39

CHAPTER-II

RF SPUTTERING TECHNIQUE AND CONDITIONS FOR

DEPOSITING MgIn2O4 FILMS

2.1 INTRODUCTION TO THIN FILMS

With recent advances in engineering, thin film technology is playing an

increasingly important role in spearheading technological advancement of future

society. These advances manifest themselves in numerous application areas: including

substrate patterning, under bump metallurgy, thin film filters and optical coatings for

fiber optic telecommunication systems, high speed machining and grinding, solid

lubrication and heat dissipation as well as in bio-active coatings for medical implant.

Aside from traditional applications, thin film technology is also closely tied to

nanotechnology, which is one of the key technologies of the near future. Nano

composite, nanophase or nanostructure bulk materials and coatings will become

tomorrow’s workhorse in new generation manufacturing and precision engineering

industries. Thin film coating is already an important part of precision engineering and

it will play an ever-increasing role in nanotechnology. Thin films play a dominant role

in modern technology like opto-electronics, microelectronics etc. The study of

surfaces and thin films overlaying them has been carried out for many years. But

recently, it has become increasingly important in several fields of study. The study of

thin surface films has the advances in solid state physics since 1930. Generally the

films have thickness between 0.1µm and 300µm and must be chemically stable,

adherent well to the surface, and uniform, pure and low density of imperfections.

There are a number of different techniques that facilitate the deposition or formation

of very thin films (of the order of micrometers or less) of different materials on a

40

silicon wafer (or other suitable substrate). These films can then be patterned using

photolithographic techniques and suitable etching techniques. Common materials

include silicon dioxide (oxide), silicon nitride, polycrystalline silicon and aluminum.

Materials that can be deposited as thin films include noble metals like gold.

Electrochromic materials are able to change their optical properties in a reversible and

persistent way under the application of a voltage pulse. These materials are currently

of interest for displays, rear-view mirrors and smart windows. Basically this arises

from the numerous inherent characteristics of nucleation and growth of thin film. The

enormous flexibility provided by the thin film growth processes allows the fabrication

of desired geometrical, topographical, physical, crystallographic and metallurgical

structures in two or lesser dimensions. These features are increasingly being exploited

to tailor make the structure sensitive to physical, mechanical, chemical and

electrochemical properties of micro materials.

The thin film deposition involves six important sequential steps:

1. The arising of atoms and molecules that absorb on the surface.

2. The diffusion of such atoms and molecules to some distance before being

incorporated into thin film.

3. The incorporation involves reaction of the absorbed species with each other

and to the surface to form the bonds of the film material.

4. This initial aggregation of the film material is called nucleation.

5. As the film grows thicker it develops a structure or morphology ranging from

amorphous to polycrystalline to single crystal.

41

6. Finally diffusion occurs within the bulk of the film and with the substrate.

Film coating is an important part of precision engineering and it will play an

ever-increasing role in nanotechnology.

The distinction between thin-film and thick-film technology is that the former

involves deposition of individual molecules, while the latter involves deposition of

particles.

2.2 APPLICATIONS OF THIN FILMS

Thin film science and technology play a crucial role in microelectronics,

communications, optoelectronics, integrated optics, photovoltaic devices and

waveguides. Thin film device for windows, mirrors, rechargeable Li ion batteries and

space applications have become an essential part of modern technology [1, 2]. A thin

material created by an atom/ molecule/ ion/cluster of species – by –atom/ molecule

/ion/ cluster of species condensation process is defined as “Thin film”. Thin film

materials may also be formed from a liquid or paste, which is called as “thick film” A

great advantage with a thin film device, is the small amount of material used and the

compact volume of the device. In as energy efficient window in a warm climate for

example, a coating i.e., transparent to visible light but reflecting in the near IR region

may be used, thereby preventing overheating inside the building [3].

2.3 VARIOUS THIN FILM PREPARATION TECHNIQUES

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

semiconductors, insulators or dielectrics etc. and for this purpose various preparative

techniques have also been developed. Newer methods are also being evolved to

improve the quality of the deposits with maximum reproducible properties and

minimum variation in their compositions [4].

42

Thin film deposition can be broadly classified as either physical vapor

deposition or chemical vapor deposition. Vacuum evaporation and sputtering comes

under physical methods, where the deposition takes place after the material to be

deposited has been transferred to a gaseous state either by evaporation or an impact

process. The gas phase chemical processes such as conventional chemical vapor

deposition, photo CVD and plasma-enhanced CVD comes under chemical methods.

Thermal oxidation is a chemical thin film process in which the substrate itself

provides the source for the constituent of the oxide. Liquid phase chemical techniques

include electrolytic deposition, electroless deposition, electrolytic anodization, spray

pyrolysis and liquid phase epitaxy. Molecular beam epitaxy (MBE) is a sophisticated,

finely controlled evaporation technique performed in ultra high vacuum. There are

other method based on the application of an ion or ionized cluster source, and these

can be treated as variants of the physical methods of film deposition. Reactive

deposition makes use of a reactive component for the deposition of compound films

[5]. The choice of a preparative technique is, however, guided by several factors

particularly the melting point of the charge, its stability, desired purity and

characteristics of deposits etc. and these can often be achieved by several methods [6].

Thus various methods can be used for MIO film deposition. But the most widely

reported techniques in the literature and most widely used in industry are DC/RF

magnetron sputtering, pulsed laser deposition and spray pyrolysis. Extract details

about the various thin film preparation techniques are given by Maisel and Glang[7]

and Chopra[8]. In this study, MIO films were produced by RF-magnetron sputtering

presented in detail. In addition, since the properties of MIO depend strongly on the

microstructure, stoichiometry and the nature of the impurities present, it is inevitable

43

that each deposition technique with its associated controlling parameters should yield

films with different characteristics. Some of these issues are discussed briefly.

Table. 2.1. Survey and Classification of Thin-Film Deposition Technologies [9]

EVAPORATIVE METHODS

Vacuum Evaporation Conventional vacuum evaporation Electron-beam evaporation

Molecular-beam epitaxy (MDL) Reactive evaporation

GLOW-DISCHARGE PROCESSES • Sputtering Diode sputtering Reactive sputtering Bias sputtering (ion plating) Magnetron sputtering Ion beam deposition Ion beam sputter deposition Reactie ion plating Cluster beam deposition (CBD)

•Plasma Processes Plasma-enhanced CVD Plasma oxidation Plasma anodization Plasma polymerization Plasma nitridation Plasma reduction Microwave ECR plasma CVD Cathodic arc deposition

GAS-PHASE CHEMICAL PROCESSES • Chemical Vapor Deposition (CVD CVD epitaxy Atmospheric-pressure CVD (APCVD) Low-pressure CVD (LPCVD) Metalorgainc CVD (MOCVD) Photo-enhanced CVD (PHCVD) Laser-induced CVD (PCVD) Electron-enhanced CVD

• Thermal Forming Processes Thermal oxidation Thermal nitridation Thermal polymerization Ion implantation

LIQUID-PHASE CHEMICAL TECHNIQUES • Electra Processes Electroplating Electroless plating Electrolytic anodization Chemical reduction plating Chemical displacement plating Electrophoretic deposition

• Mechanical Techniques Spary pyrolysis Spray-on techniques Spin-on techniques Liquid phase epitaxy

44

2.3.1. Pulsed laser deposition

One of the newer techniques for depositing thin films makes use of the

interaction of laser beams with material surfaces. Lasers were used in assorted

applications involving materials processing and surface modification before

techniques were developed to capitalize on them as a heat source for the flash

evaporation of thin films. Early experimentation with laser evaporation sources in the

1970s culminated in the successful deposition of stoichiometric, mixed-oxide films by

the late 1980s. In its simplest configuration, a high-power laser situated outside the

vacuum deposition chamber is focused by means of external lenses onto the target

surface, which serves as the evaporation source. Irrespective of laser used, the

absorbed beam energy is converted into thermal, chemical and mechanical energy,

causing electronic excitation of target atoms, ablation and exfoliation of the surface,

and plasma formation. Evaporants form a plume above the target consisting of a

motley collection of energetic neutral atoms, molecules, ions, electrons, atom clusters,

micron sized particulates and molten droplets. The plume is highly directional and its

contents are propelled to the substrate where they condense and form a film. A single

homogeneous, multielement target is usually sufficient for the deposition of

individual films, e.g., a sintered powder compact target to deposit mixed oxide films.

MIO thin films were prepared by pulsed laser deposition technique by Kudo et al. and

his coworkers [10].

2.3.2 Magnetron Sputtering

The ejection of atoms from the surface to bombardment by positive ions,

usually inert, is commonly known as (cathode) sputtering. When the ejected atoms are

made to condense on a substrate, thin film deposition takes place.

45

Magnetron sputtering is a magnetically enhanced sputtering technique

discovered by Penning and subsequently developed. For a simple planar magnetic

system, a planar cathode is backed by permanent magnets that provide a toroidal field

with field lines forming a closed path over the target (cathode) surface. The secondary

electrons generated are trapped in cycloidal orbits near the target and prevent self-

heating of the substrate. The confinement of the plasma and the resultant intense

plasma allow magnetron sputtering systems to operate at much lower pressures and

lower target voltages than are possible for RF diode sputtering. Also here the

deposition rates are relatively higher and cover large deposition areas. Low substrate

heating allows the use of a variety of substrate for a wide variety of applications.

Many coworkers [11, 12, 13, and 14] prepared MgIn2O4 thin films by sputtering

technique.

2.3.3 Spray Pyrolysis

The spray method depositing thin films is quite simple; it uses inexpensive

equipment to make coatings over large areas. Unlike the other chemical solution

deposition techniques, the film is found on a substrate kept outside the solution. The

solution is sprayed onto a heated substrate, where the film is formed either by

pyrolytic or hydrolytic chemical reaction of the liquid droplets, the hot substrate

providing the thermal energy. Different techniques for spraying that have been

employed include standard sprayers using compressed argon gas as a carrier gas and

ultrasonic spraying using nozzle sprayers. Spray pyrolysis involves spraying onto hot

substrates on aqueous solution containing soluble salts in appropriate ratio of

constituent elements. The sprayed droplets undergo an endothermic reaction on the

surface of the substrate. The heat of the substrate initiates the chemical reaction by

providing the necessary thermal energy for decomposition of reacting materials into

46

its constituents and recombination of these constituents to form desired oxide films.

The other volatile by products and excess solvents formed during this process escape

in the vapour phase. Moses Ezhil Raj et al. have deposited MIO thin films by

chemical spray pyrolysis technique [15, 16, and 17].

2.4 LITERATURES ON MAGNESIUM INDIUM OXIDE

MgIn2O4, a mixed oxide with spinel structure was prepaned by combustion

systhesis by S.Esther Dali, et al., [18]. The band gap was found to be 4.3 eV which is

higher than that of ITO. The conductivity was found as 1.56 x 10-3 Scm-1. The particle

size was found as 2-3 μ m. MIO has been prepared by solid state reaction by S.Esther

Dali et al., [19]. MgIn2O4 thin films were deposited on glass substrates by pulsed laser

deposition [15] technique by Atsushi kudo et.al [10]. The maximum electrical

conductivity of the film at room temperature reached 1.3 x 103 Scm-1. The carrier

concentration was 3.6 x 1021 cm-3 and mobility was 2.4cm2 V-1s-1.

Magnesium Indium Oxide thin films were deposited onto a silica glass plate

by the RF magnetron sputtering method by Hiroshi Un’no et.al. [12,14]. The highest

conductivity observed for the film post annealed under H2 flow was 2.3 x 102 S/cm,

with a carrier concentration of 6.3 x 1020cm-3 and a mobility of 2.2cm2 V-1s-1. Target

materials used were ceramics. Sputtering conditions were clearly illustrated. Optical

transmission measurements showed no absorption bands in the visible wavelength

region.

Proton implantation on nanocrystalline MgIn2O4 films were examined by

Hideo Hosono et al. [13]. The optical and electrical properties were examined.

Electronic conductivities in MgIn2O4 sputter - deposited films at 300K increased from

10-7 Scm-1 to 1.5 x 10-1 Scm-1 on implantation of H+. Electrical properties of Mg-In-

47

Oxide spinel solid solution were studied by Tanji et.al prepared by solid state reaction

[20].

MgIn2O4 was prepared by solid state reaction by Prasad Manjusri Sirimanne

et al. [21]. Microcrystals of MgIn2S4 were formed on sintered MgIn2O4 pellets by

sulfurizing in a H2S atmosphere. Band gap of MgIn2S4 was evaluated as 2.1eV.

Polycrystaline samples of MgIn2O4 were synthesized by solid state reaction by

Naoyuki Ueda et al [22]. Optical band gap was found to be wider than ITO (~3.4eV).

Electrical conductivity has reached almost 102 Scm-1 at room temperature with no

intentional doping.

New materials for a transparent conducting oxide film are demonstrated by

Minami et.al [11], who prepared MgIn2O4 thin films by conventional RF magnetron

sputtering. The greatest transparency was obtained in the MgIn2O4 film. The

properties of the deposited films were reported. The resistivity decreases from 2 x 10-3

to 3.9 x 10-4 Ω cm. The optical band gap energy is 3.4eV.

Conversion of insulating thin films of MgIn2O4 into transparent conductors by

ion implantation was done by Hosono et.al [23]. Electrical conductivities in RF

sputter- deposited thin films of MIO at room temperature increased from < 10-7 to ~

10Scm-1 (n-type conduction) in the as -implanted state, when implanted with H+ or

Li+ to a fluence of 2 x 1016cm-2. The efficiency of carrier generation was ~20% in the

as- implanted state and ~ 40% after annealing at 3000C for H+ and Li+ respectively.

Thickness measurements were done by stylus was ~ 1.3 μ m. The crystallite size was

5-8 nm in diameter.

48

Generation of electron carriers in insulating thin films of MgIn2O4 spinel by

Li+ implantation was done by kawazoe et.al [24]. Conductivity at room temperature

increased from σ <10-7 to ~ 101 Scm-1 upon the Li+ implantation. Two optical-

absorption bands were induced upon the implantation, one at about ~ 500nm and

another above ~ 1000nm extending to the IR region, which was attributed to plasma

oscillation of electron carriers. Chemical analysis by the inductively coupled plasma

(ICP) method on a film 200Å thick showed the In/Mg atomic ratio to be ~ 2.01.

A novel ternary oxide compound magnesium indate film, MIO, manifesting

high transparency and conductivity has been prepared by spray pyrolysis technique by

Moses Ezhil Raj et.al and his coworkers. For the first time [16, 17, 25] conductivity of

the film deposited at 450oC is in the order of 0.15-1.24 x 10-4 S cm-1. The properties

of MgIn2O4 films were first reported by Moses Ezhil Raj et al. Apart from the

optoelectronic properties, they studied the surface compositional analysis.

MgIn2O4 films were deposited on to glass and also on quartz substrates to

achieve high transmittance and high electrical conductivity by Moses Ezhil Raj et al

[17]. Room temperature electrical conductivity of 1.5 x 10-5 S cm-1 has been achieved.

Hall measurements showed n-type conductivity with electron mobility value 0.95 x

10-2 cm2 V1 s-1 and carrier concentration 2.7 x 1019 cm-3. The atomic ratio of

magnesium and indium in the film is 0.46. The optical band gap is 3.82 eV.

2.5 SPUTTERING

When a solid surface is bombarded with energetic particles, the surface is

eroded and surface atoms are removed due to collisions between surface atoms and

the energetic particles. This phenomena is named “sputter” or “sputtering”

49

Sputtering was first discovered more than 130 years ago by Grove. At one

time sputter was regarded just as an undesired drift effect which destroys the cathode

and grid in a gas discharge tube.

Today sputtering is widely applied to surface cleaning and etching, for thin

film deposition, for surface and surface layer analysis and for sputter ion sources [26].

The energetic particles which cause sputtering may be ions, neutral atoms,

neutrons, electrons or photons.

2.5.1 Basic Concept of Sputtering

When the surface of solid, i.e. target, is bombarded with ion, several

interactions of ion with the surface are expected.

1. The incident ions are reflected, probably being neutralized in the process.

2. The impact of the ion causes the target to eject a secondary electron.

3. The ion is buried in the target. This is the phenomenon of ion implantation.

4. The ion impact causes some structural rearrangements in target material.

5. The ion impact sets up a series of collisions between atoms of the target,

possibly leading to the ejection of one of these atoms. This is the phenomenon

of sputtering.

Sputtering is predominant at energy region of incident ions, 100eV to 100keV. At

higher energy region, the ion implantation becomes predominant.

2.5.2 Mechanism of Sputtering

Two theoretical models were originally proposed for sputtering [27].

1. The thermal-vaporization theory: the surface of the target is heated enough to

be vaporized due to the bombardment of energetic ions.

50

2. The momentum-transfer theory: surface atoms of the target are emitted when

kinetic moments of incident particles are transferred to target surface atoms.

Influenced factors

Sputtering is caused by the interactions of incident particle with target surface

atoms and the sputtering yield will be surely influenced by the following factors:

1. energy of incident particles

2. target materials

3. incident angles of particles

4. crystal structure of the target surface

Cathode sputtering is used for the deposition of thin films. Several sputtering

systems are proposed for thin-film deposition including dc diode, rf diode, magnetron

and ion-beam sputtering.

But here, the planar magnetron RF sputtering was used to deposit the

Magnesium Indium Oxide thin film on the glass substrate.

2.5.3 Advantages of Sputtering Technique over other Techniques

The following are the advantages of the magnetron sputtering, [28].

1. Low substrate temperatures (down to room temperature).

2. Good adhesion of films on substrate.

3. High deposition rates (up to 12µm min-1).

4. Very good thickness uniformity and high density of the films.

5. Good controllability and long-term stability of the process.

51

6. Alloys and compounds of materials with very different vapor pressures

can be sputtered easily.

7. By reactive sputtering in rare/reactive gas mixtures many compounds

can be deposited from elemental (metallic) targets.

8. Relatively cheap deposition method.

9. Scalability to large areas (up to 3x6 m2).

2.5.4 Sputtering system

Several sputtering systems are proposed for thin film deposition. Among these

sputtering systems the basic model is the dc diode sputtering system. The other

sputtering systems are improved systems of the dc diode sputtering.

2.5.4.1 dc diode sputtering

The dc diode sputtering system is composed of a pair of planar electrodes. One

of the electrodes is cold cathode and the other is anode. Top of the cathode is covered

with target materials to be deposited and reverse side of the cathode is water-cooled.

The substrates are placed on the anode. When the sputtering chamber is kept at Ar

gas of 1 x 10-1 Torr and several kilovolts of dc voltage with series resistance of 1 to 10

kΩ are applied between the electrodes, the glow discharge is initiated. The Ar+ ions in

the glow discharge are accelerated at the cathode fall and sputter the target results in

the deposition of thin film on the substrates.

In the dc diode system the sputtered particles collide with gas molecules and

then arrive at the substrates since the gas pressure is so high and the mean free path of

the sputtered particles is less than the electrode spacing. Then the amount of the

sputtered deposit on unit substrate area W is given by

52

W pl/Wk 01≈ ……………………2.1

and the deposition rate R is given by

R≈W/ρt ...………………….2.2

where k1 is a constant, W0 is the amount of sputtered particles from unit cathode area,

p is the discharge gas pressure, 1 is the electrode spacing, ρ is the density of the

sputtered films, t is the sputter time. The W0 is given by

W0= (i+/e)S. t (A/N) .....…………………2.3

where i+ is ion current density at cathode, e is the electron charge, S is sputtering

yield, A is atomic weight of sputtered materials; N, Avogadro number. The ion

current is nearly equal to discharge current Is and the sputtering yield is proportional

to the discharge voltage Vs, the total amounts of sputtered particles becomes VsIst/pl.

Thus, the sputtered deposit is proportional to the sputering power (Vs Is t) and is

inversely proportional to the (pl). Fig.2.1. depicts a simplified sputtering system

capable of depositing metal films. Inside is a pair of parallel metal electrodes, one of

which is the cathode or target of the metal to be deposited. It is connected to the

negative terminal of a DC power supply and typically, several kilovolts are applied to

it. Facing the cathode is the substrate or anode, which may be grounded, biased

positively or negatively, heated, cooled, or some combination of these. After

evacuation of the chamber, a working gas, typically argon, is introduced and serves as

the medium in which an electrical discharge is initiated and sustained. Gas pressures

usually range from a few to a hundred millitorr. After a visible glow discharge is

maintained between the electrodes, it is observed that a current flows and metal from

the cathode deposits on the substrate.

53

Microscopically, positive gas ions in the discharge strike the cathode and

physically eject or sputter target atoms through momentum transfer to them. These

atoms enter and pass through the discharge region to eventually electrons, desorbed

gases, and negative ions) as well as radiation (X-rays and photons) are emitted from

the target. The electric field accelerates electrons and negatively charged ions toward

the anode substrate where they impinge on the growing film.

2.5.3.2 RF diode sputtering

Simple substitution of an insulator for metal target in a DC diode, sputtering

cannot sustain the glow discharge because of the immediate buildup of a surface

charge of positive ions on the front side of the insulator. To sustain the glow discharge

with the insulator target, the DC voltage power supply of the DC diode sputtering is

replaced by RF-power supply. This system is called RF-diode sputtering, The RF

diode sputtering system requires an impedance-matching network between power

supply and discharge chamber.

In the RF diode sputtering, the target current density iS is given by

iS ≈ C dtdV .................…………..2.4

where C is capacitance between discharge plasma and the target, dV/dt denotes the

time variations of target surface potential. This indicates that the increase of the

frequency increases the target ion currents. In practical system the frequency used is

13. 56MHz.

It is noted in the RF discharge system the operating pressure is lowered as low

as 10-3 Torr, since the RF electrical field in the discharge chamber increases the

collision probability between secondary electrons and gas molecules. In the RF

54

sputtering system a blocking capacitor is connected between the matching network

and the target. The target area is much smaller than grounded anode and chamber

wall. This asymmetric electrode configuration induces negative dc bias on the target.

The induced dc bias causes the sputtering in the RF system. The dc bias is in the order

of peak-to-peak voltage of RF power supply.

2.5.3.3 Magnetron Sputtering

Of three magnetron configurations, the planar one with parallel target and

anode electrode surfaces is most common [19]. In this geometry a typical DC electric

field of E ~ 100 V/cm is impressed between the target and anode plates. Small

permanent magnets are arranged on the back of the target in either ellipse-like or

circular rings depending on whether the targets are rectangular of circular in shape.

When a magnetic field of strength B is superimposed parallel to the electric field E

between the target and substrate, charged particles within the dual field environment

experience the well-known Lorentz force in addition to electric field force, i.e.,

⎟⎠⎞

⎜⎝⎛ +−==

→→→→

→

BXVEqdt

VmdF ..............……………2.5

where q, m, and V are the electron charge, mass, and velocity, respectively. First

consider the case where B and E are parallel as shown in Fig. 2.2a. Only electrons

will be considered because, their dynamical behavior controls glow-discharge

processes. When electrons are emitted exactly normal to the target surface or parallel

to B and E, then V x B vanishes; electrons are only influenced by the E field and

simply accelerate toward the anode, gaining kinetic energy in the process. If, however

E = 0, and the electron is launched with velocity V at an angle θ with respect to the

uniform B field between electrodes (Fig. 2.2b), it experiences a force qVB sinθ in a

55

direction perpendicular to B. The electron now executes a circular motion whose

radius r is determined by a balance of the centrifugal (m(V sinθ)2/r) and Lorentz

forces involved, i.e.,

r = qB

mV θsin ………………………. 2.6

A spiral electron motion ensures and in corkscrew fashion the electron returns

to the same radial position around the axis of the field lines. If the magnetic field was

not present, such off-axis electrons would tend to migrate out of the discharge and be

lost at the walls.

The case where electrons are launched at an angle to parallel and uniform E

and B fields is shown in the (Fig. 2.2c). Helical motion with constant radius occurs,

but because of electron acceleration in the E field the pitch of the helix lengthens with

time. Time-varying fields complicate matters further and electron spirals of variable

radius can occur. Clearly magnetic fields prolong the electron residence time in the

discharge and enhance the probability of ion collisions.

Perpendicular Fields

Through the application of perpendicular electric and magnetic fields even

greater electron confinement is achieved. The geometry is shown in Fig. 2.2d, where

E is still normal to the cathode while B, which is directed into the page (+ z direction),

lies parallel to the cathode plane. Electrons emitted normally from the cathode ideally

do not even reach the anode but are trapped near the electrode where they execute a

periodic hopping motion over its surface. Physically, the emitted electrons are initially

accelerated towards the anode, executing a helical motion in the process; but when

they encounter the region of the parallel magnetic field, they are bent in an orbit back

56

to the target in very much the same way that electrons are deflected toward the hearth

in an electron beam evaporator. The analysis for this behavior is not difficult and

starts with the equations for electron motion in the three perpendicular directions.

Coordinate positions of the electron above and along the cathode are y and x,

respectively. Applying the Lorentz equation we have

me 2

2

dtxd = qB

dtdy ..............……………..2.7

me 2

2

dtyd = qE – qB

dtdx dx/dt ………………..............2.8

me 2

2

dtzd = 0 ………………………2.9

By solving these coupled differential equations it is readily shown that the parametric

equations of motion are

y = - 2

)cos1(

ce

c

mtqE

ϖϖ−

…………………......2.10

x = ⎟⎟⎠

⎞⎜⎜⎝

⎛−

tt

BEt

c

c

ϖϖsin

1 ……………………..2.11

where ωc= qB/me. Known as the cyclotron frequency, ωc has a value of 2.8 x 106B Hz

with B in gauss. Physically, these parametric equations describe a cycloidal motion

where electrons repeatedly return to the cathode at time intervals of π/ωc. The same

motion is traced out by a point on the circumference of a circle rolling on a planar

surface. Electron motion is strictly confined to the cathode dark space where both

fields are present; however, electrons stray into the negative glow region where ‘ is

small, they describe a circular orbit before collisions may drive them either back into

the dark space or forward toward the anode. Confinement in crossed fields prolongs

the electron lifetime over and above that in parallel fields, enhancing the ionizing

efficiency near the cathode. A denser plasma and larger discharge currents result.

57

2.6 THIN-FILM NUCLEATION

After exposure of the substrate to the incident vapor, a uniform distribution of

small but highly mobile clusters or islands is observed. In this stage the prior nuclei

incorporate impinging atoms and subcritical clusters and grow in size while the island

density rapidly saturates. The next stage involves merging of the islands by a

coalescence phenomenon which, in some cases, is liquid-like, especially at high

substrate temperatures. Coalescence decreases the island density resulting in local

denuding of the substrate where further nucleation can then occur. Crystallographic

facets and orientations are frequently preserved on islands and at interfaces between

initially disoriented, coalesced particles. Continued coalescence results in the

development of a connected network with unfilled channels in between. With further

deposition, the channels fill in and shrink leaving isolated voids behind. Finally, even

the voids fill in completely and the film is said to be continuous. This sequence of

events occurs during the early stages of deposition, typically accounting for the first

few hundred angstroms of film thickness.

Many observations of subsequent film formation have pointed to three basic

growth modes: (1) island (or Volmer-Weber), (2) layer (or Frank Van der Merwe),

and (3) Stranski Krastanov, which are illustrated schematically in Fig.2.3. Island

growth occurs when the smallest stable clusters nucleate on the substrate and grow in

three dimensions to form islands. This happens when atoms or molecules in the

deposit are more strongly bound to each other than to the substrate. Metal and

semiconductor films deposited on oxide substrates initially form such islands. The

opposite characteristics are displayed during layer growth. Here the extension of the

smallest stable nucleus occurs overwhelmingly in two dimensions, resulting in the

formation of planar sheets. In this growth mode the atoms are more strongly bound to

58

the substrate than to each other. The first complete monolayer is then covered with a

somewhat less tightly bound second layer. Providing the decrease in bonding energy

is continuous toward the bulk-crystal value, the layer growth mode is sustained. The

layer plus island or Stranski-Krastano (S-K) growth mechanism is an intermediate

combination of the preceding two modes. In this case after forming one or more

monolayers, subsequent layer growth becomes unfavorable and islands form. Film

growth by the S-K mode is fairly common and has been observed in metal-metal and

metal-semiconductor systems.

2.7 PLANAR MAGNETRON RF SPUTTERING INSTRUMENT

At present, the planar magnetron is indispensable for the fabrication of

semiconductor devices. Historically, magnetron sputtering was first proposed by

Penning in 1936. A wide variety of thin films can be made with little film

contamination and at a high deposition rate by the low-pressure sputtering technique.

The planar magnetron sputtering consists of a water-cooled cathode made of

copper on which planar targets of any material bounded to a copper backing plate can

easily be screw-mounted. The cathode is insulated from a water-cooled aluminum

shield with a Teflon spacer and is kept in position using a stainless-steel nut. The

magnets are mounted outside the shield. The whole assembly is affixed to a stainless-

steel base plate, which is placed in a bell jar in which sputtering is carried out. The

discharge voltage is 300 to 800V where the maximum sputtering yield per unit energy

is obtained.

In this present work, the sputtering instrument used for coating the MIO film

was the Planar Magnetron RF/DC HindHivac sputtering system (Model-12” MSPT)

59

in a mechanically pumped stainless steel chamber. The schematic representation of

the sputtering unit is shown in Fig.2.4.

A 2-inch diameter and 5mm thick, MIO target of 99.99% purity from “Super

Conductor Materials, Inc, USA” and a corning glass substrate is loaded inside the

chamber. The photograph of the HINDHIVAC sputtering unit used for preparing thin

films is shown in Fig. 2.5. In an AC sputtering process, the substrate is electrically

grounded while a radio frequency (13.56 MHz) AC voltage is applied to the target. A

capacitor is connected between the RF source and the sputtering system. Here, the

front surface of the cathode is covered with target materials to be deposited. The

substrates are placed on the anode. The sputtering chamber is filled with sputtering

gas, typically argon gas. The glow discharge is maintained under the application of rf

potential. The Ar+ ions generated in the glow discharge are accelerated at the cathode

fall (sheath) and sputter the target, resulting in the deposition of the thin films on the

substrates.

A specific pressure of gas, in this case about 5x10-2 torr of Ar, is maintained

during the sputtering process. The strong AC electric field accelerates electrons that

collide with Ar molecules, ionizing some of the Ar molecules and producing more

electrons to start a glow discharge (plasma). In the plasma the Ar molecules get

ionized. The Ar ions and electrons bombard the target alternatively as the AC field

changes polarity. Since electrons are much lighter and thus have greater mobility than

positive ions, the target draws a much higher electron current than ion current under

the same potential. Therefore, the capacitor in the circuit is always recharged more

rapidly by electron current than ion current. When the AC frequency is sufficiently

high (> 1MHz), the net effect of this inequality of mobility of positive ions and

electrons is a negative self-bias of the target. This self-bias is equivalent to a negative

60

“DC offset” of the AC voltage. Since the time dependence of the AC voltage is

approximately a sinusoidal function, the DC offset reduces the positive target bias to a

very small fraction of the full amplitude. The positively charged Ar thus has a net

acceleration towards the target and bombards the target almost continuously. Due to

gas law of working pressure, the sputtered particles traverse the discharge space

without collisions, which results in a high deposition rate [29].

The most important difference between RF and DC systems is that the former

requires an impedance matching network between power supply and discharge

chamber. It is also important that in RF system, adequate grounding of the substrate

assembly be ensured to avoid undesirable RF voltages, which can develop on the

surface. The schematic illustration of the RF magnetron sputtering source is shown in

the Fig. 2.6. The semicylindrical shaped deposition chamber is made up of stainless

steel, which is a cold water cooled. It includes two plane cicular sputter sources

(targets) with a diameter of 5 cm each.

Thus, the use of RF sputtering for the deposition of thin films is of great

interest because it enables more economical deposition on to substrates of large areas

[30].

The sputtering deposition system consists of a vacuum chamber, where the

deposition occurs, a pumping system, a gas supply system and a power supply system.

The semi-cylindrical shaped deposition chamber is made up of stainless steel material.

Permanent magnets are placed at the back of the sputter sources in a suitable

arrangement for the production of the required magnetic field. The sputter sources are

cooled by water circulation during the sputtering process. There is a possibility of

changing the sputter sources according to the different kind of materials to be

deposited. At a distance of about 6 cm above the sputter sources and parallel to them

61

there is a substrate holder, which can be rotated by the user. On the substrate holder,

plane circular sample holders with a diameter of 9 cm are fixed which can be easily

transferred outside of the chamber for placing and exchanging the samples, which

form the substrate for the deposited material. In our unit we are using rotary vane

pump and oil diffusion pump to attain the vacuum in the deposition chamber

approximately 10-6 mbar. The following are the internal operations of the rotary and

diffusion oil pumps.

(i) Rotary pump

The rotary piston and rotary vane pumps are the two most common devices

used to attain reduced pressure. In the rotary piston pump gas is drawn into the keyed

shaft rotates the eccentric and piston. There the gas is isolated from the inlet after one

revolution, then compressed and exhausted during the next cycle.

Hind Hivac coating unit is having direct driven rotary vane type vacuum

pump. The rotary vane pump contains an eccentrically mounted rotor with spring

loaded vanes. During rotation the vanes slide in and out within the cylindrical interior

of the pump, enabling a quantity of gas to be confined, compressed and discharged

through an exhaust valve into the atmosphere. The whole stator/rotor assembly is

submerged in suitable oil. Single stage vane pumps have an ultimate pressure of 10-2

Torr, and two stage pumps can reach 10-4 Torr. Rotary pumps are frequently used to

produce the minimal vacuum required to operate both oil diffusion and

turbomolecular pumps, which can then attain far lower pressures.

62

(ii) Vapor pump or oil diffusion pump

This is the main pump to obtain the desired high vacuum. Diffusion pumps are

designed to operate in the molecular flow regime and can function over pressures

ranging from well below 10-10 torr to about 10-2 torr.

Diffusion pumps have been constructed with pumping speeds ranging from a

few liters per second to over 20,000 lit/sec. Pumping is achieved through the action of

a fluid medium (typically silicon oil) that is boiled and vaporized in multistage jet

assembly. As the oil vapor stream emerges from the top nozzles, it collides with and

imparts momentum to residual gas molecules, which happen to bound into the pump

throat. These molecules are thus driven towards the bottom of the pump and

compressed at the exit side where they are exhausted. The pump fluid should have a

high molecular weight so that each molecule carries considerable momentum and can

therefore make effective collisions with several gas molecules before all its

momentum is lost. It should have a very low pressure. Silicon oil DC 704 satisfies

these requirements.

The sputtering gas (Ar) is introduced into the chamber for the deposition

process. The gas flow is controlled constantly throughout the sputtering process. The

pressure of the chamber can be measured by two manometers (pressure gauges) which

cover two different regions of pressure values. The pirani type manometer is used for

measuring pressures in the region from atmospheric pressure to 1×10-4 mbar, whereas

for lower pressures until 1×10-6 mbar, the penning type manometer is used.

2.8 SUBSTRATE FOR FILM DEPOSITION

The substrate selection and cleaning procedures are the special art in thin film

deposition. The cleanliness of the substrate is a pre-requisite for obtaining good

63

quality films with reproducible properties. It exerts a decisive influence on the film

growth and adhesion.

2.8.1 Substrate Materials

The nature and surface finish of the substrate are extremely important because

they greatly influence the properties of films deposited on to them. Glass, quartz and

ceramic substrates are commonly used for polycrystalline films. Single-crystal

substrates of alkali halides, mica, MgO, semiconductors etc are used for epitaxial

growth. The most important consideration in all these cases is that the deposit layers

must be adherent to the substrates and these should not peel off from the substrates

under the normal conditions of stress and strain, mechanical or thermal to which the

deposits are exposed during their uses. Substrate selection mainly depends on the type

of thin film to be deposited and also the process by which the film is prepared. Many

times a thin film scientist has to restrict his experiments because of lack of proper

substrate for the desired process. In general the following information may be noted

before selecting the substrate for the thin film deposition. [31, 32, 33]

1. Type of substrates

a. Glass or crystalline materials like silicon and germanium

b. Organics and plastics

c. Metals

2. Rigid or flexible

3. Structure

4. Melting point

5. Hardness

64

6. Thermal conductivity

7. Thermal expansion

8. Transmitting region

9. Refractive index and extinction coefficient

10. Dielectric strength

The substrate can be selected keeping the end use and the properties of the

evaporated/sputtered films.

2.8.2 Substrate Properties

The properties of glasses vary widely depending on their chemical

composition. The major advantage of glasses is that smooth surface can be achieved

directly by drawing, which is reflected in generally low cost. Individually, glasses

vary significantly in regard to volume resistivity, loss tangent and softening points.

Their poor thermal conductivity and the difficulty of obtaining intricate shapes

including holes preclude the use of glasses for many electronic applications. Glass has

high modulus and non-zero coefficient of thermal expansion. Glasses are generally

multicomponent solids that are chemically bonding such that there are no free

electrons. Therefore there is no electrical conductivity.

2.8.3 Substrate Cleaning:

The cleanliness of the substrate surface exerts a decisive influence on film

growth and adhesion. A thoroughly cleaned substrate is a pre-requisite for the

preparation of films with reproducible properties. The choice of cleaning techniques

depends on the nature of the substrate, the type of contaminants, and the degree of

cleanliness required. Residues from manufacturing and packaging, lint, fingerprints,

65

oils and airborne particulate matter are examples of frequently encountered

contaminant. It is generally desirable to limit the removal process to the contaminant

layer, but a mild attack of the substrate material itself is often tolerate and ensures

completeness of the cleaning operation. Some cleaning methods require substrate

handling or the use of solvents and must therefore be applied outside the vacuum

system. The physical cleaning methods are generally conducted in situ by installing

provisions for substrate heating or particle bombardment in the deposition system.

(i) Ultrasonic Cleaning

In ultrasonic cleaning, dissolution of residues is enhanced by the intense local

stirring action of the shock waves created in the solvent. Thus, solvent saturated with

impurities is continually carried away from the substrate surface and fresh, less

saturated liquid is admitted. Mechanical vibrations induced in the substrate further aid

in loosening gross contaminants such as particulate-matter flakes. The parameters

which affect the efficiency of ultrasonic cleaning are numerous. The frequency of

vibration, applied power, type and temperature of the solvent, its surface tension and

viscosity, and the presence of nucleating particles and dissolved gases are factors

which play a role. Low frequency ultrasonic agitation was most effective in removing

gross surface contaminants such as particles and fingerprints.

(ii) Chemical treatment

This method is likely to attack the substrate surface. Acid cleaners react with

contaminants such as grease and some oxides to convert them into more soluble

compounds. The effectiveness of the solvent is the probably more dependent on the

ability to wet the substrate than the solvent action.

66

(iii) Vapor greasing

The sample to be cleaned is to be inserted in the vapor degreasing chamber.

The samples are kept in the bath until they reach the vapor temperature and then they

are removed. The residual alcohol on the surface is evaporated immediately and the

sample is ready for use.

(iv) Glow-discharge Cleaning

This is the most widely used technique to clean substrates in situ and

immediately prior to film deposition. It is affected by exposing the substrates to the

plasma of a glow discharge.

The substrate itself is not part of the glow-discharge circuit as it is in sputter

cleaning. Although the latter is an effective cleaning method, it involves

bombardment of the substrate with high-energy particles, sputtering and possibly

roughening of the substrate surface, as well as deposition of foreign material from the

counter electrode. If it is felt that these effects are not detrimental and sputter cleaning

is preferred over glow-discharge cleaning, it will generally be necessary to employ RF

voltage because of the insulating nature of most substrates.

In glow-discharge cleaning, removal of impurities and other beneficial

changes of the substrate surface are brought about by one or more of the following

mechanisms:

1. Straight forward heating due to impingement of charged particles and their

recombination.

2. Impurity desorption through electron bombardment.

3. Impurity desorption resulting from low-energy ion or neutral-particle

bombardment.

67

4. Volatization of organic residues by chemical reaction with dissociated oxygen.

5. Modification of glass surfaces through the addition of oxygen.

6. Enhanced nucleation during subsequent film deposition.

Very often a substrate material has desirable bulk properties but its surface is

unsatisfactory because of roughness or lack of chemical stability. In these cases, it

may be possible to upgrade the surface by an extra finishing step. The principle of

these procedures is to coat the substrate with a layer of another material which allows

well and adds the desired property without losing the attractive features of the original

substrate.

(v) Washing and Drying

Cleaning in detergent solution was less effective than either of the other

techniques. The drying of wet cleaned substrates is also critical because

recontamination can occur unless stringent precautions are taken. Typically, the last

step in substrate cleaning is rinsing in deionized water. As the latter may contain

traces of salts or organic matter, withdrawal of the substrate should be conducted such

that a minimum of liquid adheres to the surface. If storage cannot be avoided, dust-

free containers with a lid or desicators may be utilized.

In the present work, the corning 7059 glass plates were chosen as substrates

for the deposition of MIO thin films for their high optical and low electrical

conduction characteristics.

The glass substrates were degreased by the following cleaning procedure to

remove the unwanted impurities normally present on the surface of the glass plates

when exposed to the atmosphere.

68

(i) The glass substrates are washed in Sodium thiosulphate solution (extran) by

scrubbing the surfaces with the cotton swab till they pass the breathe figure

test to remove oil, grease etc.

(ii) The glass slides are then rinsed thoroughly in deionized water to remove any

traces of the sodium thiosulphate solution left on the surface.

(iii) Then the substrates are soaked in chromic acid and heated to about one hour to

dissolve the fine silica layer formed on the surface and to make a new surface

for deposition of the film.

(iv) Finally the substrates are placed in the Ultrasonic cleaner then they are rinsed

thoroughly in deionized water and dried with acetone. Now the glass

substrates are ready for the deposition of the films.

2.9 SUBSTRATE TEMPERATURE

The temperature of the substrate surface is important, yet difficult to control.

In conventional sputtering systems, the substrate is mounted on a temperature-

controlled substrate holder. However, the heat of the target heats the surface of the

substrate. Moreover, bombardment by high-energy secondary electrons also heats the

substrate. In order to reduce the effect of the heat, the surface of the target must be

cooled. Bombardment by the secondary electrons is avoided by negatively biasing the

substrate.

It is noted that the temperature rise of the substrate depends on the type of

sputtering system. The temperature rise at magnetron is lower than that of the RF

diode, since the secondary electrons from the target are trapped by the transverse

magnetic field near the surface of the target. The temperature rise at the magnetron

system for laboratory scale is less than 300 °C.

69

It is clearly that the mobility of adatoms and clusters, which is in proportion

to their energy, on the substrate will be increased with increasing the substrate

temperature. Hence, the substrate temperature influences the microstructure and the

orientation of MIO films.

2.9.1 Substrate Temperature Monitoring

The substrate temperature can be measured using a thermocouple. A

thermocouple mounted on the substrate surface measures the substrate temperature. If

a thermocouple cannot be mounted on the substrate surface, dummy substrates can be

used for measuring substrate temperature: the temperature differences between the

substrate and the substrate holder are measured for the dummy substrates. The

temperature of the substrate holder estimates the substrate temperature. If the

substrates are closely mounted on to the substrate holder, the temperature of the

substrate holder controls the substrate temperature.

2.10 TARGETS

At present, targets of virtually all important materials are commercially

available for the production of sputtered thin films. Targets come in a variety of

shapes and in assorted sizes. Ceramic and oxide targets are generally prepared by hot

pressing of powders. The elemental and metal targets generally have purities of

99.99% or better, whereas those for the nonmetals have typical upper purity limits of

99.9%. For microelectronic purposes purities an order of magnitude higher are

demanded together with stringent stoichiometries. While the metal targets usually

attain theoretical densities, lower densities are achieved during powder processing.

These metallurgical processing realities are particularly evident in lower sintered-

density targets, which are prone to greater arcing rates, emission of particulates, and

70

release of trapped gases, nonuniform target erosion, and deposition of generally

inferior films. Prior to use, targets must be bonded to a cooling backing plate to avoid

thermal cracking during sputtering.

In the present study, the Magnesium Indium Oxide (MgIn2O4) target is

purchased from “Super Conductor Materials, Inc, USA” a 2- inch diameter and 5mm

thick, MIO target of 99.99% purity.

2.11 MgIn2O4 THIN FILM PREPARATION

Transparent and conducting films were prepared by RF magnetron sputtering

technique using HINDHIVAC 12” MSPT RF/DC magnetron sputtering unit which

contains 500W RF generator. The de-greased and well cleaned glass plates were used

as the substrates and mounted on the substrate holder of the sputtering chamber which

is facing the sputtering source and parallel to the surface of the target and having the

heating elements to control the substrate temperature during the film deposition. The

target was placed on the base plate of the sputtering source. The target was fixed on

the magnetron sputtering source with a stainless steel backing plate which is designed

for holding the 2 inch target and with the screws.

The whole magnetron source with the target was covered with the guard ring

cover of the magnetron. There is a shutter which separates the target and the

substrates. This helps in preventing the contamination of the target during substrate

loading and unloading, protects the substrates during pre-sputtering. Initially the

shutter was closed.

Now the chamber was closed and evacuated using the rotary and oil diffusion

pumps. After achieving the required vacuum i.e. high vacuum of the order of 5 x 10-6

mbar, the sputtering process was started. As the magnetron source and the wall of the

71

sputtering chamber were water cooled, the water cooler and the water circulating

pump should be switched on before 30 minutes prior to sputtering. Argon (99.999%)

being an inert gas which does not react with either the target or the substrate is

introduced into the chamber at a specified pressure. This gas serves as the source of

Ar+ ions. RF power supply is switched and stabilized to the required specific power

which induces the required DC bias levels. This bias is an indication of sheath

potential and is good sign of ion bombardment energy. With the shutter closed, the RF

power, Ar gas flow and the substrate temperature were stabilized with the constant

glow discharge, since all the parameter depends on each other. Once the above

parameters were stabilized, the target was pre sputtered for 15 mins prior to every

deposition. After the stabilization and pre-sputtering shutter was opened and allowed

to deposit on the substrates which is maintained in a constant temperature. Several

films were deposited by varying the substrate temperature and the RF power. The

deposited films were pin hole free, uniform, transparent and well adhesive nature with

substrates. The RF power was varied as 50W, 100W, 150W, 200W. The substrate

temperature was kept as RT, 100 0C, 150 0C, 200 0C and 250 0C. The target to

substrate distance was kept 6cm throughout all the coating. Duration of the deposition

was 30mins for all the coatings.

2.12 CONCLUSION

This chapter extensively summarized the detailed mechanism of the sputtering

method and describes about the functions of various pumps that have been employed

in the coating unit to create the desired vacuum. The substrate and the cleaning

selections clearly demonstrate some basic qualifications for the substrates. A detailed

method for the preparation of MIO films is also described in this chapter.

72

The various preparative conditions such as source to substrate distance,

deposition rate pressure level, and substrate temperature were altered and optimized

for good quality and device oriented films. The sequential characterization studies on

the films have been carried out to explore their various properties and are summarized

in the proceeding chapters.

73

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