves. Although these waves cancel one another out in most directions through

destructive interference, they add constructively in a few specific directions, determined by

Bragg's law:

2dsinθ = nλ

Here d is the spacing between diffracting planes, θ is the incident angle, n is any integer, and λ

is the wavelength of the beam. These specific directions appear as spots on the diffraction

pattern called reflections. Thus, X-ray diffraction results from an electromagnetic wave (the X-

ray) impinging on a regular array of scatterers (the repeating arrangement of atoms within the

crystal).

X-rays are used to produce the diffraction pattern because their wavelength λ is

typically the same order of magnitude (1–100 Ångströms) as the spacing d between planes in

the crystal. In principle, any wave impinging on a regular array of scatterers produces

diffraction

The idea that crystals could be used as a diffraction grating for X-rays arose in

1912 in a conversation between Paul Peter Ewald and Max von Laue in the English Garden in

Munich. Ewald had proposed a resonator model of crystals for his thesis, but this model could

not be validated using visible light, since the wavelength was much larger than the spacing

between the resonators. Von Laue realized that electromagnetic radiation of a shorter

wavelength was needed to observe such small spacing’s, and suggested that X-rays might have

a wavelength comparable to the unit-cell spacing in crystals. Von Laue worked with two

technicians, Walter Friedrich and his assistant Paul Knipping, to shine a beam of X-rays

through a copper sulfate crystal and record its diffraction on a photographic plate. After being

developed, the plate showed a large number of well-defined spots arranged in a pattern of

intersecting circles around the spot produced by the central beam.Von Laue developed a law

that connects the scattering angles and the size and orientation of the unit-cell spacings in the

crystal, for which he was awarded the Nobel Prize in Physics in 1914.

Basic Features of Typical XRD Experiment:

Production

Diffraction

Detection

Interpretation

Fundamental Principles of X-ray Powder Diffraction (XRD):

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 toward 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 sin θ). 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 2θangles, 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.

All diffraction methods are based on generation of X-rays in an X-ray tube.

These X-rays are directed at the sample, and the diffracted rays are collected. A key

component of all diffraction is the angle between the incident and diffractedrays.

XRD working:

X-ray diffractometers consist of three basic elements: an X-ray tube, a sample holder,

and an X-ray detector.

X-rays 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 = 1.5418Å. 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 output 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.

Applications:

X-ray powder diffraction is most widely used for the identification of unknown

crystalline materials (e.g. minerals, inorganic compounds). Determination of unknown solids

is critical to studies in geology, environmental science, material science, engineering and

biology.

Other applications include:

characterization of crystalline materials

identification of fine-grained minerals such as clays and mixed layer clays that are

difficult to determine optically

determination of unit cell dimensions

measurement of sample purity

With specialized techniques, XRD can be used to:

determine crystal structures using Rietveld refinement

determine of modal amounts of minerals (quantitative analysis)

characterize thin films samples by:

determining lattice mismatch between film and substrate and to inferring stress and

strain

determining dislocation density and quality of the film by rocking curve measurements

measuring superlattices in multilayered epitaxial structures

determining the thickness, roughness and density of the film using glancing incidence

X-ray reflectivity measurements

make textural measurements, such as the orientation of grains, in a polycrystalline

sample

Strengths and Limitations of XRD:

Strengths:

Powerful and rapid (< 20 min) technique for identification of an unknown mineral

In most cases, it provides an unambiguous mineral determination

Minimal sample preparation is required

XRD units are widely available

Limitations:

Homogeneous and single phase material is best for identification of an unknown

Must have access to a standard reference file of inorganic compounds (d-spacings,

hkls)

Requires tenths of a gram of material which must be ground into a powder

For mixed materials, detection limit is ~ 2% of sample

For unit cell determinations, indexing of patterns for non-isometric crystal systems is

complicated

Peak overlay may occur and worsens for high angle 'reflections'

Data Collection, Results and Presentation:

Data Collection The intensity of diffracted X-rays is continuously recorded as the sample and

detector rotate through their respective angles. A peak in intensity occurs when the mineral

contains lattice planes with d-spacings appropriate to diffract X-rays at that value of θ.

Although each peak consists of two separate reflections (Kα1 and Kα2), at small values of 2θ

the peak locations overlap with Kα2 appearing as a hump on the side of Kα1. Greater

separation occurs at higher values of θ. Typically these combined peaks are treated as one. The

2λ position of the diffraction peak is typically measured as the center of the peak at 80% peak

height.

Data Reduction

Results are commonly presented as peak positions at 2θ and X-ray counts (intensity) in the

form of a table or an x-y plot (shown above). Intensity (I) is either reported as peak height

intensity, that intensity above background, or as integrated intensity, the area under the peak.

The relative intensity is recorded as the ratio of the peak intensity to that of the most intense

peak ( relative intensity = I/I1 x 100 ).

Observations:

The XRD pattern shows no sharp peaks at all angles being scanned which shows that the La2O3

thin film is amorphous in nature.

4. SUMMARY:

Thin film is layer with high surface to volume ratio . Range of thickness of thin

film is from a few Å to about 5000 Å depending on the properties to be investigated.

Based on the thickness of the thin film we can classsify as ultra thin , thin ,

comparitively thicker. Thin films possess advantages of high compatibility and high

reliability. Because of these advantages thin films are used as capacitors, inductors , solar

cells, resistors, sensors and anti reflection coatings. They are prepared by various Physical and

Chemical methods.

La2O3 is a chemical compound which contains Lanthanum and Oxygen . It is

used to develop Ferro electric and optical materials. It is mostly applicable as anti reflection

coating in the fabrication of solar cells and it has a hexagonal structure.

We have prepared La2O3 thin film by resistanec heating evaporation method

and we have used the vacuum about 10-6 torr by using rotary pumps as the primary pump

and diffusion pump as the secondary pump . The temperatures were about 1500 0 C. The boat

used was made of Tantalum which is a refractory metal. The substrate used was made up of

glass. Pirani gauge was used to measure the vacuum.

The film characterisation is done by Tolansky method to find the thickness, UV

visible spectroscopy to study optical properties and XRD to study the structural properties .

The thickness was measured to be about 500 A 0 using the Tolansky method. The

Band Gap was obtained form the UV visible spectrophotometer as 4.08 ev. The graphs

obtained from XRD indicate that the thin film deposited has amorphous nature.

All the obtained characteristics indicated that the La2O3 could be a promising

candidate for Anti reflection coating in solar cell.

The instruments we have used :

UV visible spectrometer

X Ray Diffractometer