FABRICATION OF SMART GLASS ELECTROCHROMIC DEVICE ...

FABRICATION OF SMART GLASS ELECTROCHROMIC DEVICE

USING RF MAGNETRON SPUTTERING

SITI ASHRAF BT ABDULLAH

A project report submitted in partial

fulfillment of the requirement for the award of the Degree of Master of Electrical

Engineering

Faculty of Electrical and Electronic Engineering

Universiti Tun Hussein Onn Malaysia

JANUARY 2014

v

ABSTRACT

Electrochromic device is an important functional device to control the amount of light

through a glass. It usually used in sunlight control window glazing for buildings and

automobile. The important feature of electrochromic glass is the ability to response

toward the apply voltage in shortest time, and endurance to maintain in color shape after

apply voltage. In this thesis, the oxygen gas percentage is optimized during the

fabrication of tungsten trioxide (WO3) as an electrochromic glass for window glazing

application by using RF magnetron sputtering. The oxygen flow rate for the deposition

is varied from 10sccm -22sccm which is 25%, 27%, 30%, and 35% of oxygen flow. The

structures of WO3 were investigated using X-Ray diffraction, Field effect scanning

electron microscopy (Fe-Sem) and Atomic force microscopy (AFM). The

electrochromic properties were characterized by a cyclic voltammogram and UV-Vis

absorption spectra. The results show that nanocrystalline film with particle size of

51.54nm was deposited at 27% oxygen flow rate has the largest charge capacity and

coloration efficiency among the others. The time respond taken for complete coloration

at 4V is 2second. This result is a starting point for future work such as optimizing the

film thickness or doping by other metals.

vi

ABSTRAK

Filem nipis elektrokromik adalah alat yang sangat penting untuk mengawal jumlah

cahaya yang melalui cermin kaca. Ia biasanya digunakan pada bangunan dan kenderaan

untuk mengawal kemasukan cahaya didalam bangunan atau kenderaan. Ciri-ciri yang

penting dalam elektrokromik adalah keupayaan untuk bertindak balas pada aliran voltan

dalam masa yang singkat, dan ketahanan untuk mengekalkan bentuk warna selepas

dikenakan sejumlah voltan pada sampel. Dalam tesis ini, gas oksigen telah di optimum

dalam menghasilkan tungsten trioksida (WO3) sebagai filem nipis elektrokromik untuk

aplikasi tingkap kaca dengan menggunakan RF magnetron sputtering. Kadar aliran

oksigen untuk pemendapan WO3 adalah 10sccm-22sccm iaitu 25%, 27%, 30%, dan

35% aliran oksigen. Struktur WO3 disiasat menggunakan X-Ray diffraction, Field effect

scanning electron microscopy (Fe-Sem) dan Atomic force microscopy (AFM).Sifat

elektrokromik telah dikenal pasti oleh cyclic voltammogram dan UV-Vis absorption

spectra. Kajian ini mendapati filem nanocrystalline dengan saiz zarah 51.54nm

didepositkan pada 27% kadar aliran oksigen, mempunyai kapasiti caj lebih besar dan

kecekapan warna yang tinggi berbanding dengan filem-filem yang lain. Masa untuk

filem bertindak balas kepada warna diambil pada 4V adalah 2saat. Keputusan ini adalah

satu titik permulaan untuk kerja-kerja masa depan seperti mengoptimumkan ketebalan

filem atau doping dengan logam lain.

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CONTENTS

TITTLE i

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

CONTENT vii

LIST OF TABLES x

LIST OF FIGURES xi

LIST OF SIMBOLS AND ABBREVIATIONS xiii

LIST OF APPENDICES xiv

CHAPTER 1 INTRODUCTION 1

1.1 Introduction to electrochromic material 1

1.2 Problem statement 6

1.3 Motivation 6

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1.4 Objective 6

1.5 Scope of project 7

1.6 Thesis organization 7

CHAPTER 2 LITERATURE REVIEW 8

2.1 History 8

2.2 Background study 10

2.3 Principle operation of electrochromic thin film 13

2.4 WO3 thin films properties 14

2.5 RF Magnetron Sputtering method 17

CHAPTER 3 METHODOLOGY 20

3.1 Introduction 20

3.2 Fabrication process 22

3.2.1 Substrate cleaning process. 23

3.2.2 Deposition 24

3.2.3 WO3 film measurement 26

3.2.3.1 Alpha-Step IQ Surface Profiler 26

3.2.3.2 UV-Visible Spectroscopy (UV-Vis) 27

3.2.3.3 Two point probe (IV characteristic) 28

3.2.3.4 Field emission scanning electron 29

microscope (FE-SEM)

3.2.3.5 Atomic force microscope (AFM) 30

3.2.3.6 X-Ray diffraction (XRD) 31

3.2.4 Fabricate Gold (Au) electrode using 32

DC Sputtering.

3.3 Testing process 33

3.3.1 Inject electrolyte onto WO3 thin film using 33

lithium perchlorate (LiClO4) into propylene

ix

carbonate (PC).

3.3.2 Apply Voltage into WO3 thin film using 35

DC supply

CHAPTER 4 RESULT AND DISCUSSION 37

4.1 Result and discussion 37

4.1.1 Measurement of deposition rate of WO3 using 37

Surface profile

4.1.2 Measure IV-characteristic of WO3 using 39

two-point probe

4.1.3 Relationship between deposition rate and resistivity 43

4.1.4 Measure crystalline of WO3 using XRD 44

4.1.5 Measure surface morphology of WO3 using 46

FE-SEM

4.1.6 Measure roughness of WO3 using AFM 49

4.1.7 Measure transmittance of WO3 using UV-Vis 52

CHAPTER 5 CONCLUSSION AND FUTURE RECOMMENDATION 54

5.1 Conclussion 54

5.2 Future recommendation 55

REFERENCES 56

APPENDICES 62

x

LIST OF TABLES

3.1 Parameter for RF Magnetron Sputtering used to fabricated WO3 25

xi

LIST OF FIGURES

1.1 Electrochromic device configuration consist of two substrates 1

divided by electrolyte.

1.2 Electrochromic Smart window used in a bulding 2

1.3 General schematic of a spray pyrolysis deposition process 3

1.4 General schematic of chemical vapor deposition process. 4

1.5 plasma ionization created during fabrication of Magnetron 5

sputtering process.

2.1 Configuration of electrochromic glass 9

2.2 XRD patterns of the WO3 films deposited using the W target on 11

the glass substrate. RF power:50W, O2: Ar =5:5 [15].

2.3 X-ray diffraction patterns of WO3 films prepared with varying 12

different oxygen gas flow rates [14].

2.4 Schematic structure of the electrochromic process in electrochromic 14

(EC) and ion movement during coloration process.

2.5 Monoclinic crystal structure of WO3 15

2.6 XPS spectra of the valence band and the occupied part of the 16

conduction band for evaporated tungsten oxide films

2.7 Ionization process in Magnetron sputtering chamber during fabrication. 18

3.1 Flow chart of the process for fabricating tungsten trioxide (WO3) 21

as electrochromic films.

3.2 Basic design of electrochromic device including LiClO4+PC 22

electrolyte to transport ions.

3.3 Equipment used to cleaning substrate process 23

3.4 Magnetron sputtering machine 24

3.5 Alpha-Step IQ Surface Profiler machine 26

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3.6 UV-Visible Spectroscopy (UV-Vis) machine 27

3.7 Two point probe machine 28

3.8 FE-SEM machine. 29

3.9 Atomic force microscope (AFM) 30

3.10 X-Ray diffraction (XRD) 31

3.11 DC Sputtering 32

3.12 Measurement of 10.639g of LClO4 using analytical balance 33

3.13 10.639g LIClO4 mixed into 100ml of propylene carbonate (PC) solution 34

3.14 Overlapping of two pieces films with LIClO4/PC electrolyte in the middle 35

3.15 Different coloration of WO3 thin film using various oxygen percentage; 36

(a) 25%, (b) 27%, (c) 30%, (d) 35%

4.1 Deposition rate of WO3 thin films deposited at different oxygen content 38

4.2 IV characteristic for WO3 fabricated at 25% oxygen flow rate 39




4.6 Dependence of the electrical resistivity of WO3 thin films using 42

Various oxygen flow rate.

4.7 Relation between electrical resistivity and deposition rate using 43

various oxygen flow rate.

4.8 XRD pattern of the WO3 thin films using various oxygen flow rate 44

4.9 Surface morphology FE-SEM at 25% oxygen flow rate at 5k and 46

50k scan rate


50k scan rate


50k scan rate

4.12 Surface morphology FE-SEM at 35% oxygen flow rate at 48

5k and 50k scan

4.13 Surface roughness of WO3 thin film at 25% oxygen flow rate 49

4.14 surface morphology of WO3 thin film at 27% oxygen flow rate 49

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4.17 The optical transmittance spectra at transparent state of the WO3 52

thin films deposited on ITO at various oxygen content.

4.18 The optical transmittance spectra at blue color of the WO3 thin films 53

deposited on ITO at various oxygen content.

xiv

LIST OF SYMBOLS AND ABBREVIATIONS

EC - Electrochromic

WO3 - Tungsten trioxide

NiO - nickel oxide

MoO3 - molybdenum trioxide

IrO3 - iridium trioxide

W - Tungsten

Ti - Titanium

V - Vanadium

Nb - Niobium

Ta - Tantalum

Mo - Molybdenum

Cr - Chromium

Mn - Manganese

Fe - Iron

Co - Cobalt

Ni - Nikel

Rh - Rhodium

Ir - Iridium

CVD - chemical vapor deposition

LiClO4 - Lithium perchlorate

PC - Propylene carbonate

NiOOH - Nickel oxyhydroxide

HCl - Hydrochloric acid

NaOH - Sodium hydroxide

xv

IZO - Aluminum doped zinc oxide

FTO - fluorine-doped tin oxide

FE-SEM - Field emission scanning electron microscope

AFM - Atomic force microscope

XRD - X-Ray diffraction

UV-Vis - UV-Visible Spectroscopy

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LIST OF APPENDICES

Gantt Chart 62

Journal 1: The colored and blenched properties of tungsten oxide 63

electrochromic films with different substrate conductivities.

Journal 2: Electrochromic Phenomenon in indium-tin oxide thin films 67

deposited by RF magnetron sputtering

CHAPTER 1

INTRODUCTION

1.1 Introduction to electrochromic material

Electrochromic (EC) glass is a device that can change color when apply some voltage to

the film. EC device consist of two EC layers separated by an electrolytic layer,

conducting electrodes are used on both EC layers. Figure 1.1 show the basic structure of

EC device embodies six superimposed layers on two substrates. First substrate consists

of four layers as working electrode and the other substrate consist of two layers acting as

counter electrode. Both substrates are then separated by electrolyte in a laminated

configuration.

Working electrode usually made from mixed conductor, it acting as ion-storage layer

and conduct ions and electrons. Optical absorption occurs when electrons move into the

EC layers from the transparent conductors along with charge balancing ions entering

from the electrolyte.

Figure 1.1: Electrochromic device configuration consist of two substrates divided by

electrolyte.

2

Electrochromic (EC) becomes a whole wide attention among scientists since 40

years ago. However, electrochromism has remained an active area for basic and applied

research, with large possibilities for applications in emerging technologies. The interest

was boosted in the mid-1970s with the realization that electrochromisms was of much

interest in fenestration technology as a means to achieve energy efficiency in buildings

[1]. The application of electrochromic smart glass include transmittance modulation of

sunlight control window glazing for buildings, optical display, and reflectance

modulating automobile rear view mirrors [2].

Figure 1.2: Electrochromic Smart window used in a bulding [3]

Electrochromic (EC) materials are able to change their optical properties by

changing the electrical voltage supply. Transition metal such as tungsten trioxide (WO3),

nickel oxide (NiO), molybdenum trioxide (MoO3), and iridium trioxide (IrO3) have been

widely studied for used in electrochromic materials [4]. The electrochromic (EC)

material will change their optical properties when charge insertion and this may cause

the material change color or its opacity. Materials that change color upon insertion are

called cathodic while a material that change color upon extraction called anodic. Metal

oxide of W, Ti, V, Nb, Ta and Mo exhibit cathodic electrochromism and oxides of V,

3

Cr, Mn, Fe, Co, Ni, Rh and Ir are anodic electrochromism [5]. An amount of voltage

needed to change its opacity, however once the glass change color, no electricity is

needed for maintaining the particular shade which has been reach.

Method for preparing this electrochromic films include sputtering, spray

pyrolysis, chemical vapor deposition (CVD), electrodeposition and sol-gel deposition.

Figure 1.3 show the general schematic for spray pyrolysis, spray pyrolysis is a process

in which a thin film is deposited by spraying a solution on a heated surface, where the

constituent react to form a chemical compound. The chemical compound is volatile at

the temperature of deposition. The process is a particularly useful for the deposition of

oxides and has long been a production method for applying a transparent electrical

conductor [6].

Figure 1.3: General schematic of a spray pyrolysis deposition process [7]

4

Chemical vapor deposition (CVD) is a chemical process used to produce high

quality, high-performance, solid materials. The majority of its applications involve

applying solid thin-film coatings to surfaces, it has been used to deposit a very wide

range of materials. CVD also has a number of disadvantages. One of the primary

disadvantages lies in the properties of the precursors. Ideally, the precursors need to be

volatile at near-room temperatures [8]. Figure 1.4 illustrated the general process of

CVD.

Figure 1.4: General schematic of chemical vapor deposition process [9].

Among those methods, reactive magnetron sputtering widely used to obtain WO3

with good electric flow and optical properties. Sputtering is the process when an atom

with enough energy bombarded into particles and produces an ion. The momentum

transfer from the particles to the surface atoms can impart enough energy to allow the

outer electron to escape from its orbit. Once ejected, these ion can travel to a substrate

and deposit as a film. So in sputtering, the target material and the substrate is placed in a

vacuum chamber. A voltage is applied between them so that the target is the cathode and

the substrate is attached to the anode.

5

Plasma is created by ionizing a sputtering gas, generally a chemically inert,

heavy gas like Argon. Figure 1.2 illustrate ionization process that creates plasma, the

sputtering gas which is Argon ion will bombards the target and the target ion will

deposit into substrate. Ions can be generated by the collision of neutral atoms with high

energy electrons. The interaction of the ions and the target are determined by the

velocity and energy of the ions, since ions are charged particles, electric and magnetic

fields can control these parameters. The process begins with a stray electron near the

cathode is accelerated towards the anode and collides with a neutral gas atom converting

it to a positively charged ion. The process results in two electrons which can then collide

with other gas atoms and ionize them creating a cascading process until the gas breaks

down.

Figure 1.5: plasma ionization created during fabrication of Magnetron sputtering

process [10].

6

The breakdown voltage depends on the pressure in the chamber and the distance

between the anode and the cathode. At too low pressures, there aren’t enough collisions

between atoms and electrons to sustain plasma. At too high pressures, there so many

collisions that electrons do not have enough time to gather energy between collisions to

be able to ionize the atoms.

1.2 Problem statement

An attempt to fabricate a high efficiency electrochromic glass using tungsten oxide WO3

is increasing extensively due to high color efficiency study by researchers. However, the

electrochromic films cannot change color simultaneously in time even though the film is

supposed to change, most of the eletrochromic devices are completely change color after

15 minutes [24]

1.3 Motivation

By changing the oxygen percentage would create defect in the WO3 structure, and

changing the performance of electrochromic device.

1.4 Objectives

Objective for this research is to fabricate electrochromic thin film using tungsten target

using RF magnetron sputtering method and LiClO4+PC as electrolyte. Oxygen flow of

RF magnetron sputtering chamber was change to improve the performance of the

device.

This research embarks on the following objectives:

a) To fabricate a fast color response of electrochromic WO3 thin films

b) To get a low transmittance for blench state and color state of films after test

voltage.

7

1.5 Scope of project

The main scope of this project is to investigate characterization of tungsten trioxide as

electrochromic material. The scopes include:

1. To fabricate WO3 film using pure 99.99% tungsten target using RF magnetron

sputtering

2. To change parameter of oxygen flow to get high efficiency based on time respond

of electrochromic WO3 thin film.

3. To investigate the properties of WO3 thin film deposited at various oxygen flow by

using FE-SEM, AFM, XRD and two point probe.

1.6 Thesis Organization

This thesis consist of five chapter, the first chapter of this thesis consist of introduction

to electrochromic material, this part explained about general overview of electrochromic

smart glass based on first discovery, application and suitable material to produce this

smart glass. Next section is about background study, problem statement, objectives,

motivation, and scope of project and thesis organization. Second chapter is literature

review consist of History, principle operation of electrochromic trioxide (WO3)

development, WO3 thin films properties, and RF magnetron sputtering system. For third

chapter, it consist of methodology on fabricating the WO3 thin film using RF magnetron

sputtering and introduce to the apparatus used to measure the characteristic of WO3

materials. Chapter four consist of discussed about result and discussion of experiment

and the last chapter is about conclusion to this research and future recommendation for

future good. This thesis ended by references used during this research and Appendix for

this thesis.

CHAPTER 2

LITERATURE REVIEW

This chapter consists of historical method and technology development for preparing

electrochromic thin films, fabrication method, tools, and characterization equipment.

This chapter may help to understand this project through basic theory.

2.1 History

Smart glass or switchable glass also called smart windows or switchable windows, it is

refer to glass that changes light transmission properties under the application of voltage,

heat or light. The glass blocking certain or all wavelength of light to pass through and it

change into two conditions which are colored or blenched. Smart glass technologies

include electrochromic, photochromic, thermochromic, suspended particle, micro-blind

and liquid crystal devices [11] become interested among researcher.

The basic concept behind all smart windows is the same, they can be made in

several different ways, each with a different method and properties for blocking light.

Critical aspects of smart glass include material costs, installation costs, electricity costs

and durability, as well as functional features such as the speed of control, fastest

response, possibilities for dimming, and the degree of transparency.

An electrochromic window has a double-sandwich of thin layers, a separator in

the middle, two electrodes act as electrical contacts on either side of the separator, and

then two transparent electrical contact layers on either side of the electrodes. The basic

working principle involves lithium ions acting as positively charged lithium atoms with

9

missing electrons that migrate back and forth between the two electrodes through the

separator.

Normally, when the glass is clear, the lithium ions reside in the innermost

electrode. When a small voltage is applied to the electrodes, the ions migrate through the

separator to the outermost electrode where they scatter away most of the incoming light

and turn the glass opaque. They remain there by themselves until the voltage is reversed,

causing them to move back so the glass turns transparent once again. This process can

be illustrated by figure 2.1below. The power only needed to change them from one state

to the other and no power is needed to maintain the windows in color or opaque state.

The general EC phenomena of WO3 due to the formation of tungsten bronze (MxWO3)

according to the equation below where is , , , or .

(Transparent) (blue)

Figure 2.1: How electrochromic glass work

10

When installed in the envelope of buildings, smart glass creates climate adaptive

building shells, with the ability to save costs for heating, air-conditioning and lighting

and avoid the cost of installing and maintaining motorized light screens or blinds or

curtains. Most smart glass blocks ultraviolet light, reducing fabric fading.

2.2 Background study

EC material was found since early 1970 by researcher, researcher early found that

evaporated tungsten trioxide amorphous layers commonly used in EC displays actually

have the composition WO2.7Hy (0.2<y<0.5). Then researcher emphasize that coloration

of virgin transparent films can be obtained without injection of any external ion into the

layer [12]. Tungsten trioxide WO3 widely treated as electrochromic (EC) material

because of it has rich special physics and chemical properties [13], photochromic

[14,15],

gasochromic [16], catalyzed [17], and hide material [18], and it even has potential as a

superconducting material [19]. Researchers nowadays are focused on WO3 (coloring

under charge insertion) this is due to the advantage among the others in terms of

reversibility, stability and color efficiency.

Tien-syh Yang in his paper said some crystallization dispersed in the WO3 thin

film possesses the better coloration efficiency. Larger internal volume with optimal

nanocrystal-size is essential to conduct ions and electrons for electrochromic

intercalation [20]. On the other hand, Hiroharu Kawasaki in his paper said, in general,

the amorphous WO3 thin film has a good performance as an electrochromic display.

Some sample deposited at different parameter using same magnetron sputtering method.

Figure 2.2 explained at total gas pressure 3 Pa, a crystalline WO3 (022) peak can be

observed. With increasing total gas pressure, the crystalline peaks disappear that means

the films are amorphous. [21].

Apart from that, C. Chananonnawathorn believe that amorphous WO3 films are

more suitable than crystalline WO3 films for electrochromic applications. A crystalline

structure is less favorable for ions to diffuse through because of the densely packed

atomic structure, because the lithium ion movement through the film is obstructed by the

11

dense structure leading to a lower response time. Figure 2.3 shown WO3 films were

poor crystallinity or amorphous structure shown at different deposition oxygen flow rate

for 5sccm, 10sccm, 15sccm and 20sccm. However, these amorphous or nanocrystaline

structure of WO3 films resulted in better electrochromic property than crystal structure

[22].

Figure 2.2: XRD patterns of the WO3 films deposited using the W target on the glass

substrate. RF power: 50W, O2: Ar =5:5 [15].

12

Figure 2.3: X-ray diffraction patterns of WO3 films prepared with varying different

oxygen gas flow rates [14].

The transmittances at 550nm usually observed due to visible spectrum (400nm-

700nm) where for Violet, Indigo, Blue, Green, Yellow, Orange and Red for 400nm,

445nm, 475nm, 510nm, 570nm, 590nm, 650nm respectively. The best transmittance

should be higher than 80% and below 80% after gives some voltage.

Furthermore, electrolyte effect was study by researchers, different type of

electrolyte will give different color to the film. Usually researchers used NiOOH [23],

HCl, NaOH, and LiClO4. Among this electrolyte LiClO4 become concentration among

researcher, this is due to the fast chemical reaction. The color of the films changes to

dark blue when positive Lithium ions and electron are electrochemically injected into

these films [24-26]. The films reported show a good coloration behavior in the visible

and NIR region when it was deposited at 60% oxygen content, it show a good

electrochromic stability as Li⁺ ion can be almost inserted and extracted reversibly and

good reverse phenomena between the coloration and blenched state [27].

Other than that, in theory band gap energy for bulk WO3 is about 2.62eV. The

maximum optical band gap of 3.14eV was reported for the film deposited at 3.1 x 10⁻²

13

mbar, and it decreased by decreasing oxygen sputter pressure. The lowest band gap was

reported is 2.97 at bleached state at 1.5 x 10⁻² mbar [28]. The best EC film is having a

short response times below 1 minute and it can be achieved by obtained optimum band

gap energy. The crystalline reported to be increase by increasing temperature, however

the response for electrochromic at room temperature is faster than at temperature 500ᵒC.

2.3 Principle operation of electrochromic thin films

Electrochromic is materials which are changing color reversibly when they kept in a

different electronic state through either by reduction (absorbing electrons) or oxidation

(loosing electrons). The phenomenon of color change is called electrochromism.

Electrochromic materials are divided into different groups by their physical state at

room temperature. In this way electrochromic materials are divided into 3 broad

categories. The first category has electrochromic materials that are soluble in neutral

states even after electron transfer. Examples of such a compound are Molecular dyes

(ethyl viologen). Second category has electrochromic materials that are soluble in

neutral state and forms solid on electrode surface (which is insoluble) after the electron

transfer, for example heptyl viologen.

Third category has electrochromic materials that are solids in neutral state and

remain in solid state after the electron transfer. Examples are metal oxide films and

conducting polymers. This system can be viewed as rechargeable electrochemical cell in

which electrochromic electrode(where oxidation reduction takes place) and charge

balancing counter electrode are separated by solid electrolyte (generally polymeric) or

liquid electrolyte. Color changes in these devices occur by charging and discharging of

the cell on application of electricity [29].

When a voltage is applied on EC thin films, ions are inserted into WO3 film in

the device, thereby the optical properties are changed. It switches reversibly from

transparent to dark blue upon electrochemical redox reactions. The insertion/extraction

process can be represented by:

WO3 (transparent) + xLi⁺ (or H, Na, K) + xe⁻⇔ LixWO3 (blue)

14

This reaction goes to the left or right depending on the sign of the applied voltage.

When ions are inserted, both XPS and electron spin resonance (ESR) show that W5+ as

well as W6+ is present, and that the W5+ density scales with the intensity of the EC

absorption peak [30].

Figure 2.4: Schematic structure of the electrochromic process in electrochromic (EC)

and ion movement during coloration process.

2.4 WO3 thin films properties

WO3 shows different crystal structures at different temperatures. The most common

monoclinic crystal structure of WO3 is represented in Figure 2.5. There is another stable

tungsten oxide compound, also existing in the tungsten-oxygen shape.

15

Figure 2.5: Monoclinic crystal structure of WO3 [31]

Electrochromic device employ both amorphous as well as crystalline WO3 film

owing to their highly catalytic behavior both in oxidation and reduction processes. The

amorphous films have their applications in transmittance modulation devices in the

visible region, whereas the crystalline WO3 film, due to its better reflection modulation

in near-infrared region of the spectrum, is used for thermal modulation. The coloration

in WO3 thin film occurs due to ionic insertion but the coloration mechanism in

crystalline WO3 films is different from amorphous thin films.

Figure 2.6 shows XPS spectra of evaporated W oxide films intercalated to

different values of x from work byHashimoto and Matsuoka [32, 33]. The intensity goes

towards zero at higher energy, but then a narrow peak appears just below the Fermi

level. This peak increases with the amount of ion intercalation. The reason for this

feature is probably that the electrons that are inserted together with the ions occupy the

lower part of the conduction band. The minimum between the two peaks corresponds to

the band gap.

16

Figure 2.6: XPS spectra of the valence band and the occupied part of the conduction

band for evaporated tungsten oxide films. [32]

The optical properties of the various electrochromic materials differ

substantially. Since tungsten oxide is the most common material that deal with it in more

detail than the other materials. Upon the insertion of small positive ions for example

protons, Li+, K+ or Ag+ , tungsten oxide absorbs light with a maximum around 950 nm

This gives rise to a deep blue color of the material. The disorder in the material is large

enough to cause localization of the injected electrons. The optical properties can be

understood using small polaron theory crystalline tungsten oxide has a higher lattice

order which give rise to a more free-electron like behavior [34]

In contrast the tungsten oxide film absorbs most in the visible part of the

spectrum. This results in a greyish to bronze color. Prussian blue has optical properties

that are close to amorphous tungsten oxide, with the big difference that it colors when

the ions are removed from the film. It can be combined with tungsten oxide in devices

where both layers color or bleach at the same time [35]

17

2.5 RF Magnetron Sputtering method

Recent years, variety method and techniques were used to fabricate electrochromic thin

film as a smart glass or smart window using different material such as V2O5-z [36], IZO

[37], Nb2O5 [38], fluorine-doped tin oxide (FTO) [39], NiOx [40,41], Nb2O5:MoO3

[42], and tantalum oxide [43]. Some method to fabricate electrochromic thin films

include sol gel chemistry [44], solid-state metal-oxide-metal capacitor development

[45], poly(3,4-ethylenediox-ythiophene)(PEDOT) [46], magnetron sputtering [47,48],

layer by layer assembly (lbl) [49], and chemical vapor deposition [50].

W. R. Grove was the first person to study about sputtering process on 1852

whereas others had observed the effect while studying glow discharges. Since that most

of the physical vapour deposition is closely associated with the development of vacuum

technology, the first piston type vacuum pump was invented by Otto van Guericke at

1640 in order to pump water out of mines. However, the first person to apply the

vacuum pump in order to form a glow discharge in a vacuum tube was M. Faraday in

1838. Since then, magnetron sputtering was studied by several researcher and the first

uses of radio frequency (RF) to sputter material was investigated in the 1960s [51]

In 1966, Davidse and Maiseel used RF sputtering to produce dielectric films

from dielectric materials. However, RF sputter deposition is not used widely due to their

high cost and the introduction of high temperatures, due to the high self-bias voltage

associated with RF power. RF sputtering was then developed to sputter de-posited hard

coatings on tools at mid-1970s and became commercially available at early 1980s [52].

Magnetron sputtering is a technique that requires a high energy of ions for

bombardment, amounting to a few hundred of electron volts around the cathode. Due to

the bombardment of the ions towards the cathode, fast metal atoms are formed along

with secondary electrons. Basically, during the sputtering process, gas ions at the plasma

were accelerated towards the target material that to be deposited. The material will then

detached from the target and deposited on the substrate.

Basically, the advantages of reactive sputtering compared to chemical vapor

deposition, reactive DC sputtering and sol-gel chemistry techniques is due to reactive

sputtering deposit thin film at low deposition rates and the oxygen content can be

18

controlled to produce specifically. Other than that, the sputter deposited films have a

composition close to that of the source material. The difference is due to different

elements spreading differently because of their different mass (light elements are

deflected more easily by the gas) but this difference is constant.

Figure 2.5: Ionization process in Magnetron sputtering chamber during fabrication.

Sputtering is the process ion or atoms from inert gas bombarded into metal

surface and produce a metal ion. The movement of an ion illustrated in figure 2.4. The

ejecting material from a target will be depositing on a substrate such as a silicon wafer,

glass, Indium Tin Oxide (ITO) and Fluorine doped Tin Oxide (FTO) Substrates are

placed in a vacuum chamber and are pumped down to a prescribed process pressure.

Sputtering starts when a negative charge is applied to the target material causing a

plasma or glow discharge. Positive charged gas ions generated in the plasma region are

19

attracted to the negatively biased target plate at a very high speed. This collision creates

a momentum transfer and ejects atomic size particles from the target. These particles are

deposited as a thin film into the substrates surface [53].

Sputtering is extensively used in the semiconductor industry to deposit thin films

of various materials in integrated circuits processing. Thin anti reflection coatings on

glass, which are useful for optical applications are also deposited by sputtering. Because

of the low substrate temperatures used, sputtering is an ideal method to deposit contact

metals for thin film transistors. This technique is also used to fabricate thin film sensors,

photovoltaic thin films (solar cells), electrochromic thin films, metal cantilevers and

interconnects.

CHAPTER 3

METHODOLOGY

This chapter describe the method used to fabricate thin films, it emphasize on procedure

and process carried out during preparation and fabrication of electrochromic thin film.

3.1 Introduction

In this research, RF magnetron sputtering technique was used to fabricate WO3 thin

film. During the process, a 99.99% tungsten metal was used as a sputtering target and

ITO glass with sheet resistance 180Ω/cm² as a substrate. Several parameters were fixed

while oxygen parameter changed during fabrication process. The WO3 thin film will be

analyzed using surface analysis equipment such as field emission scanning electron

microscope (FE-SEM), atomic force microscope (AFM), X-Ray diffraction (XRD), UV-

Visible Spectroscopy (UV-Vis), Alpha-Step IQ Surface Profiler, and two point probe(IV

characteristic). The standardized process to analyzed and fabricate the WO3 as

electrochromic glass was according to the flow chart in figure 3.1.

21

Figure 3.1: Flow chart of the process for fabricating tungsten trioxide (WO3) as

electrochromic films.

22

3.2 Fabrication process

Tungsten trioxide (WO3) was constructed using RF magnetron sputtering to perform as

electrochromic glass. Normally electrochromic films have two pieces of glass, overall it

consist of 5 layers as shown in figure 3.2. The first piece of electrochromic thin film has

3 layers consist of glass and Indium Tin Oxide (ITO) as substrate to the film, followed

by WO3. Second piece consist of Indium Tin Oxide (ITO) only. LiClO4+PC act as

electrolyte and Au electrode was fabricated at both films using DC sputtering. First film

act as working electrode and second film act as counter electrode.

Figure 3.2: Basic design of electrochromic device including LiClO4+PC electrolyte to

transport ions.

23

3.2.1 Substrate cleaning process.

A 2×6cm Indium Tin Oxide (ITO) glass was cut using diamond cutter into 3 parts.

Acetone and ethanol was used to clean the contamination on the ITO surface using

ultrasonic bath in 10minutes. The sample then dried in nitrogen gas flow. (see figure

3.3)

Diamond cutter Ultra sonic bath Nitrogen gas

Figure 3.3: Equipment used to cleaning substrate process

24

3.2.2 Deposition

Step 1: Conductivity of ITO sample was checked using multimeter then ITO substrate

plastered using heat resistant tape onto substrate holder on Magnetron sputtering

Step 2: Fabrication of WO3 was started using Magnetron Sputtering machine (See

figure 3.4) followed manual attached at APPENDIX.

Magnetron sputtering Sputtering chamber

Figure 3.4: Magnetron sputtering machine

To perform a better electrochromic material, several parameter needed was optimized.

Oxygen flow rate was change from 25%, 27%, 30% and 35%. The parameter used to

fabricate WO3 thin films shown in table 1.

56

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