Development of Magnetized Plasma Sputtering Source for

Effective Target Utilization with Various Magnet Setups

September 2018

Department of Science and Advanced Technology

Graduate School of Science and Engineering

Saga University, Japan

MD. AMZAD HOSSAIN

Development of Magnetized Plasma Sputtering Source for

Effective Target Utilization with Various Magnet Setups

By

MD. AMZAD HOSSAIN

A dissertation submitted in partial fulfillment of the

requirements for the degree of

Doctor of Engineering (Dr. Eng.)

in Electrical and Electronic Engineering

Department of Science and Advanced Technology

Graduate School of Science and Engineering

Saga University, Japan

September 2018

i

Acknowledgement

First of all, Alhamdulillah, I would like to express my praise and gratitude to the Almighty,

Gracious and Merciful Allah who makes me capable to successfully complete this doctor study.

His presence is the main source of my inspiration, without his help; I would not achieve this goal.

With great pleasure, I am profoundly indebted to my supervisors Professor Dr. Yasunori

Ohtsu, Professor Dr. Toshiyuki Oishi, Professor Dr. Tooru Tanaka and Associate Professor Dr.

Satoshi Ihara of Saga University, Japan, for their expertise guidance, extraordinary kindness,

inspiration and all kinds of supports throughout my present study. Their sufficient cooperation

made me confidential enough in each step of my doctoral works.

Md. Amzad Hossain is grateful to Dr. Tatsuo Tabaru, National Institute of Advanced

Industrial Science and Technology, Professor Hirotaka Toyoda, Nagoya University, Professor Dr.

Michael A. Lieberman, University of California at Berkeley, Professor Dr. Ken Yukimura,

National Institute of Advanced Industrial Science and Technology, Japan, Dr. Julian Schulze, Ruhr

University Bochum, Germany and West Virginia University, USA for fruitful discussions.

I am thankful to Mr. Yusuke Takada, Mr. Tsubasa Ide, Mr. Koichirou Ikari, Mr. Yutaro

Nakamura, Mr. Kosei Sugawara, and Mr. Masaya Takasaki, Plasma Electronics Lab, Saga

University, Japan for their heartfelt cooperation. Warmest and sincere thanks to my entire

laboratory members who have extended their kind helps to me during my study. I would like to

expand my gratitude to the Ministry of Education, Culture, Sports, Science and Technology

(MEXT), Japan for supporting his scholarship.

I would like to thank Jessore University of Science and Technology, Jessore – 7408,

Bangladesh for allowing me the opportunity to continue my studies at the doctoral level. Special

thanks to Professor Dr. Md. Mortuza Ali, Professor Dr. Md. Rafiqul Islam Sheikh, and Professor

Dr. Md. Faruk Hossain at Rajshahi University of Engineering & Technology, Rajshahi,

Bangladesh who introduced me with plasma engineering, thin film technology, as well as research

opportunities in Japan. Special thanks to Md. Hasanuzzaman, Khulna University of Engineering

& Technology, Md. Abdul Majed Patwary, Comilla University, and Md. Mahbubur Rahman

Bhuiyan, Jahangirnagar University, Bangladesh.

Needless to mention, my beloved wife who has encouraged me always, whose love, support,

ii

patience, encouragement and sacrifice allowed me to finish my doctoral study. I am thankful to

my lovely son and daughter whose unconditional love to me and for his love, patience and sacrifice.

All of my heartfelt love to them. Finally, I deeply grateful to my parents, my father, is the person

who put the fundament my learning character, showing me the joy of intellectual pursuit ever since

I was a child; and my mother, is the one who sincerely raised me with her caring and gently love.

for their patience as well as moral support and endless love in long journey of my life. Their prayers,

encouragement, and advice have been and will always be a fortune for my life.

Finally, I would like to thank all of my laboratory friends for their supports as well as express

my gratitude to all Bangladeshi friends, Saga Moslem Society and international students in Saga

for making my Japan life more comfortable and enjoyable. This thesis is dedicated to my beloved

mother, father, wife, and my lovely son, ABMA Ishmam Amzad.

iii

Abstract

A high-density radio frequency (RF) magnetized sputtering plasma source with a rotational

square-shaped magnet arrangement for uniform target utilization has developed. Eight neodymium

rod magnets of 30 × 5 × 3 mm, where the connection between N-pole and S-pole magnets is one

side of the square, are mounted on a circular iron yoke disc and an iron cover of 5 × 3 × 1 mm is

also used for magnetic shielding of otiose magnetic fields from the permanent magnets. The

magnetic field simulation, the measurement of the target erosion and the time-averaged ion flux to

the target have been investigated for case (a) without iron cover, no air gap between N-pole and

S-pole magnets, case (b) with iron cover, no air gap, and case (c) with iron cover, 5 mm air gap,

respectively. It is found that the iron covers suppress the horizontal magnetic flux density and the

copper target utilization percentage increases from 74.15 % to 87.49 %. Moreover, by decreasing

the air gap between the shielded magnets, the copper target utilization percentage rises from

83.85 % to 87.49 %. The target utilization as well as the time-averaged ion flux to the target are

optimum for case (b).

A gyratory square-shaped capacitive radio-frequency (RF) discharge plasma sputtering

source is proposed for materials processing and functional film preparation, composed of magnet

arrangements consisting of eight neodymium bar magnets of dimensions 30 mm × 5 mm × 3 mm.

In order to evaluate its performance, two square-shaped magnetic arrangements were investigated:

case (a) without iron shielding and case (b) with iron shielding of dimensions 5 mm × 3 mm × 1

mm. The magnetic field simulation is analyzed, while the plasma discharge characteristics and the

film properties are measured. The film thickness and the resistivity profiles of case (b) are more

uniform than their corresponding profiles in case (a). The lowest electrical resistivity of the film

is 4.33 × 10-8 m at r = 30 mm for case (b), which is of the same order as the bulk resistivity of

the copper. The roughnesses of the film thickness profile for cases (a) and (b) are 24.4% and

7.2%, respectively. Using atomic force microscopy (AFM) analysis, the film surface for case (b)

was observed to show an improved smooth surface with reduced needle-shaped grain size, as well

as a lower surface roughness than that of case (a). The surface roughness of the films is

approximately 3.73 nm and 2.49 nm for case (a) and case (b), respectively. From the X-ray

iv

diffraction (XRD) patterns, the film texture, the relative intensity ratios of the (111) peak to the

(200) [I(111)/ I(200)] were found to be 13.76 and 4.08 for the cases (a) and (b), respectively.

To improve the target erosion near the edge, the outer ring-shaped RF magnetized plasma is

produced near the chamber wall by a monopole magnet scheme. Three monopole magnet schemes

such as the setups (a) R = 5 mm, (b) R = 20 mm and (c) R = 35 mm has been investigated are

chosen, where “R” is the gap distance between magnets in consecutive circles. Distributions of the

2D magnetic flux lines, absolute value of the horizontal magnetic flux density and discharge

voltage are investigated for the proposed setups to produce outer ring-shaped plasma. A high

luminous ring-shaped plasma is observed for (b) R = 5 mm, whereas multi-ring discharges are

observed for (b) R = 20 mm and (c) 35 mm. It is found that the electron temperature decreases

with increasing gas pressure for the all cases. The electron temperatures were 2.42, 1.71 and 1.15

eV at Ar gas pressure of 4 Pa for the setup (a), (b), and (c), respectively. The plasma density is

approximately same for the setups (b) and (c) at all gas pressure. The highest plasma densities

were 6.26×1015, 1.06×1016 and 1.11×1016 m-3 at 5 Pa for the setups (a), (b), and (c), respectively.

It is found that, the electron mean free path is 41.4, 63.17 and 84.66 mm at Ar gas pressure of 5

Pa for the setups (a), (b), and (c), respectively. Electron neutral collision frequency for case (a) R

= 5 mm is higher than that for case (b) R = 20 mm and case (c) R = 35 mm at a constant RF power

of 40 W and z = 13 mm axial distance from the target surface. Radial profile of ion saturation

current for case (b) R =20 mm is more uniform than that for case (a) R = 5 mm and case (c) R =

35 mm set up.

The capacitively coupled RF outer ring-shaped magnetized plasma discharge is developed

with a concentrically monopole arrangement of magnets to erode the target in a specific area, in

especial, near the chamber wall. The three concentric monopole magnet arrangements with a center

magnet, and magnets in setups (a) three circles, (b) two circles, and (c) one circle were investigated.

From the magnetic flux lines profiles, it was found that the magnetic flux density in component

parallel to the target surface has a peak magnitude in the outer circle of magnets for all setups.

Ring-shaped plasma in the specific outer area was observed. The ion saturation current, Iisat were

0.6 mA, 0.79 mA, and 0.46 mA, for setups (a), (b), and (c), respectively at r = 47 mm, where r =

0 mm is the center of the target. It was found that, Iisat is very high in the outer target region near

the chamber wall for setups (a) and (b), where Iisat for setup (c) decreases slowly. The results

showed that the target utilization could be controlled in the outer specific area near the wall.

v

A pulsed direct current (DC) discharge ring–shaped plasma source has been proposed using

single pole magnet arrangements, including a center magnet, with magnets in the setups (a) one

circle, (b) two circles, and (c) three circles. The 2D magnetic flux lines profiles, larmor–radii and

Hall parameters of the electrons and ions, electrical discharge characteristics, ion saturation current

profiles were investigated to characterize the proposed plasma. The electron larmor–radii, re were

0.17, 1.64, 5.82 mm for setups (a), (b), and (c), respectively. It was found that the highest electron

Hall parameters are approximately 561 at r = ±21 mm for setup (a), 544 at r = ±36 mm for setup

(b), and 297 at r = ±50 mm for setup (c). The strong ring–shaped plasma discharges was observed

for all setups. The typical discharge voltages were 1.0, 0.6, and 0.6 kV for setups (a), (b), and (c),

respectively. The ion saturation currents, Iisat were 1.44, 2.88, and 2.2 mA for setups (a), (b), and

(c), respectively at r = 45 mm and t = +10 μs. The Iisat of setup (b) is less fluctuating, whereas Iisat

of setup (c) is highly variable in all radial positions. Setup (b) has the best profile among the three

setups.

vi

Table of Contents

Acknowledgements i

Abstract iii

Table of contents vi

List of Symbols x

Chapter 1. Introduction 1

1.1. Plasma and Its Typical Features 1

1.1.1. Plasma Sheaths and Debye Length 4

1.1.2. Cyclotron Frequency and Larmor Radius 5

1.1.3. E × B Drift Motion 6

1.2. Thin Film Deposition by Plasma Sputtering 7

1.2.1. Principle of Plasma Sputtering 7

1.2.2. Merits and Demerits of Plasma Sputtering 9

1.3. Background Research on Plasma Sputtering 10

1.3.1. Synthesis of Functional Films by Plasma Sputtering 10

1.3.2. Previous Research for Improving Target Utilization and

their Problems

12

1.4. Problems of the Plasma Sputtering and Objectives of this Thesis 13

1.5. Structure of this Thesis 14

References 17

Chapter 2. Measurement Techniques and Methods 18

2.1. Major Experimental Apparatus 18

2.2. Ion Saturation Current, Plasma Density, and Electron

Temperature Measurement

19

2.3. Measurement of the Eroded Target Profile and Target Utilization 22

vii

Rate Calculation

2.4. Resistivity of the Prepared Film 24

2.5. Magnetic Field Analysis Simulation 26

2.6. Measurement of the Film Property 26

References 27

Chapter 3. Rotational Square-Shaped Arrangement of Rod Magnets for

Uniform Target Utilization

28

3.1. Introduction 28

3.2. Experimental Setup 29

3.3. Results and Discussions 33

3.3.1. Magnetic Field Profile Analysis of Square-Shaped

Schemes

33

3.3.2. Plasma Emissions and Discharge Characteristics 38

3.3.3. Profiles of the Target Erosion Depth and Ion Flux to the

Cu target

41

3.4. Conclusion 45

References 46

Chapter 4. Performance of a Gyratory Square-Shaped Capacitive Radio

Frequency Discharge Plasma Sputtering Source for Material Processing

48

4.1. Introduction 48

4.2. Experimental Setup Details 50

4.3. Results and Discussions 54

4.3.1. Magnetic Shielding Effect Square-Shaped Magnet

Arrangement

54

4.3.2. Plasma Discharge Voltage and Emission for Square-

Shaped Magnetized Plasma

57

4.3.3. Thickness and Resistivity Profiles of the Deposited Film

of the Gyratory Square-Shaped RF Magnetized Plasma

Sputtering Source

59

viii

4.3.4. Surface Morphology of the Film Deposited by the

Gyratory Square-Shaped RF Magnetized Plasma

Sputtering Source

61

4.3.5. Aluminum-Doped Zinc Oxide (AZO) Film Prepared by

RF Magnetized Plasma Sputtering Source with Square-

Shaped Rod Magnets

64

4.4. Conclusion 67

References 68

Chapter 5. Ring-Shaped Plasma for Target Utilization Obtained with Circular

Magnets Monopole Arrangement

70

5.1. Introduction 71

5.2. Experimental Methods 71

5.3. Results and Discussions 75

5.3.1. Magnetic Field Profile Analysis 76

5.3.2. Discharge Characteristics 79

5.3.3. Plasma Characteristics 82

5.3.4. Ion Saturation Current Profile 87

5.4. Conclusion 89

References 90

Chapter 6. Outer Ring-Shaped Radio Frequency Magnetized Plasma Source for

Target Utilization in Specific Area

92

6.1. Introduction 92

6.2. Experimental Setup Details 94

6.3. Results and Discussions 96

6.3.1. Profiles of Magnetic Field of the Monopole Setups 96

6.3.2. Emission of Outer Ring–Shaped Plasma 98

6.3.3. Outer Ring–Shaped Plasma Characteristics 100

6.3.4. Ion Saturation Current in Outer Ring–Shaped Plasma 103

6.4. Conclusion 105

ix

References 106

Chapter 7. Characteristics of a Ring–Shaped Pulsed DC Discharge Plasma

Source Using Single Pole Magnet Setups

108

7.1. Introduction 108

7.2. Experimental Arrangements 110

7.3. Results and Discussions 113

7.3.1. Simulation of Single Pole Magnet Setups for Ring-

Shaped Pulsed Discharge

113

7.3.2. Electrical Characteristics of the Ring–shaped Pulsed

Discharge Plasma

121

7.3.3. Discharge Characteristics of the Ring–shaped Pulsed

Discharge Plasma

123

7.3.4. Ion Saturation Currents Profiles 126

7.4. Conclusion 129

References 131

Chapter 8. Summary and Suggestions for Future Plan 134

8.1. Summary and Conclusions 134

8.2. Suggestions for Future Works 137

List of Refereed publication papers and conferences 138

x

List of Symbols

Parameter Definition Unit

a Maximum of target erosion height µm

A Probe area m2

Af Cross section of the deposited film resistance m2

B Radial interval between consecutives target erosion depth m

B Magnetic flux density G

Bx Horizontal magnetic flux density to target G

Bz Vertical magnetic flux density to target G

E Electronic charge C

H Hall parameter Unit less

he Electron hall parameter Unit less

hi Ion hall parameter Unit less

Ie Electron current detected by the probe mA

Iesat Electron saturation current detected by the probe mA

Iisat Ion saturation current detected by the probe mA

Ip Probe current mA

K Boltzmann’s constant J/K

m Electron mass kg

M Ion mass kg

n Charge number density m-3

nAr Argon gas number density m-3

nCu Atomic number density of the copper target m-3, cm-3

ne Electron number density m-3, cm-3

ni Ion number density m-3, cm-3

q Electric charge C

xi

r Radial position from center of the target mm

R Gap distance between magnets in consecutive circles mm

Ri Ammeter internal resistance Ω

Rv Voltmeter internal resistance Ω

Rf Deposited film resistance Ω

rL Larmor radius m

rLe Electron larmor radius m

rLi Ion larmor radius m

Rsp Sputtering rate µm/hour

Si Area occupied by erosion at target m2

Sp Area of the probe used to detect the ion saturation current m2

Te Electron temperature eV

TU Target utilization percentage %

V Charge velocity component m/s

ve Electron velocity component m/s

VB Biasing voltage V

VRF RF discharge voltage (peak - to - peak value) Vpp

VSB Self-biased voltage V

λD Debye length m

ωc Cyclotron frequency Hz

ϵo Permittivity of free space F/m

γsp Yield of sputtered atoms per incident ion Unit less

νm Charged particle neutral collision frequency Hz

νme Electron neutral collision frequency Hz

νmi Ion neutral collision frequency Hz

1

Chapter 1. Introduction

1.1. Plasma and Its Typical Features

A plasma is a collection of free charged particles (electrons and ions) moving in a

random direction that are electrically neutral (ne ≈ ni) where ne and ni are the electron and ion

densities, respectively [1-2]. Artificially plasma can be generated by heating or be subjecting a

neutral gas to a strong electromagnetic field to the point an ionized gaseous particles become

electrically conductive, and the behavior of the matter is dominated by the electromagnetic

fields in long-range [3]. Plasma is matter heated beyond its gaseous state, heated to a

temperature so high that atoms are stripped of at least one electron in their outer shells so that

what remains are positive ions in a sea of free electrons. That is, as the temperature increases,

the atoms become more energetic and transform matter in the sequence: solid, liquid, gas, and

finally plasma which justifies the title, "fourth state of matter".

Fluorescent lights contain plasmas and Lightning is an example of plasma present at

Earth's surface. Aside from those, we do not often encounter plasmas in everyday life. Outside

the earth in the ionosphere or outer space, however, almost everything is in the plasma state.

Some examples are aurora borealis, solar winds, magnetospheres of earth and Jupiter, gaseous

nebulae, galactic arms, quasars, pulsars, novas, black holes, fluorescent lamp, fires and TVs

[2]. The plasma ion engine is used for the flight of the space probe "Hayabusa". On the other

hand, plasma processing is the most widely used chemical process in microelectronic industry

for thin film deposition and etching. Its application expands to surface modification, flat panel

display fabrication, solar cell, plasma spray and plasma micro-discharge and many other

growing areas [4]. The studies such as sterilization and sterilization, the growth stimulation of

the plant are pushed forward recently. It is highly advanced the imminent product of all of you

by this plasma process technology.

Figure 1.1 shows the schematic view of a plasma showing charged particles movements

in the random directions. In general, plasmas have the following features [1]: (1) they are driven

by voltage or current source, electrically; (2) charged particles collisions with neutral gas

molecules are significant; (3) there are boundaries at which surface losses are significant; (4)

in the steady state ionization of neutrals sustains the plasma; and (5) the electrons and the ions

are not in thermal equilibrium.

2

Figure 1.1. Schematic view of a plasma showing charged particles movements in the andom

directions.

Figure 1.2 shows a typical radio-frequency (RF) plasma discharge system. It involves

a voltage source that flows current through a low-pressure gas between two parallel conducting

plates. The gas break down to form a typical plasma, usually weakly ionized. Plasma is formed

between the electrodes. As shown in the Fig. 1.2, the process chamber can be surronded by dc

multipole magnetic fields to improve plasma confinement near the chamber surfaces, while

providing a magnetic near-field-free plasma atmosphere at the wafer. Such arrangements are

often called remote sources [1]. Sometimes, the source and process chambers are more complex.

For example, the wafer is kept very near to the source exit, to obtain increased ion and radical

fluxes, reduced spread in ion energy, and improved process uniformity. But the wafer is then

exposed to higher levels of damaging radiation. Although the need for low pressures, high

fluxes, and controllable ion energies has motivated high-density source development, there are

many issues that need to be resolved [1]. A critical issue is achieving the required process

uniformity over 300 mm wafer diameters. In contrast to the nearly one-dimensional geometry

of typical rf diodes (two closely spaced parallel electrodes), high-density cylindrical sources

can have length-to-diameter ratios of order or exceeding unity. Plasma formation and transport

in such geometries are inherently radially nonuniform [1].

3

Figure 1.2. A typical radio-frequency plasma discharge system.

There is a wide range of densities and temperatures for both artificially processed and

space plasmas. Low-pressure plasma discharges characterized by plasma density, n ≈ 1014 -

1019 m-3, and electron temperature, Te ≈ 1 – 10 eV, which are used as chemically reactive

etchants, functional film depositions precursors. In general, while energy is delivered in the

form of bombarding ions to substrate surface, the energy flux enhances the chemistry at the

substrate surface, do not heat the substrate. The functional gas pressure for these discharges are

≤ 1 Pa 133.3 Pa and subjected of this thesis.

The high-pressure discharges have Te ≈ 0.1 – 2 eV and n ≈ 1020 – 1025 m-3 are also used

in plasma processing. The light and heavy particles are more nearly in thermal equilibrium

satisfying the condition Ti ≤ Te. These discharges are used mostly to distribute heat to the

substrate surface to increase surface reaction rates, to melt, evaporate materials, to weld

refractory materials. The operating pressures are approximately atmospheric pressure,

1.01325×105 Pa. Shock tubes, focus, high-pressure arcs and laser plasma are the examples of

high-pressure discharges have plasma density approximately n ≈ 1020 m-3. Fusion reactor

plasmas (n ≈ 1020 – 1021m-3, and Te ≈ 3.5 – 4 eV), alkali metal plasmas (n ≈ 1015 – 1018 m-3,

and Te ≈ 1 eV), flames (n ≈ 1014 m-3, and Te ≈ 1 eV), earth ionosphere (n ≈ 1010 1014 m-3, and

Te ≈ 2 eV) and laser plasma (n ≈ 1025 – 1027m-3, and Te ≈ 2 – 3 eV), are typical examples of

the plasma discharges.

4

1.1.1. Plasma Sheaths and Debye Length Figure 1.3 shows the profile of the plasma densities after formation of the two sheaths

near the chamber wall. The condition ne ≈ ni is called as quasi-neutrality and the most important

characteristics of a plasma [1-2]. Plasmas are quasi-neutral (ne ≈ ni), and are joined to chamber

wall surfaces across thin positively charged layers called sheaths. The plasma potential varies

slowly in the plasma but rapidly in the sheath. The plasma density is relatively flat in the center

and falls sharply near the sheath edge. The electron densities fall rapidly at chamber wall

surface. If the sheaths potential drops are unequal, the electron fluxes will be unequal. The

Bohm criterion states that ions must steam in the sheath with a velocity at least / / [2].

Figure 1.3. Profile of the plasma densities after formation of the two sheaths near the

chamber wall.

A fundamental characteristic of the behaviors of a plasma is its ability to shield out

electric potentials of individual charged particles that are applied to it. Debye length indicates

that the distance scale over which significant charge densities can spontaneously exist. The

5

characteristic length scale in a plasma is the Debye length can be expressed by the following

equation [1-2].

⁄

(1.1)

It is assumed that plasma having equilibrium densities ne = ni = n0. In practical units, it can be

written as following form [1]:

743 ⁄ (1.2)

With Te in electron volts and ne in cm-3. For example, Debye length is 0.14 mm for Te = 4 eV

and ne = 1010 cm-3.

1.1.2. Cyclotron Frequency and Larmor Radius

If the plasma is inserted in a DC magnetic field (B-field) the motions of the charged

particles are affected by the B-field and makes the plasma an anisotropic medium, with a

preferred direction along B. That is, the charged particle, electron rotate along the B-field as

shown in figure 1.5. As long as the ion or electron of charge q is moving, it experiences a

Lorentz force qv×B, which is perpendicular to both velocity and the field. This force has no

effect on the velocity component to B, but in the perpendicular plane, it powers the particle to

rotate in a cyclotron orbit. The frequency of this circular motion, the cyclotron frequency is

represented by the following equation [2].

⁄ (1.3)

The radius of the circle of rotation called Larmor radius or gyro-radius . The cyclotron

frequency is independent of velocity, however, gyro-radius depend on velocity. If is the

velocity component in the plane perpendicular to B, a particle completed an orbit a length 2

in a time 2 ⁄ , so , alternatively can be written by the following formulae [2].

6

⁄ (1.4)

⁄ (1.5)

That is, the presence of B-field controls charged particles, in especial, electrons so that

electrons are magnetized. The magnetized electrons play an important role to produce a high-

density plasma that can enhance the deposition rate for plasma processing such as plasma

sputtering. Figure 1.4. shows the rotation of electron in the presence of the magnetic field. The

Larmor radius can be controlled by the velocity, v and the magnetic field, B, respectively.

Figure 1.4. Rotation of electron in the presence of the magnetic field.

1.1.3. E × B Drift Motion

The direction of electric field E and the behavior of magnetic flux density B plays an

important role in performing the magnetized discharge based on E×B drift motion, where E

and B are the electric field perpendicular to the target and magnetic flux density parallel to the

target, respectively. The direction of E × B drift motion in the presence of electric and magnetic

field is shown in Fig. 1.5. Moreover, it can be demonstrated by the right hand rule. If the

magnetic fields are so strong that both ions and electrons have Larmor radii much smaller that

the plasma radius. So that the particles guiding centers drift across B-field, in response to

applied electric fields E, the perpendicular component to B-field. The drift velocity can be

7

found using the following Eq. (1.6). The velocity component parallel to B-field is unaffected

by E-field. Moreover, the drift velocity, v is perpendicular to both E-field and B-field and have

the same value for ions and electrons.

| |

(1.6)

Figure 1.5. Direction of E × B drift motion in the presence of electric and magnetic field.

1.2. Thin Film Deposition by Plasma Sputtering 1.2.1. Principle of Plasma Sputtering

Sputter deposition is a physical vapor deposition (PVD) method of thin film deposition.

This process consists of ejecting material from a target surface onto a substrate. Sputtered

atoms ejected from the target surface have a wide energy distribution, typically up to tens of

eV (100,000 K). The number of atoms ejected or “Sputtered off” from the target surface is

called the sputter yield. The sputter yield depends on the energy and incident of angle of the

bombarding ions, the relative masses of the ions and target atoms, and the surface binding

energy of the target atoms. Plasma processing such as plasma CVD and sputtering are widely

used for fabrication of microelectronic thin film preparation. Magnetron sputtering has become

the process of choice for the deposition of a wide range of industrially important coatings.

Figure 1.6 shows the principle of magnetron sputtering demonstrating the direction of electric,

magnetic field, charged particle motions and target erosion. Magnetron Sputtering is a Plasma

8

Vapor Deposition (PVD) process in which a plasma is created and positively charged ions from

the plasma are accelerated by an electrical field superimposed on the negatively charged

electrode or "target". The positive ions are accelerated by potentials ranging from a few

hundred to a few thousand electron volts and strike the negative electrode with sufficient force

to dislodge and eject atoms from the target. These atoms will be ejected in a typical line-of-

sight cosine distribution from the face of the target and will condense on surfaces that are

placed in proximity to the magnetron sputtering cathode. A magnetic field is applied at right

angles to the electric field by placing large magnets behind the target. This traps electrons near

the target surface and causes them to move in a spiral motion until they collide with an Ar atom. To increase deposition rates, magnets are used to increase the percentage of electrons that take

part in ionization events, increasing the ionization efficiency. Orbital motion of electrons

increases probability that they will collide with neutral species and create ions.

9

Figure 1.6. Principle of magnetron sputtering demonstrating the direction of electric,

magnetic field, charged particle motions and target erosion.

1.2.2. Merits and Demerits of Plasma Sputtering

Magnetron Sputtering Sources are very familiar plasma source for functional thin film

preparation such as metal, oxide, nitride and nano-materials thin films. The merits are high-

speed deposition rate, high-quality thin film property. However, the major demerits are non-

uniform target erosion profile, lower target utilization rate such as 30 - 40% due to localized

ring-shaped high-density plasma. So that, it is needed to develop a novel plasma sputtering

source with improved target utilization percentage, erosion rate and uniform film property.

10

1.3. Background Research on Plasma Sputtering

1.3.1. Synthesis of Functional Films by Plasma Sputtering

Cu films of 290–350 nm were prepared by DC magnetron sputtering, the target voltage

of 400 V, and Ar gas pressure was introduced at 0.5, 1.0, and 1.5 Pa, respectively by H. Qiu et

al [5]. It was found that the target current increased from 69 to 200 mA with increasing Ar

pressure, and the target voltage was settled at 400 V. The amount of larger grains decreases

with an increase in Ar pressure while the resistivity of the films increases with increasing Ar

pressure. The roughness of the film surface was approximately 5 nm, which was independent

of Ar pressure [5]. The crystalline orientation of the prepared Cu film shifted slightly from the

[1 1 1] direction to the [2 2 0] direction with increasing Ar pressure [5].

Cu doped ZnO films at various doping concentrations of Cu (0, 5.1, 6.2 and 7.5%) by

simultaneous RF and DC magnetron were prepared by A. Sreedhar et al [6]. The structural,

electrical and optical properties of ZnO films was discussed in detail. X-ray diffraction (XRD)

patterns show that the films were polycrystalline in nature wurtzite structure toward c-axis and

Atomic force microscopy (AFM) results indicate that the films displayed needle-like shaped

grains throughout the substrate surface. The electrical resistivities were found to be increased

with an increase of copper content from 0 to 7.5% [6]. Films showed an average optical

transmittance about 80% in the visible region and optical band gap values decreased from 3.2

to 3.01 eV with increasing of Cu doping content from 0 to 7.5%, respectively [6]. However,

the optical band gap values were decreased from 3.2 to 3.01 eV with increasing the copper

doping concentration from 0 to 7.5% in the ZnO host lattice respectively [6].

The size effect on the resistivity of evaporated copper films, ranging in thickness from

9 nm to 167 nm, was determined experimentally from the sheet resistance and the physical

thickness [7]. In combination, to determine the mean grain size of the grains in the plane of the

film, the Electron Back Scatter Diffraction (EBSD) and the XRD methods were used, A. E.

Yarimbiyik et al proposed Matthiessen’s rule can be used to measure the thickness of a copper

film and [7]. The resistivity of these films increased with decreasing film thickness [7].

Al-doped ZnO (AZO) thin films have been prepared by radio frequency (RF)

magnetron sputtering and applied RF power was varied in the range 600–1200 W by A.

Spadoni et al [8]. The effects of RF power on structural, electrical and optical properties were

examined by XRD analysis, Hall measurements and UV–vis–NIR spectrophotometry

respectively. It was found that increasing the RF power, AZO films having a preferential

growth orientation along (002) direction, showed a decrease of the lattice distance indicating a

11

less defected structure [8]. It was observed that the increase of the RF power made a higher

optical absorption by free carriers, coupled with an increase of the band gap value. Resistivity

varied from 1.1 × 10−3 Ω cm at 600 W down to a minimum value of 5.6 × 10−4 Ω cm at 1200

W, whereas the carrier density increased up to 1 × 1021 cm−3. Lattice defect variation of AZO

films was analyzed by photoluminescence (PL) measurements. It was hypothesized that at

higher RF power a more effective diffusion phenomenon can give more effective Al doping

and less amount of zinc vacancies. As a consequence, Al atoms are more effectively trapped

into the structure [8].

S. Rahmane et al deposited Al-doped ZnO (AZO) films on glass and silicon substrates

by RF magnetron sputtering technique at room temperature [9]. The effects of thickness on the

structural, optical and electrical properties were investigated. It was found that the electrical

resistivity decreases with the increase of the film thickness and the smallest measured value

was 8 ×10-4 Ω cm for the 1500 nm thick film. All the deposited films showed a crystalline

wurtzite structure with a strong preferred (002) orientation. It was observed that the intrinsic

compressive stress decreased with the increase of the film thickness [9]. The obtained AZO

films had an average transmittance greater than 90% in the visible region and films have an

optical band gap between 3.32 and 3.49 eV depending on the film thickness [9].

Cu film has been proposed because of its low resistivity, high chemical stability, and

excellent electromigration resistance B. H. Wu et al [10]. The films deposited on a silicon

substrate by varying the substrate bias voltage using high power pulsed magnetron sputtering

(HPPMS). It was found that the substrate bias increased from - 17.3 V to - 100 V, the electron

current decreases slowly, while the ion current increases, and steadying at - 50 V [10]. It was

observed that the Cu films prepared at - 50 V, and -100 V exhibited a higher tensile stress and

a superior (111) texture. The electrical resistivity of the deposited Cu films was found to be a

minimum value of 1.79 μΩcm at - 100 V [10].

A. S. Christiansen et al investigated the nitrogen dissociation and plasma parameters

during radio frequency sputtering of lithium phosphorus oxynitride thin films in nitrogen gas

by mass appearance spectrometry, electrostatic probes, and optical emission spectroscopy [11].

Despite lower plasma density, the film grows quicker at a lower pressure where the higher

plasma potential, translated into higher energy for imposing ions on the substrate which

resulted in a compact and smooth film structure. It was found that low pressure (5 mTorr) and

moderate power (100 W) were most beneficial for the growth of good quality films with a high

ionic conductivity [11]. Increasing the RF power (300W) resulted in a poor film quality due to

cracks and dislocation of large clusters from the target material, while increasing the nitrogen

12

pressure (50 mTorr) resulted in a lower deposition rate and a rough microstructure with

volcano-shaped structures on the surface. Higher pressures showed much less nitrogen

dissociation and lower ion energy with thinner films, less ionic conductivity and poor film

structure with large roughness [11].

A highly transparent field emitter was achieved by Ar+ ion irradiation onto highly

transparent and conducting ZnO films deposited on glass substrates by Zurita Zulkifli1 et al

[12]. The deposited flat ZnO films before ion irradiation, which showed 90% transmittance and

186 Ω/cm sheet resistance, showed no field emission current up to 15 μVm-1 [12]. The

nanocone size was less than the wavelength of visible light and the transmittance was

maintained at 86% for the ion-irradiated ZnO film [12]. It was observed that the field emission

properties of the transparent nanoconed ZnO were promising compared to other nontransparent

nanostructured ZnO [12].

An inverted gapped-target magnetron sputtering device for deposition of ferromagnetic

thin films under energetic conditions has been developed by P. Poolcharuansin et al [13]. The

side view and the front view pictures during the inverted plasma discharge with a power of 80

W and with a pressure of 2 Pa were discussed [13]. No cracking of the film surfaces was found

in the sputtering of nickel films without substrate heating or biasing. Langmuir probe and ion

energy measurements confirmed that plasma ions with a density of around 1016 m-3 and the

almost energies of approximately 200 eV can be reached at the substrate [13].

1.3.2. Previous Research for Improving Target Utilization and their Problems

A wider eroded and higher target utilized magnetron sputtering system for

ferromagnetic nickel (Ni) target using a large, tall and eccentrically rotating tilted center

magnet was proposed by T. Iseki et al [14]. The target utilization was found approximately

49% for 5-inch and 4-mm thick Ni target, which is 8% better than when a non-tilted center

magnets were used. It was observed that the rotating mechanism of a yoke magnet could be

separated from the cooling water and the target could be cooled more effectively and the center

non-eroded area was decreased [14].

T. Iseki et al investigated the dependence of the magnetic flux density, erosion

uniformity, and target utilization on the yoke magnet tilt angle in a planar magnetron sputtering

system, using a rotating, tilted, unbalanced, asymmetrical magnet [15]. The magnetic flux

density distributions were measured two-dimensionally on the target surface. As the yoke

magnet tilt angle increased from 0 to 8 degree, utilization of a 5-inch target linearly increased

from 60 to 80%. On the other hand, with an elliptical outer yoke, the target utilization was

13

approximately 70%, regardless of the yoke magnet tilt angle [15]. It was found that the

deposition rate when using the elliptical outer yoke was 1.2 times faster than that of when using

the circular outer yoke at the same magnet tilt angle [15]. However, to make a tilt angle in a

yoke surface, an unbalanced magnet is arranged on the magnet holder surface. The number of

magnets are different from one side to another side. These effect make short system life.

Y. Ohtsu et al proposed the racetrack-shaped RF magnetron plasma with weak rubber

magnets (ferrite and neodymium) for the full utilization of the circular target and the reduction

of the magnet weight [16]. The magnetic field simulations for the ferrite rubber magnets, the

neodymium rubber magnets, and the neodymium rubber magnets including the neodymium

metal magnets were investigated [16]. The radial profile of the erosion depth was roughly

constant for r = 20 mm and then decreases slowly away from the center for an RF power of 40

W, Ar gas pressure of 2 Pa and a sputtering time of 4 h. It was found that the target utilization

was approximately 72% estimated from the target erosion profile [16].

1.4. Problems of the Plasma Sputtering and Objectives of this Thesis

Magnetron Sputtering Sources are very familiar plasma source for functional thin film

preparation such as metal, oxide, nitride and nanomaterials thin films. Recently, RF magnetized

plasma sources have been widely used in microelectronics, such as magnetic films, surface

treatment and cleaning, diamond-like carbon, biomaterial thin films, flat panel display

fabrication, transparent conductive oxide film preparation for solar cells and mobile phones,

and many other rapidly growing areas. In particular, RF magnetron plasma sources have

become an attractive tool for functional film preparation. In the conventional magnetron system,

the target material is not effectively used because high-density plasma is localized on the target

surface. The target utilization is very low approximately at 20-30% [5].

From the practical viewpoint of the limited resources, the utilization of the target

material is necessary. The symmetrical magnets magnetron sputtering method with one inner

magnet and two outer annular magnets facing each other were investigated. The maximum

target erosion rate was 57% [17]. The rotating magnet sputtering has also been proposed by

rotating helical magnets for increasing the target utilization efficiency. Using a rotating

unbalanced and asymmetrical magnet, a flat erosion-sputtering method has been developed.

The estimated target utilization had a value of 80% and 77% for five, four-inch aluminum target

material, respectively [13-14]. The target utilization efficiency was increased from 73.6% to

86.3% when iron pole pieces were used in the rotating cruciform arrangement of neodymium

magnets [17-18].

14

In general, near the chamber wall, the plasma potential and the plasma density is very

small because an ion sheath exists near the wall [1, 19]. The plasma potential varies slowly in

the plasma, but rapidly in the sheath region. Only in the sheath region, the quasineutrality

property cannot be satisfied. The plasma density profile is relatively flat in the center and falls

sharply near the sheath edge. It is required to produce (1) outer ring-shaped and (2) specific

area plasmas for obtaining (1) uniform, (2) high-density plasma as well as (3) convenient outer

target area erosion profile near the chamber wall. Moreover, plasma processing has various

problems on plasma equipment, thin film preparation and so on. In practical industrial

application, the outer width of the target is large.

Therefore, it is required high-density plasma discharge in a specific area and also in the

outer region of the chamber to obtain convenient outer target area erosion profile near the

chamber wall. Moreover, to deposit a functional thin film in a specific area and near the

chamber wall, the target utilization in a specific area is required from the viewpoint of target

utilization. However, the conventional magnetron plasma has an issue that the target erosion is

not uniform owing to high-density localized plasmas.

In this thesis, the developments of novel plasma source equipment with uniform target

utilization are investigated for the growing plasma processing applications. The following aims

and objectives of the research are listed below.

Finding the way of increasing target utilization percentage

Making a uniform target erosion over the entire area of the target

Depositing a uniform functional film property

Producing a specific area plasma for target utilization near the chamber wall

Obtaining a pulsed dc discharge plasma for target utilization

1.5. Structure of this Thesis

The structure of the thesis is organized into separate chapters. Chapter 2 discusses the

experimental methods and measurement techniques used during the doctoral study as well as

simulation and analysis methods used to obtain and interpret information. Various kinds of

measurement tools and diagnostics are presented.

Chapter 3 describes a high-density radio frequency (RF) magnetized sputtering plasma

source with a rotational square-shaped magnet arrangement for uniform target utilization. The

experimental setup for the proposed RF magnetized plasma sputtering system is explained in

detail. The magnetic field profiles obtained based on the squared-shaped magnets, the plasma

15

emission and discharge characteristics, the copper target erosion profile and ion saturation

currents are investigated explicitly. Finally, the results obtained in this chapter are summarized.

Chapter 4 is dedicated to investigating the performance of a gyratory square-shaped RF

magnetized plasma sputtering source for materials processing and different functional film

preparations. A radial profile and microstructure of the deposited films are investigated under

two square-shaped magnet schemes. The experimental setup, the results and discussions on the

effects of the magnetic shielding material in a square-shaped magnet arrangement on the

discharge characteristics, plasma emission luminescence, thickness and resistivity profile, as

well as the surface morphology of the film deposited by the gyratory square-shaped RF

magnetized plasma sputtering source is explained in detail. Finally, the results obtained in this

chapter are summarized.

Chapter 5 is focused to make an RF magnetized outer ring-shaped plasma sputtering

source with a concentrically monopole arrangement of magnets at various different magnet-

gap distances for the specific area target utilization. The experimental setup, the effects of

monopole schemes, the discharge characteristics, electron temperature, plasma density and

electron collision frequency in the monopole arrangements are explained in detail. Finally, the

results obtained in this chapter are summarized.

Chapter 6 is devoted to producing outer ring-shaped and specific area plasmas for

obtaining high-density plasma and outer target erosion profile near the chamber wall. The

proposed experimental setup, the results and discussions on the effects of the three setups are

investigated in a monopole circular magnet arrangement of the magnetic flux lines and their

profiles, discharge characteristics, plasma discharge luminescence, plasma density, and ion

saturation currents are explained in detail. To conclude, the results obtained in this chapter are

summarized. Chapter 7 is dedicated to characterizing a ring–shaped pulsed DC discharge plasma

using single pole magnet setups in a specific area for obtaining outer target erosion profile near

the chamber wall. The proposed experimental setup of pulsed DC discharged plasma, the

results and discussions on the effects of the three setups are investigated in a single pole magnet

arrangement, 2D magnetic flux lines and their profiles, gyro–radius of the electrons and ions,

Hall parameters of the electrons and ions, discharge characteristics, ion saturation current

profiles are explained in detail. As a final point, the results obtained in this work are

summarized.

16

Chapter 8 is summarized based on chapter 1 to chapter 7. Few possible suggestions for

future works are recommended at the end of this thesis. List of refereed publication papers and

contributed conferences and symposiums are listed.

17

References

[1] M. A. Lieberman, and A. J. Lichtenberg, “Principles of Plasma Discharges and Material

Processing”, 2nd ed. John Wiley & Sons, Inc., New York, 2005.

[2] F. F. Chen, and J. P. Chang, “Lecture Notes on Principles of Plasma Processing”, Plenum,

New York, 2002.

[3] A. I. Morozov, “Introduction to Plasma Dynamics”, CRC Press, 2012.

[4] T. Makabe “Advances in Low Temperature RF Plasmas, Basis for Process Design, Elsevier,

2002.

[5] H. Qiu, F. Wanga, P. Wua, L. Pana, and Y. Tiana, Vacuum, 66, 447–452, 2002.

[6] A. Sreedhar, J. H. Kwon, J. Yi, J. S. Kim, and J. S. Gwag, Materials Science in

Semiconductor Processing, 49, 8–14, 2016.

[7] A. E. Yarimbiyik, H. A. Schafft, R. A. Allen, M. D. Vaudin, and M. E. Zaghloul,

Microelectronics Reliability, 49, 127–134, 2009.

[8] A. Spadoni, and M.L. Addonizio, Thin Solid Films, 589, 514–520, 2015.

[9] S. Rahmane, M. S. Aida, M. A. Djouadi, and N. Barreau, Superlattices and Microstructures,

79, 148–155, 2015.

[10] B. H. Wu, J. Wu, F. Jiang, D. L. Ma, C. Z. Chen, H. Sun, Y. X. Leng, and N. Huang,

Vacuum, 135, 93 – 100, 2017.

[11] A. S. Christiansen, E. Stamate, K. Thyden, R. Younesi, and Peter Holtappels, Journal of

Power Sources, 273, 863 – 872, 2015.

[12] Z. Zulkifli1, S. Munisamy, M. Z. M. Yusop, G. Kalita, and M. Tanemura, Japanese Journal

of Applied Physics, 52, 11NJ07, 2013.

[13] P. Poolcharuansin, P. Laokul, N. Pasaja, A. Chingsungnoen, M. Horprathum, P.

Chindaudom, and J. W. Bradley, Vacuum, 141, 41-48, 2017.

[14] T. Iseki, H. Maeda, T. Itoh, Vacuum, 82, 1162–1167, 2008.

[15] T. Iseki, Vacuum, vol. 84, pp. 339–347, 2010.

[16] Y. Ohtsu, S. Tsuruta, T. Tabaru, and M. Akiyama, Surface Coatings Technology, 307,

1134–1138, 2016.

[17] Y. Ohtsu, M. Shigyo, M. Akiyama, T. Tabaru, Vacuum, 101, 403 – 407, 2014.

[18] T. Ide, M. A. Hossain, Y. Nakamura, and Y. Ohtsu, Journal of Vacuum Sci. Technol. A:

Vacuum, Surfaces, and Films, 35, 061312, 2017.

[19] P. J. Kelly, R. D. Arnell, Vacuum, 56, 159 – 172, 2000.

18

Chapter 2. Measurement Techniques and Methods

2.1. Major Experimental Apparatus

Major apparatus used to do the experiment in this research showing matching network,

vacuum system, external motor circuits, oscilloscope, and plasma discharge chamber are

shown in Fig. 2.1. The experiments were performed in a stainless-steel cylindrical vacuum

chamber with an outer diameter of 235 mm, an inner diameter of 160 mm and a height of 195

mm. The discharge chamber was evacuated to a base pressure of 10-5 Pa by a turbo molecular

and an oil rotary pump [1-3]. Argon (Ar) gas was used as sputtering gas by regulating a flow

meter. An RF power source at 13.56 MHz was applied to the target via an impedance matching

network. A copper plate of 3 mm thickness was used as the target to measure the sputtering

characteristics. An aluminum disc was only used to measure plasma parameters in order to

avoid thin film deposition on the Langmuir probe. The discharge voltage between the RF

powered electrode and the grounded vacuum wall was measured by a high-voltage probe

connected with oscilloscope.

The iron yoke is set up with a gap of 1 mm, and 0 mm from the copper target plate for

rotational case and stationary case, respectively. The circular iron yoke disk with the magnet

arrangement was rotated by an iron yoke shaft by an external motor. A precision surface tool

was used to measure the erosion depth of the copper target as a basis to calculate the target

utilization percentage. The plasma parameters such as the time-averaged ion saturation currents

are measured to obtain the ion flux to the target by a tiny tungsten wire probe of 1.0 mm in

diameter and 10 mm in length. In order to avoid the influence of the RF potential oscillations

on the probe current-voltage characteristics, the probe wire was connected to an LC filter circuit

[1-2]. The ion flux to the target is calculated from the ion saturation current by negatively

biasing the probe including a resistance of 100 Ω.

19

Figure 2.1. Major apparatus used to do the experiment in this research showing matching

network, vacuum system, external motor circuits, oscilloscope, and discharge chamber.

2.2. Ion Saturation Current, Plasma Density and Electron Temperature

Figure 2.2 shows ion saturation current and current-voltage characteristics curve

measuring equipment. Langmuir probe and DC power supply were used. The biasing voltage

was changed from -70 V to +70 V. The Langmuir probe was compensated with an LC parallel

filter circuit to avoid the influence of the RF plasma potential fluctuations on the probe current-

voltage characteristics. The probe measurements were carefully performed so as to minimize

the disturbance to the stable plasma [1-3].

20

Figure 2.2. Ion saturation current and current-voltage characteristics curve measuring

equipment.

Figure 2.3 shows a practical current-voltage characteristics curve showing its various

parts. The point at which the curve crosses the V axis is called the floating potential Vf. At any

potential, where electron saturation currents start is defined as Vp. The electron temperatures,

the plasma density are estimated using current-voltage characteristics curve. According to the

probe theory [1-2], the ion saturation current is proportional to the ion flux to the target.

Moreover, the plasma density can be estimated from ion saturation, which can be expressed as

the following equation.

0.6 / (2.1)

21

Figure 2.3. A practical current-voltage characteristics curve showing its various parts.

At floating potential, electrons and ions currents are equal that is Ie = Ii. So that total

current is zero. The electron current can be expressed as Eq. (2.2). Taking logarithm and

making simplification Eq. (2.2) can be written in the following form in Eq. (2.3) and (2.4). So

that the inverse slope of the logarithmic electron probe current with respect to biasing voltage,

VB (in volts) gives electron temperature, Te directly.

exp (2.2)

ln (2.3)

ln

(2.4)

The measurement of ion saturation current, Iisat is the simplest and best way to

determine the plasma density, n. The sheath around a negatively biased probe is so thin that the

area of the sheath edge is essentially the same as the area of the probe tip itself at higher plasma

densities more than 1016 m-3. Eq. (2.1) indicates a constant ion saturation current, which can

occur only for flat probes in which the sheath area cannot increase as the probe is functioned

22

more and more negative. In practice, ion saturation current, Iisat usually has a slope to it because

the Iisat has to come from a disturbed volume of plasma (the presheath). Plasma density can be

calculated from the following Eq. (2.5).

0.6 /

(2.5)

2.3. Measurement of the Eroded Target and Target Utilization Rate Calculation

A precision surface profile tool (Mitutoyo SJ-400) was used to measure the radial

profile of the thickness of the deposited Cu film. Figure 2.4 shows the target erosion depth and

film thickness measuring apparatuses showing eroded target and moving step profiler. For

erosion depth measurement, measurements are carried out using a step meter (Mitutoyo SJ-

400). A precision tape was attached to a part of the target material. The target material was set

for the experiment for plasma sputtering. After the experiment, the tape was taken off. The

mechanical step at the tape boundary between tape part and the sputtered part to measure the

difference. Scanning was done in the direction perpendicular to the target surface. Scan in the

direction of the erosion surface from the part where it is eroded. The thin film thickness is

obtained from the output data obtained by the stylus profilometer. In fact, it captures the fine

unevenness of the material surface with high definition.

The target utilization percentage was estimated based on the ratio of the target erosion

volume to the volume calculated under the assumption that the target is completely eroded at a

maximum erosion depth [4]. Target erosion depth profile and calculation of the target

utilization percentage is shown in Fig. 2.5. The target utilization percentage is calculated using

following Eq. (2.6).

2 / 100 % (2.6)

23

Figure 2.4. Target erosion depth and film thickness measuring apparatuses showing eroded

target and moving step profiler.

24

Figure 2.5. Target erosion depth profile and calculation of the target utilization percentage.

2.4. Resistivity of the Prepared Film

The resistivity of the deposited films was measured by a standard four terminals probe

method at a room temperature. Separate pairs of current-carrying and voltage-sensing

electrodes were used to make more accurate measurements by four-terminals probe method.

Figure 2.6 shows Four terminals probe method to measure deposited film resistivity mentioning

(a) schematic diagram and (b) electrical circuit diagram. The deposited film resistance can be

calculated using current voltage characteristics curve. However, the film resistance is expressed

as following Eq. (2.7), where Af is the deposited film surface. The resistivity, ρ can be calculate

using the Eq. (2.8), where Rf is the resistance of the prepared functional film.

(2.7)

(2.8)

25

(a)

(b)

Figure 2.6. Four terminals probe method to measure deposited film resistivity (a) schematic

diagram and (b) electrical circuit diagram.

26

2.5. Magnetic Field Analysis Simulation

The two-dimensional magnetic flux lines and their density profiles (Bx and Bz) are

analyzed by conventional Electromagnetic field analysis software poisson superfish – 7.18 [5]

developed at Los Alamos National Laboratory. It was analyzed by selecting the dimensions

and the magnetic properties of the permanent magnet. It was created an input coding that runs

on Poisson Superfish with TeraPad. It was analyzed by creating a mesh area by command

Automesh, and WSFplot command for display of mesh creation and confirmation. Magnetic

field analysis was performed by commanding Pandira. Output electronic text format data of

the magnetic field was made by the commanding SF 7. Using MS Excel text format data was

analyzed, and horizontal, Bx, and vertical, Bz magnetic field density profiles were evaluated.

2.6. Measurement of the Film Property

Atomic force microscopy (AFM) scanning was performed to observe the surface

morphology of the prepared films. In order to investigate the structure of the deposited film,

X-ray diffraction (XRD) standards (card no. ICDDPDF#04-0836) were used. Four-terminal

method was used the measure the film resistance of the prepared film.

27

References

[1] F. F. Chen, J. P. Chang, “Lecture Notes on Principles of Plasma Processing”, Plenum, New

York, 2002.

[2] M. A. Lieberman, and A. J. Lichtenberg, “Principles of Plasma Discharges and Material

Processing”, 2nd ed. John Wiley & Sons, Inc., New York, 2005.

[3] P. Chabert and N. Braithwaite, “Physics of Radio- Frequency Plasmas”, Cambridge

University Press, Cambridge, 2011.

[4] Y. Ohtsu, S. Tsuruta, T. Tabaru, and M. Akiyama, Surface Coatings Technology, 307,

1134–1138, 2016.

[5] http://laacg.lan.gov/laacg/services/serv_codes.phtml.

28

Chapter 3. Rotational Square-Shaped Setups of Rod Magnets for Uniform

Target Utilization

3.1. Introduction

Plasmas and their particles play an important role in nanotechnology and

semiconductor manufacturing [1-2]. Various kinds of plasma sources have been developed for

plasma processing. In particular, CCP driven by RF power supplies at 13.56MHz are widely

used for plasma processing, because their setup and maintenance are simple [3-4]. However,

CCP sources suffer from the fact that (1) productivity and deposition rates are low due to the

low plasma density of less than 109 cm-3 (depends on discharge conditions) and (2) the plasma

density and the energy of ions at the electrodes cannot be controlled separately in single

frequency CCPs [5]. On the other hand, it is well known that high-density plasmas can be

produced by hollow cathode discharges [6-18] because electrons are effectively confined in the

hollow trench.

The RF magnetized sputtering plasma source is widely utilized in the microelectronic

industry for functional thin film deposition [19-25] and to produce magnetic films [26-27]

because the setup is simple and their thin films are not conductive. Its application extends to

surface treatment and cleaning [13-14], flat panel display fabrication; transparent conductive

oxides film preparation [19-20] for solar cells and mobile phones and many other rapidly

growing areas [2]. In particular, the RF magnetron sputtering process has become popular for

the deposition of a variety of industrial surface coatings. This is because, at a RF input power

of more than 1000 W, the magnetron discharge plasma attains a high-density of charged

particles of 1010 - 1011 cm-3 at low gas pressures around 1 Pa due to plasma confinement by the

E×B drift motion [2], where E and B are the electric field perpendicular to the target and the

magnetic flux density parallel to the target, respectively.

The target surface is continuously eroded during the magnetron operation, but the

target material is not used effectively due to the non-uniformity of the plasma density. Thus,

the sputtering occurs highly localized in the region of the electron confinement. This leads to

the formation of a narrow and deep erosion groove and a low target material utilization [2]. In

conventional magnetron sputtering sources, the target utilization is approximately 20 to 30%

due to the narrow and deep groove erosion of the target [28]. It was reported that the target

erosion influences the ion distribution function in the near-cathode region as well as the

deposition rate to the substrate [29].

29

The estimated utilization of the target is increased to approximately 60-70% for planar

magnetron sputtering using a rotating tilted unbalanced and asymmetrical yoke magnet [30],

whereas RF magnetron plasmas based on stationary monopole arrangements attain a target

utilization of up to 59% for a magnet spacing of 10 mm [31]. Rotating the permanent magnet

in a circular cathode is a common technique for expanding the eroded area and increasing the

target utilization. However, at the center and outside the target, some non-eroded areas remain.

From the viewpoint of the practical operation of resources, improvement of the target material

utilization, uniform target erosion and high-density plasmas, as well as the prediction of

functional thin film deposition rates is required.

A high-density RF magnetized sputtering plasma source with a rotational square-

shaped magnet arrangement for uniform target utilization has been developed. This chapter

discusses a method to increase the target utilization and deposition rate for uniform erosion

over the entire area of the target. It is based on the rotation of a neodymium permanent rod

magnet of square-shaped arrangement with respect to the circular target. In section 3.2, the

experimental setup for the proposed RF magnetized plasma sputtering system is explained in

detail. In section 3.3.1, the magnetic field profiles obtained based on the squared-shaped

magnets are described. In section 3.3.2, the plasma emission and discharge characteristics are

discussed. In section 3.3.3, the copper target erosion profile and ion saturation current are

investigated explicitly. In section 3.3.4, the results obtained in this work are summarized.

3.2. Experimental Setup

The experiments are performed in a stainless-steel cylindrical vacuum chamber with

an outer diameter of 235 mm, an inner diameter of 160 mm and a height of 195 mm such as

shown in Fig. 3.1. In this experiment, a Cartesian coordinate system is used, because the

magnetic field is not axisymmetric. In this setup, before starting the experiment, an initial

vacuum pressure in the chamber of 2.8 10-4 Pa is realized based on a turbo molecular and an

oil rotary pumps. Argon (Ar) gas at 1.03 Pa is used by regulating a flow meter. An RF power

of 50 W at 13.56 MHz was applied to the target via an impedance matching network The

electrode (target) dimensions are 160×160×3 mm and the grounded wall. A copper plate with

3 mm thickness was used as the target to measure the sputtering characteristics based on

rotational square-shaped magnet schemes in the proposed high-density RF magnetized plasma

sputtering source. An aluminum disc was only used to measure plasma parameters in order to

avoid thin film deposition on the Langmuir probe. The discharge voltage between the RF

30

powered electrode and the grounded vacuum wall was measured by a high-voltage probe. The

typical discharge voltage changed from 800 to 600 Vpp (peak-to-peak value) with increasing

gas pressure from 1 Pa to 5 Pa at an RF input power of 50 W. The sputtering time period was

4 hours to measure the erosion depth of the target and to perform a calculation to obtain the

target utilization percentage. The origins of axial z and radial r positions are defined at the

surface and at the center of the RF powered target, respectively.

Eight Neodymium rectangular rod magnets of 30 mm length, 5 mm width, and 3 mm

height, where the connection between N-pole and S-pole magnets is one side of the squares,

are mounted on a circular iron yoke disk. The iron yoke is set up with a gap of 1 mm from the

copper target plate. An iron (Fe) cover of 5×3×1 mm is used for magnetic shielding of otiose

magnetic fields from the permanent magnets. The three magnet arrangements including

magnetic shielding material and the iron cover of 1 mm thickness are shown in Fig. 3.2 for the

proposed high-density RF plasma sputtering system. Details of the three magnet arrangements

will be explained later.

Figure 3.1. Experimental setup for the proposed high-density RF magnetized plasma

sputtering source.

31

The circular iron yoke disk with the magnet arrangement was rotated by an iron yoke

shaft at a speed of 40 rpm by an external motor. A precision surface tool was used to measure

the erosion depth of the copper target as a basis to calculate the target utilization percentage.

Experimental conditions are shown in Table – 3.1. The plasma parameters such as the time-

averaged ion saturation currents are measured to obtain the ion flux to the target by a tiny

tungsten wire probe of 1.0 mm in diameter and 10 mm in length. The time-averaged ion

saturation currents were measured by moving the L-shaped needle probe in axial z and radial r

directions as shown in Fig. 3.1. In order to avoid the influence of the RF potential oscillations

on the probe current-voltage characteristics, the probe wire was connected to an LC filter circuit

[1-5]. The ion flux to the target is calculated from the ion saturation current by negatively

biasing the probe including a resistance of 100 Ω positioned at z = 10 mm.

Table 3.1

Practical Experimental Conditions

Item Specification

Initial vacuum base pressure [Pa] 3.2 10-4

Argon gas introduce pressure [Pa) 1.03

Sputtering time [hours) 4

RF input power [W] 50

RF discharge [Vpp) 796

Self-biased DC voltage [V] -389

Rotational speed for iron yoke [rpm] 40

Neodymium magnet size [mm] 30 5 3

Copper target size [mm] 160 160 3

32

(a)

(b)

33

(c)

Figure 3.2 (a)-(c). Proposed square-shaped magnet arrangement (a) without iron (Fe) cover

and no air gap, (b) with iron (Fe) cover and no air gap and (c) with iron (Fe) cover and 5 mm

air gap. Here, the measured magnetic flux lines are also superimposed on these figures.

3.3. Results and Discussions

3.3.1. Magnetic Field Profile Analysis of Square-Shaped Schemes

In order to investigate the effect of the magnetic field pattern induced by the square-

shaped permanent rod magnet schemes on the plasma production, three arrangements of

magnets are used: (a) no iron (Fe) cover, no air gap between the N-pole and the S-pole magnets,

(b) with iron (Fe) cover, no air gap and (c) with iron (Fe) cover, 5 mm [see Fig. 3.2(a)-(c)].

Eight Neodymium rectangular rod magnets of 30 × 5 × 3 mm dimension are positioned in a

square-shape on a circular iron yoke disk of 140 mm in diameter. The iron yoke is set up with

a gap of 1 mm from the copper target plate. The surface magnetic flux density of the rectangular

rod magnets and the thickness of the iron yoke disc is 313 mT and 1 mm, respectively.

As shown in Fig. 3.2(a) - (c), the rectangular rod magnets are placed at a distance of

33 mm away from the central position. The measured magnetic flux lines parallel to the target

are superimposed on the magnet illustrations in Fig. 3.2. Figure 3.2 shows that the magnetic

flux lines are directed from the N-pole to the S-pole. As shown in Fig. 3.2 (a), the direction of

34

the electric field, E, and the behavior of the magnetic flux density, B, play an important role

for generating the magnetized discharge based on the E×B drift motion, where E and B are the

electric field perpendicular to the target and the magnetic flux density parallel to the target,

respectively. The direction of the E×B drift on the surface of the iron yoke disc is outward [see

Fig. 3.2(a)]. Four different E × B drift motions occur in the central region at each contact zone

between the N-pole and the S-pole magnets such as shown in Fig. 3.2 (a).

In this chapter, we propose to improve the uniformity of the target erosion by rotating

the magnets on the iron yoke. When the magnets are rotated at a fixed angular frequency, ,

the rotating speed is v = r, where r is the radial position. At the central region of the target,

the interactional time of ions to the target becomes longer, while for the edge region the

interactional time becomes shorter. This inward drift motion increases the plasma density in

the central region of the target so that the erosion depth is higher than that in the edge region

of the target. Thus, the suppression of the inward drift motion is important. In order to realize

a uniform target erosion, the inward drift motion should be suppressed. The E×B drift motion

can be prohibited by a magnetic shielding material, i.e. an iron cover of 1 mm thickness. The

iron cover of 5 mm in length, 3 mm in width and 1 mm in thickness was used for the magnetic

shielding such as shown in Figs. 3.2 (b) and (c). The influence of the iron cover mounted on

the contact zone between the N-pole and the S-pole magnets [see Fig. 3.2 (b)] and the air gap

between the N-pole and the S-pole magnets [see Fig. 3.2 (c)] on the magnetic field profile are

investigated to suppress the magnetic flux density which enhances the inward drift motion of

the plasma particles.

The two-dimensional magnetic flux lines and their profiles of one–pair magnet in the

square-shaped magnet scheme are analyzed by the conventional magnetic-field analysis

software Poisson Superfish developed in Los Alamos National Laboratory [32]. Figure 3.3

shows the two-dimensional distributions of the magnetic flux lines near the target surface for

cases (a), (b) and (c), respectively. For all square-shaped magnet arrangements and based on

the two-dimensional distributions of the magnetic flux lines near the target surface, the

magnetic field lines starting at the N-pole of the magnets pass through the target, airspace, iron

yoke and then return to the S-pole.

35

(a)

(b)

36

(c)

Figure 3.3. Two-dimensional distributions of magnetic flux lines near the target surface (a)

without iron (Fe) cover and no air gap, (b) with iron (Fe) cover and no air gap, and (c) with

iron (Fe) cover and 5 mm air gap.

Figure 3.4 shows radial distributions of the absolute value of the horizontal magnetic

flux density Bx for the magnet arrangements of cases (a), (b), and (c) at the axial position z

=10 mm. The shape of their peaks is almost the same for all square-shaped magnet

arrangements and the radial profile of the magnetic flux density has three peaks. The

amplitudes of two peaks positioned at x = 3.5 and 3.5 cm are similar for all magnet

arrangements because their position is away from the iron cover. The peak amplitude of |Bx|

for case (b) is the lowest among all magnet arrangements. The magnetic shielding material, i.e.

the iron cover, reduces the horizontal magnetic flux density at x = 0 from approximately 65 to

50 mT. On the other hand, the peak amplitude of |Bx| with the iron cover at r = 0 increases as

a function of the air gap between the magnets. Thus, the cover effect is useful to decrease the

E×B drift towards the center of the target. Thus, the uniformity of the copper erosion profile

will be improved by the presence of the iron cover. The copper target utilization for case (b) is

maximum. From the magnet axis, the direction of the magnetic flux density |Bx| is reversed on

the left and right side such as shown in Fig. 3.4. The ring-shaped plasma formed around the

magnet drifts in opposite directions. These two ring discharges interact with each other. This

37

interaction determines the formation and the shape of the final ring discharge as well as the

position of its center.

Figure 3.5 shows the radial distributions of the absolute value of the axial component

|Bz| the magnet arrangements of cases (a), (b) and (c) at the axial position z =10 mm. It is found

that |Bz| has two peaks and that, based on Fig. 3.5, case (b) corresponds to the lowest amplitude

whereas case (a) corresponds to the highest magnitude among all square-shape magnet schemes.

Thus, case (b) is the best arrangement, because the axial component does not play a role in the

E B drift motion.

Fig. 3.4. Horizontal magnetic flux density as a function of x position at z = 10 mm.

38

Figure 3.5. Vertical magnetic flux density as a function of x position at z = 10 mm.

3.3.2. Plasma Emissions and Discharge Characteristics

Figure 3.6 shows the plasma emission for case (b) at an RF power of 50 W. The plasma

emission was measured by a low-resolution digital camera. The yellow and white colored

rectangles mark the positions of the permanent rod magnets. Strong emission is observed in an

outward direction at the four intersections between the four vertically placed magnets. As

mentioned in the magnetic field section, the plasma emission in outward direction is caused by

the E B drift motion effect. It is also found that a square-shaped plasma is observed in the

center region. At the top of the square-shaped plasma, plasma emission towards the center is

observed. The square-shaped plasma is formed by the effect of the magnetic field resulting

from each magnet. It seems that the plasma density at the edge of the target is higher than that

at the center. For the other magnet schemes such as cases (a) and (c), the plasma emission

images are similar at an argon gas pressure of 1 Pa and a RF power of 50 W. As described in

the magnetic field analysis section, for case (b) we also observe that four E × B drift motions

in the central direction are suppressed compared with the other cases. These inward drift

motions as well as the plasma density at the central direction of the target are prohibited by the

magnetic shielding material, i.e. the iron cover of 5×3×1 mm dimension, in order to achieve a

uniform target erosion.

39

Figure 3.6. Plasma emissions for (b) iron (Fe) cover, no air gap setup at RF power of 50 W.

The RF discharge voltage between the RF powered electrode and the grounded

chamber wall is measured by a digital oscilloscope and a high-voltage probe. The RF power is

fixed at 50 W. Argon gas is introduced into the chamber at 1 to 5 Pa. Fig. 3.7 shows the RF

discharge voltage VRF (peak-to-peak value) as a function of the Ar gas pressure for case (a),

(b), and (c). It is seen that the RF discharge voltage decreases gradually from approximately

760 to 600 Vp-p with increasing the argon gas pressure for all cases. This is because the electron

collision frequency increases with increasing Ar gas pressure so that the ionization rate

increases. According to the absorbed power relation, where ohmic heating is assumed in the

sheath [2], at a fixed absorbed power, the RF discharge voltage is roughly inversely

proportional to the electron-collision frequency.

The absolute value of the dc self-bias voltage, |Vsb|, of the cathode is shown in Fig. 3.8

for all square-shaped magnet schemes. The self-bias voltage of the cathode is negative. Its

absolute value decreases from approximately 400 to 270 V with increasing the gas pressure for

all cases. This tendency is attributed to the decrease in the RF discharge voltage between the

cathode and the grounded chamber wall. It is found from Fig. 3.7 and Fig. 3.8 that the case (a)

40

has the lowest amplitude and the case (c) has the highest magnitude among all square-shape

magnet schemes. These results are similar for the copper target erosion depth profile, which

will be discussed later. For the sputtering of a copper target, the Ar gas pressure of 1 Pa is

selected as a conventional sputtering gas pressure.

Figure 3.7. RF Discharge voltage between the cathode and grounded chamber wall as a

function of argon gas pressure.

41

Figure 3.8. DC self-bias voltage on the cathode as a function of argon gas pressure.

3.3.3. Profiles of the Target Erosion Depth and Ion Flux to the Cu target

The copper sputtering experiments were done by rotating the iron yoke disk at a speed

of 40 rpm for the square-shaped magnet arrangements of cases (a) - (c). Figure 3.9 shows the

radial profiles of the copper target erosion depth for the different magnetic arrangements at an

RF power of 50W and a processing time of 4 hours. As shown in Fig. 3.9, the highest target

erosion depth is observed at r = 30 mm for cases (a) and (b), whereas for case (c), the highest

target erosion depth is found at r = 25 mm. The utilization percentage of the copper target was

estimated based on the ratio of the copper target erosion volume to the volume calculated under

the assumption that the target is completely eroded at a maximum erosion depth. The target

utilization percentages are 74.15, 87.49 and 83.85 % for case (a), (b) and (c), respectively. For

the case (b), the copper target utilization percentage reaches the highest value of 87.49% among

all square-shaped magnet arrangement schemes.

It is seen that the copper target erosion homogeneity is increased by decreasing the air

gap compared with cases (b) and (c) in Fig. 3.9. The magnetic shielding characteristic has been

investigated in order to realize a uniform target erosion depth. The copper target utilization

percentage is increased from 74.15% to 87.49% by using the magnetic shielding iron (Fe) cover

of 1 mm thickness. Moreover, by decreasing the air gap between the shielded magnets, the

copper target utilization percentage is increased from 83.85% to 87.49%. However, a high

42

target erosion depth with some fluctuation and non-uniformity is observed for the case (c) in

Fig. 3.9. The uniformity of the target erosion depth has been improved by using the iron (Fe)

cover and decreasing the air gap between the shielded magnets. Thus, the iron (Fe) cover is

effective to prohibit an inward E×B drift motion to the center of the target so that the uniformity

of the copper erosion profile as well as the target utilization is improved by the addition of the

iron cover and reducing the air gap between one–pair shielded magnet.

In order to study the ion flux to the target, the ion saturation current is measured as a

function of the radial position by a Langmuir probe. Aluminum disks were used to avoid the

deposition of thin films to the probe for this measurement. As shown in Fig. 3.6, since the

plasma distribution is not axially symmetry, the ion current detected by the probe changes

temporally when the magnets are rotating. Temporal variations of the ion saturation current

were measured at each radial position from r = 0 to 45 mm. The time-averaged ion saturation

current is estimated from these temporal variations.

Figure 3.9. Radial profiles of the copper target erosion depth at RF power of 50 W.

43

Figure 3.10 shows the radial profiles of the time-averaged ion saturation current, Iisat

for cases (a) - (c). The ion saturation current is measured by a tiny probe biased at Vp = - 70 V

at z = 10 mm. The RF power was fixed at 50 W. According to the corresponding probe theory

[1], the ion saturation current is proportional to the ion flux to the target. It is shown that the

uniformity of the ion saturation current for case (b) is better than for the other square-shaped

magnet arrangement. For case (a), the time-averaged ion saturation current is lower than for

the other proposed scheme as shown in Fig. 3.10. For case (b), the radial profile of the ion flux

to the target is approximately similar to the radial profile of the copper target erosion depth as

shown in Fig. 3.9. The sputtering rate Rsp of the copper target is expressed using the following

Eq. (3.1), where is the yield of sputtered atoms per incident ion. , and e are the area

of the probe used for detecting the time-averaged ion saturation current, the atomic density of

the copper target and the electronic charge, respectively.

(3.1),

Therefore, the erosion depth profile is consistent with the ion flux profile. The plasma

density is calculated from the time-averaged saturation current for cases (a) - (c) using the

Bohm sheath criteria [1]. Measurements were performed at a position of x = 0 mm and z =10

mm. The calculated plasma densities are 8.8×109 cm-3, 1.1×1010 cm-3, 1.0×109 cm-3 for cases

(a) - (c), respectively. The plasma density was the highest for case (b) among all square shaped

arrangements.

44

Figure 3.10. Time-averaged ion saturation current as a function of the radial position detected

by Langmuir probe at various square-shaped magnet arrangements.

45

3.4. Conclusion

A square-shaped magnet arrangement consisting of eight neodymium rod magnets of

30 × 5 × 3 mm has been investigated in order to realize a uniform utilization of a copper target

in an RF magnetized sputtering plasma. A copper plate of 160 × 160 × 3 mm and an aluminum

disc of 160 mm in diameter were used as the target to measure the sputtering characteristics

based on rotational square-shaped magnet schemes in the high-density RF magnetized plasma

sputtering source. The magnetic shielding material, iron (Fe) cover, suppresses the horizontal

magnetic flux density |Bx| at x = 0 from approximately 65 to 50 mT. It is also found that the

case (b) with iron (Fe) cover and no air gap has the lowest amplitude of the horizontal magnetic

flux density among all square-shape magnet schemes. Strong plasma emission in an outward

direction is observed at four intersections between the four vertically placed magnets. It is seen

that the RF discharge voltage and the absolute value of the dc self-bias voltage decrease

gradually from approximately 760 to 600 Vp-p and approximately 400 to 270 V, respectively,

with increasing argon gas pressure for all cases. As the sputtering rate is determined by the ion

flux to the target, it is found that the target erosion profile approximately agrees with the ion

flux profile for case (b). The uniformity of the target erosion depth and ion flux profile has

been improved by using the iron (Fe) cover and decreasing the air gap between the shielded

magnets. The iron (Fe) cover is effective to prohibit the inward E×B drift motion to the center

of the target so that the uniformity of the copper erosion profile is improved by the addition of

the iron cover and reducing the air gap between the one–pair shielded magnet and the target

utilization reaches the highest value of 87.49%.

46

References

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48

Chapter 4. Performance of a Gyratory Square-Shaped Capacitive Radio

Frequency Discharge Plasma Sputtering Source for Material Processing

4.1. Introduction

Plasma processing has recently drawn significant attention owing to its promising

applications, including in semiconductor and manufacturing industries around the world [1-2].

Many plasma sources and experiments developed in the past four decades have been discussed

for the purposes material processing and functional film preparation [2-3]. CCPs functioned by

a RF power supply at 13.56 MHz operating in various gases are, in principle, some of the

simplest, and easily maintained methods of large-diameter substrate processing, and are

important in many high-tech applications, including etching, deposition of thin films,

modification of surface properties and microelectronic device fabrication [4–8]. In particular,

CCPs have some limitations, such as a low plasma density of less than 109–1010 cm-3, a low

deposition rate, the standing wave effect [8] on the powered electrode (target), as well as the

difficulty of controlling external parameters and energized ions at the target independently [4–

9]. Therefore, the conventional CCPs are not a tool suitable for plasma processing. The physics

and chemistry of capacitively coupled RF plasmas have been the subject of many theoretical

and experimental studies via modeling; in order to overcome the limitations, a number of

solutions have been employed [1, 10–12]. Some solutions include very high-frequency (VHF)

operation to increase the plasma density at a given input power [1, 8, 12], optimization of the

gas mixing to maximize the ionization probability [14], and the use of the high-secondary

electron emission electrodes [6, 13]. Two mechanisms [15-16] of electron heating, such as (1)

ohmic heating due to collisions of electrons with neutral gases and (2) stochastic heating—

often referred to as Fermi heating—due to momentum transfer from the oscillating RF sheath,

pressure and ambipolar electron heating [17], as well as voltage waveform tailoring [18], play

a major role in capacitively coupled RF discharges. Different methods [19–20] have been

proposed in order to study electron heating phenomena in capacitive discharges. Experiments

have also been performed to investigate stochastic heating [21–23]. Many authors have

investigated the characteristics of dual-frequency (DF) inductively coupled plasma (ICP) [24],

CCP discharges, experimentally [25–26]. Moreover, a novel type of DF-CCP operating at

consecutive harmonics instead of at substantially different driving frequencies was proposed

based on the electrical asymmetry effect (EAE), in order to avoid the limitations of the

independent control of sputtering parameters, such as the density and energy of ions incident

on the target [27]. In general, it is well known that hollow-cathode discharges [9, 28–32] can

49

produce high-density plasma using only a simple structure for the hollow electrode because a

hollow trench can confine electrons effectively. Recently, RF magnetized plasma sources have

been widely used in microelectronics [33–35], such as magnetic films [36], surface treatment

and cleaning [30], diamond-like carbon [37], biomaterial thin films, flat panel display

fabrication, transparent conductive oxide film preparation [33, 35] for solar cells and mobile

phones, and many other rapidly growing areas [1]. In particular, RF magnetron plasma sources

have become an attractive tool for functional film preparation. However, the conventional

magnetron plasma has an issue that the target erosion is not uniform owing to high-density

localized plasmas.

In our previous work, a high-density RF magnetized sputtering plasma source with a

rotational square-shaped magnet arrangement was developed for uniform target utilization

about 88 % uniformity [38]. The results revealed that the uniformity of the target erosion depth

and ion flux profile is improved by using magnetic shielding cover made from iron (Fe). It was

also found that the iron cover suppresses the horizontal magnetic flux density and effectively

prohibits the inward E×B drift motion towards the center of the target [38]. Copper films are

fabricated using different techniques, such as DC magnetron sputtering, pulsed laser deposition

(PLD), RF magnetron sputtering, ion beam assisted DC magnetron reactive sputtering, and

molecular beam epitaxy (MBE) [39-41]. The RF magnetized plasma sputtering method is

selected because it has many advantages, such as the possibility of large area deposition, a

simple setup arrangement, and uniform deposition capabilities.

The objectives of this paper are to investigate the performance of a gyratory square-

shaped RF magnetized plasma sputtering source for materials processing and different

functional film preparations. For example, Cu films have also been deposited on Si wafer

substrates by our developed RF magnetized sputtering plasma source at an RF power of 100

W with a processing time of 1.5 h. A radial profile and microstructure of the deposited films

are investigated under two square-shaped magnet schemes. In Section 4.2, the experimental

setups are explained in detail. In Section 4.3, the results and discussions on the effects of the

magnetic shielding material in a square-shaped magnet arrangement on the discharge

characteristics, plasma emission luminescence, thickness, and resistivity profile, as well as the

surface morphology of the film deposited by the gyratory square-shaped RF magnetized plasma

sputtering source are presented. In Section 4.4, the results obtained in this chapter are

summarized.

50

4.2. Experimental Setup Details

The schematic of the gyratory square-shaped capacitive RF discharge plasma

sputtering source for plasma processing is shown in Fig. 4.1. The experiments were performed

in a stainless-steel cylindrical vacuum chamber with an outer diameter of 235 mm, an inner

diameter of 160 mm and a height of 195 mm. Eight Neodymium rectangular permanent bar

magnets with a volume of 30 mm × 5 mm × 3 mm are mounted on a circular iron yoke disk so

that the line joining the N-pole and S-pole magnets comprises one axis of the square. Figure

4.2 shows this one axis of the proposed square-shaped magnet scheme for case (a), without

iron shielding, and case (b), with iron shielding of dimensions 5 mm × 3 mm × 1 mm. The

chamber was evacuated by a combination of a turbo molecular and an oil rotary pump, and the

base pressure in the chamber of 1.6 10-5 Pa is obtained before starting the experiment. Argon

gas was introduced into the vacuum chamber as the sputtering at a working gas pressure of

1.02 Pa through a regulating flow meter. The iron yoke was set up at a distance of 1 mm from

the target surface. An iron cover of 5 mm × 3 mm× 1 mm is used for magnetic shielding in

order to suppress the magnetic flux lines between the contact zone of the N-pole and S-pole

magnets, as shown in Fig. 4.2. The circular iron yoke disk mounted by the magnets was rotated

at a speed of 40 rpm by an external motor drive. A cooling fan was used to cool the magnets.

An RF power of 100 W at 13.56 MHz was applied to the electrode (target) via an impedance-

matching network and a blocking capacitor, while the vacuum chamber wall acted as a

grounded electrode. An aluminum disc target was used, and the substrate holder and Si wafer

substrate shown in Fig. 4.1 were also removed to measure the plasma emission image and

discharge characteristics in order to avoid film deposition on the glass window at the bottom.

A copper plate of 160 mm × 160 mm × 3 mm was used as the target to deposit a copper film

on the Si wafer substrate in the proposed gyratory square-shaped magnet arrangements in the

RF magnetized plasma sputtering source. The Si substrate was ultrasonically cleaned in

distilled water for 7 min and water drops were removed before inserting it into the deposition

chamber. The Si substrate was fixed onto the substrate holder and the distance between the

target and the substrate was 45 mm. A precision surface profile tool was used to measure the

radial profile of the thickness of the deposited Cu film.

51

Figure 4.1. Schematic of the gyratory square-shaped RF magnetized plasma sputtering

source set up for materials processing.

Figure 4.2. One axis of the proposed square-shaped magnet scheme for case (a), without iron

shielding, and case (b), with iron shielding of dimensions 5 mm × 3 mm × 1 mm.

The resistivity of the deposited Cu films was measured by a standard four-point probes

technique at room temperature. The surface morphology of the Cu films was observed using

AFM. The thin film deposition processing time period was 1.5 h. The RF discharge voltage

between the RF powered electrode and the grounded vacuum wall, as well as a DC self-biased

voltage, was measured by a high-voltage probe with an attenuator of high input impedance and

a digital oscilloscope with a sampling frequency of 2 Giga sampling/sec. As shown in Fig. 4.1,

the origins of the axial z and radial r positions are defined at the surface and at the center of the

RF powered target, respectively. The plasma density is estimated from the ion saturation

current detected by a negatively biased [4] cylindrical tungsten probe of 1.0 mm in diameter

and 10 mm in length. The estimation of plasma density from the ion saturation current is done

52

by moving the L-shaped probe to the radial direction, as shown in Fig. 4.1, and measurements

were taken at r = 5 mm and z = 10 mm. The probe wire was compensated with an LC filter

circuit in order to minimize the influence of the RF plasma potential oscillations on the probe

current-voltage characteristics [7]. A Cartesian coordinate system is used because the magnetic

field is not axisymmetric. The typical experimental conditions used to investigate the

performance of a gyratory square-shaped capacitive RF discharge plasma sputtering source for

materials processing are shown in Table 4.1.

Table 4.1

Typical experimental conditions of a proposed plasma

sputtering source for materials processing.

Parameters Specification

Initial base pressure [Pa] 1.6 10-5

Target electrode Al/Cu

Substrate Si wafer

Sputtering gas Ar

Argon gas pressure [Pa] 1.02

RF input power [W] 100

Rotational speed of yoke [rpm] 40

Sputtering time [h] 1.5

Distance between the target and rotatory

yoke with magnets [mm] 1

Target - Substrate distance [mm] 45

RF discharge voltage [Vpp] 1220

DC self-biased voltage [V] - 608

The plasma discharge image is taken by a low-resolution digital camera. A typical

plasma discharge image with for case (b) at an RF power of 100 W is shown in Fig. 4.3, and

also clarifies the magnet arrangement. The positions of the permanent rod magnets are

indicated by the red and blue colored dashed rectangles. The gray colored squares represent the

iron cover. Eight Neodymium rectangular permanent bar magnets of 30 mm × 5 mm × 3 mm

are mounted on a circular iron yoke disk so that the line joining the N-pole and S-pole magnets

53

comprised one axis of the square. Strong emissions of the plasma discharge are observed in the

outward direction at the four intersections of the four vertically placed magnets and in the

inward direction at the four connections between N-pole and S-pole magnets, which are marked

by eight arrows. According to right-hand rule, the plasma discharge in the inward and outward

directions is caused by the E B drift motion effect.

Figure 4.3. A typical plasma discharge image for case (b) at an RF power of 100 W, a

gyratory speed of 40 rpm, and Ar gas pressure of 1 Pa. Eight arrows denote the directions of

E×B drift motion.

54

4.3. Results and Discussions

4.3.1. Magnetic Shielding Effect Square-Shaped Magnet Arrangement

The horizontal and the vertical magnetic flux densities, and their radial profiles with

one–pair magnets as one axis of the square-shaped permanent rod magnet scheme, are analyzed

by the conventional magnetic-field analysis software, Poisson Superfish [42], in order to

investigate the effect of the magnetic shielding on the plasma discharge, thickness and

resistivity profiles of deposited copper thin film. Two magnetic arrangements were used: case

(a) without iron shielding and case (b) with iron shielding of 5 mm × 3 mm × 1 mm iron

material. As shown in Fig. 4.3, eight Neodymium rectangular rod magnets of 30 mm × 5 mm

× 3 mm in dimensions are placed at a distance of 33 mm away from the central position in a

square-shape on a circular iron yoke disk of 140 mm in diameter and 1 mm in thickness.

However, the Cu target covers an effective radial area from r = −50 mm to r = +50 mm inside

the vacuum chamber because the inner diameter of the hole of a circular insulator is 100 mm.

The surface magnetic flux density of the rectangular rod magnets of 3130 G was used. The

direction of the electric field, E, and the parallel component of the magnetic flux density, B,

are considered to significant for the magnetized plasma discharge based on the E×B drift

motion, where the electric field, E, and the magnetic flux density, B, are perpendicular to the

target and parallel to the target, respectively.

It was shown in our previous work [38] that four inward and outward E × B drift

motions occur in the central region at each contact zone between the N-pole and the S-pole

magnets and on the surface of the iron disc, respectively. The suppression of these inward drift

motions was necessary, because inward drift motion increases the plasma density in the central

region of the target so that the erosion depth was higher than that in the edge region of the

target. In order to deposit uniform film, these inward drift motion should be suppressed. These

four inward E×B drift motions can be prevented by a magnetic shielding material, the iron

cover. The iron covers positioned on the contact zone between the N-pole and the S-pole

magnets, such as the magnetic field profile, are investigated to suppress the magnetic flux

density that enhances the inward drift motion of the plasma particles. In our previous work [38],

it was shown that, near the target surface, in the distributions of the two-dimensional magnetic

flux lines, the magnetic field lines starting at the N-pole of the magnets pass through the target,

airspace, iron yoke and then return to the S-pole for both cases. Figures 4.4 and 4.5, respectively,

show the distributions of the absolute value of the horizontal, Bx, and the vertical, |Bz|,

magnetic flux densities for the magnet arrangements of cases (a) and (b), at the axial position

55

z = 5 mm from the copper target. It is observed that |Bx| and |Bz| have three and two peaks,

respectively for both cases. The direction of the magnetic flux density, |Bx,| is reversed on the

left and right sides of the magnet axis, as shown in Fig. 4.4. The peak amplitude of |Bx| for

case (b) is lower than that for case (a) and the amplitudes of the three peaks are located at x =

35, 0 and 35 mm, where x is the center of the target. The peaks of |Bz| are positioned at x =

45, 15, 45 and 15 mm. The magnetic shielding material reduces the horizontal magnetic flux

density at x = 0 from approximately 1120 to 824 Gauss and the axial component from

approximately 824 to 706 Gauss at x = 15 and 15 mm.

Figure 4.4. Distributions of the absolute value of the horizontal, Bx, magnetic flux densities

for the magnet arrangements of cases (a) and (b), at the axial distance z = 5 mm from the

target.

56

Figure 4.5. Distributions of the absolute value of the vertical, |Bz|, magnetic flux densities

for the magnet arrangements of cases (a) and (b), at the axial distance z = 5 mm from the

target.

The cyclotron angular frequency and larmor radii of charged particles such as

electrons and ions are important parameters for magnetized plasma. These cyclotron

frequencies decrease in a magnetic field reduced by the shielding material of the iron cover

according to the equation: , where q and m are the electronic charges and masses of

charged particles, respectively. Moreover, the larmor radius, rL, with the shielding material,

will be larger than those without the shielding material in accordance with , where v is

the particle movement speed perpendicular to magnetic field, B. Therefore, the magnetic

shielding effect caused by the iron cover is suitable for decreasing the E×B drift of ions toward

the center of the target so that the profile of the sputtered copper atoms will be uniform. The

uniformity of the radial profile of the thickness and the resistivity of the deposited Cu film will

be improved by the use of the shielding material in a gyratory square-shaped RF magnetized

plasma sputtering source. Consequently, case (b) will be the effective arrangement for

functional film preparation and material processing.

57

4.3.2. Plasma Discharge Voltage and Emission for Square-Shaped Magnetized Plasma

The RF discharge voltage, VRF, between the RF powered electrode and the grounded

chamber wall, and the self-bias dc voltage, Vsb, of the electrode are measured by a digital

oscilloscope and a high-voltage probe at various Ar gas pressures. Figures 4.6 and 4.7 show

the discharge voltage, VRF, and absolute value of the DC self-biased voltage, |Vsb|, respectively,

of the gyratory square-shaped RF magnetized plasma sputtering source as a function of the

sputtering gas pressure for cases (a) and (b) at a gyratory speed of 40 rpm and an RF power of

100 W. The RF discharge voltages, VRF, were 1160, 1125, and 1100 Vp-p and the absolute value

self-bias dc voltages, |Vsb|, were 573, 548, and 532 V with the argon gas pressures of 1, 2 and

3 Pa, respectively, for case (b) at an RF power of 100 W. These decreasing tendency results

are compatible with our previous work [38] at an RF power of 50 W. This is because the

electron collision frequency, as well as the ionization rate, increases with an increasing Ar gas

pressure, and the discharge voltage is approximately inversely proportional to the electron

collision frequency at a fixed absorbed power [1–2]. From the time-averaged ion saturation

current, the plasma density was also estimated using the Bohm sheath criteria [1–2] for both

cases. The measurements were positioned at r = 5 mm and z = 10 mm. The estimated plasma

densities were 8.4 × 1015 m-3 and 1.1 × 1016 m-3 for cases (a) and (b), respectively.

In the sputtering deposition, the RF power and sputtering Ar pressure were fixed at 100

W and 1 Pa, respectively. The RF discharge voltages, VRF, (peak-to-peak value) were 1080 and

1160 Vp-p for cases (a) and (b), respectively. The absolute values of the self-bias dc voltages,

Vsb, of the electrode were 528 and 573 V for cases (a) and (b), respectively. A typical star-

shaped plasma is also observed for both square-shaped cases. The star-shaped plasma is formed

by the effect of the magnetic field produced by each permanent rod magnet. As described in

the magnetic shielding effect section, four E × B drift motions in the central direction are

suppressed by the iron cover. In order to deposit a uniform Cu film, these inward drift motions,

as well as the plasma density, toward the central point of the target are prohibited by the

magnetic shielding material of the iron cover of 5 mm × 3 mm × 1 mm in dimension.

58

Figure 4.6. Discharge voltage, VRF, of the gyratory square-shaped RF magnetized plasma

sputtering source as a function of the sputtering gas pressure for cases (a) and (b), at a

gyratory speed of 40 rpm and an RF power of 100 W.

Figure 4.7. The absolute value of the DC self-biased voltage, |Vsb|, of the gyratory square-

shaped RF magnetized plasma sputtering source as a function of the sputtering gas pressure

for cases (a) and (b), at a gyratory speed of 40 rpm and an RF power of 100 W.

59

4.3.3. Thickness and Resistivity Profiles of the Deposited Film of the Gyratory Square-

Shaped RF Magnetized Plasma Sputtering Source

The performance of the gyratory square-shaped RF magnetized plasma sputtering

source was analyzed for materials processing on the microstructure of deposited films. Figure

4.8 shows the film thickness as a function of the radial position for cases (a) and (b) at an RF

power of 100 W, a sputtering gas pressure of 1.02 Pa, a gyratory speed of 40 rpm, and a

processing time of 1.5 h. We have shown in our previous work [38] that the copper target

utilization percentage is increased from 74.15% to 87.49% by using a magnetic shielding iron

cover of 1 mm thickness. As shown in Fig. 4.8, the highest and lowest thicknesses of the copper

thin film are observed at r = 30 and r = 40 mm for both cases, respectively. In Fig. 4.8, it can

be observed that the uniformity of the deposited film thickness is obviously improved by using

the iron shielding material on four paired magnets. The thickness of the deposited film for case

(b) is around 0.6 m for 0 < r < 30 mm and then decreases at the edge of the target, whereas

the thickness profile is fluctuated significantly for case (a). The roughness is estimated based

on the ratio of the highest thickness minus the lowest thickness and the highest thickness plus

the lowest thickness of the film. The roughnesses of the thickness profiles of the films are

approximately 24.4 % and 7.2 % for case (a) and (b), respectively. It is considered that the

thickness profile of the thin film is affected by the copper target erosion depth profile. These

findings are consistent with those reported in our previous work [38], in which the uniformity

of the copper target erosion depth profile was improved by the use of the shielding material,

and which predicts the uniformity of the copper thin film thickness profile.

60

Figure 4.8. Film thickness as a function of the radial position for cases (a) and (b), at an RF

power of 100 W, a sputtering gas pressure of 1.02 Pa, a gyratory speed of 40 rpm, and a

processing time of 1.5 h.

In order to evaluate the electrical properties of the deposited film at an RF power of

100 W with a sputtering duration of 1.5 h, the resistance and the resistivity measurements are

carried out at room temperature by the four-point probe method. The resistivity of the film

under room temperature as a function of the radial position for cases (a) and (b), where films

were deposited at an RF power of 100 W, a gyratory speed of 40 rpm, a sputtering gas pressure

1.02 Pa, and a processing time of 1.5 h, is shown in Fig. 4.9. It can be seen that the resistivity

profile of case (b) is more uniform than that of case (a). The lowest electrical resistivity of the

deposited film is 4.33 × 10-8 m at r = 30 mm for case (b), which is virtually the same as the

bulk resistivity of the copper. The resistivity decreases from 9.46 × 10-8 to 4.69 × 10-8 m at

r = 20 mm when using the shielding material. The resistivities are 8.16 × 10-8 and 4.64 × 10-8

m at r = 0 mm for cases (a) and (b), respectively.

The film property can be affected by the energy of the sputtered copper atoms incident

on the film. The energy of Cu atoms depends on the energy of Ar ions incident on the Cu target,

which is determined by the discharge voltage, that is, the self-biased voltage on the target. As

mentioned in above, the discharge voltage and the self-biased voltage for case (a) are almost

the same as those for case (b). In this deposition, the difference in the resistivity between cases

(a) and (b) is not caused by the effect of the sputtered Cu atom energy. The difference will be

61

discussed in the section on surface morphology. The uniformity of the electrical resistivity, as

well as the thickness of the copper thin film, has been achieved by using the iron cover. This

behavior could be explained by the copper target erosion depth profile uniformity and through

analysis of the AFM image and XRD patterns of the thin film as will be mentioned later.

Figure 4.9. The resistivity of the film under room temperature as a function of the radial

position for cases (a) and (b), where films were deposited at an RF power of 100 W, a

gyratory speed of 40 rpm, a sputtering gas pressure 1.02 Pa and a processing time of 1.5 h.

4.3.4. Surface Morphology of the Film Deposited by the Gyratory Square-Shaped RF

Magnetized Plasma Sputtering Source

The performance of the gyratory square-shaped RF magnetized plasma sputtering

source for materials processing on the microstructure of deposited films have been analyzed

using AFM and XRD. Figures 4.10 (a)–(b) show three-dimensional AFM images of the

deposited films for cases (a) and (b), respectively. The AFM is used to observe the surface

morphology of the films prepared at an RF power of 100 W, an Ar pressure of 1.0 Pa and a

processing time of 1.5 h. As shown in Figs. 4.10 (a)–(b), the AFM images of the copper thin

films illustrate that their surface morphologies are strongly affected by the use of the iron cover.

It can be clearly observed that the iron cover mounted on the magnets for magnetic shielding

in the magnetized plasma sputtering deposition has a significant impact on the grain size and

surface roughness of the copper thin film, which demonstrates the evidence of the structural

62

deformation. Figure 4.10 (a) shows that the films for case (a) exhibit sharp needle-shaped

grains vertical to the substrate with a mean grain size of approximately 40 nm. However, for

case (b) the film surface shows an improved smooth surface [see Fig. 4.10 (b)] with a reduced

grain size, as well as a mean grain size of around 20 nm. Inspecting Figs. 5.10 (a)–(b), it can

be observed that the surface coverage increases and the isolated nanoparticles coalesce together

to form a more uniform film when using the iron cover in the magnetized plasma sputtering

deposition. In addition, the surface roughness of the Cu film has been observed. The

roughnesses of the Cu films are about 3.73 nm and 2.49 nm for cases (a) and case (b),

respectively. Therefore, the surface properties of the AFM images of the Cu film in Fig. 4.10

are in good agreement with the results of the film thickness and the resistivity profiles as shown

in Figs. 4.8 and 4.9.

63

Figure 4.10 (a) – (b): Three-dimensional AFM micrographs of the film prepared at an RF

power of 100 W, a gyratory speed of 40 rpm, a sputtering gas pressure 1.02 Pa and a

processing time of 1.5 hour for case (a), without iron shielding and for case (b), with iron

shielding.

In order to investigate the performance of the gyratory square-shaped RF magnetized

plasma sputtering source on the structure of the Cu film, X-ray diffraction (XRD) standards

(card no. ICDDPDF#04-0836) were used. Figure 4.11 shows the XRD patterns of Cu films

prepared by the gyratory square-shaped RF magnetized plasma sputtering source for cases (a)

and (b). It is seen that both samples exhibit a face-centered-cubic structure. For (004)Si, the

strong diffraction peak was observed at 69.2° and also at 32.94° with a small magnitude. This

is caused by double diffraction. The diffraction positions are (200), (220), (311), (200), (222),

(400), (311), and (420). The peak position is in good agreement for both cases. Two typical

texture components of (111) and (200) were observed in XRD patterns. The relative intensity

of the (111) peak to the (200) [I(111)/ I(200)] is often defined as the film texture. Considering the

ratio of randomly oriented Cu powder, the I(111)/ I(200) values are 13.76 and 4.08. The full width

at half maximum (FWHM) values of the diffraction peaks were 0.42° and 0.66° for cases (a)

and (b), respectively.

64

Figure 4.11. X-ray diffraction (XRD) patterns of Cu films deposited by the gyratory square-

shaped RF magnetized plasma sputtering source for cases (a) and (b).

4.3.5. Aluminum-Doped Zinc Oxide (AZO) Film Prepared by RF Magnetized Plasma

Sputtering Source with Square-Shaped Rod Magnets

The aluminum-doped zinc oxide (AZO) films were prepared with a square-shaped

magnet setup using a high-density radio frequency (RF) magnetized sputtering plasma source

for uniform target utilization. The glass substrate was used. The glass substrates were cleaned

using ultra sound cleaning technology. Eight neodymium permanent rod magnets of 30 × 5 ×

3 mm, where the connection between N-pole and S-pole magnets is one side of the square. The

two-dimensional magnetic field profiles, the film thickness, the resistivity of the films, atomic

force microscopy (AFM), the spectroscopy, and the x-ray diffraction (XRD) profiles have been

investigated for case (a) stationary and case (b) rotational speed of 45 rpm. The experiments

were performed in a stainless-steel cylindrical vacuum chamber with an outer diameter of 235

mm, an inner diameter of 160 mm and a height of 195 mm. Argon (Ar) gas at 0.5 Pa was used

by regulating a flow meter, an RF power of 120 W at 13.56 MHz was applied to the target via

an impedance matching network. The sputtering processing time was 2.25 h. The film thickness

and resistivity of the prepared AZO films were measured at r = 0 mm for both case (a) and (b).

Prepared aluminum-doped zinc oxide (AZO) films on the glass substrate with a square-shaped

magnet setup for case (a) stationary and case (b) rotational speed of 45 rpm are shown in Fig.

4.12. It is found that the AZO films were not transparent because of high input RF power and

long processing time.

65

(a)

(b)

Figure 4.12. Prepared aluminum-doped zinc oxide (AZO) films on the glass substrate with a

square-shaped magnet setup for case (a) stationary and case (b) rotational speed of 45 rpm.

Figure 4.12 shows the AFM photographs of the AZO films prepared on the glass

substrate with a square-shaped magnet setup for case (a) stationary and case (b) rotational speed

of 45 rpm. The sharp needle-shaped grains vertical to the substrate were observed for case (b).

However, for case (a) the film surface shows an improved smooth surface with a reduced grain

size. Later, the scanning force microscopy (SEM), raman spectroscopy, x-ray photoelectron

spectroscopy (SPS), x-ray diffraction (XRD), optical emission spectrometer (OES), Ultraviolet

Visible Near-Infra red (UV-Vis-NIR) spectroscopy and Hall measurement techniques will be

analyzed for the deposited AZO films.

66

Figure 4.12. AFM photographs of the AZO films prepared on the glass substrate with a

square-shaped magnet setup for case (a) stationary and case (b) rotational speed of 45 rpm.

67

4.4. Conclusion

In order to assess the developed RF magnetized plasma sputtering source with a

gyratory square-shaped magnet arrangement consisting of eight neodymium bar magnets of 30

mm × 5 mm × 3 mm, the thickness and the resistivity profiles and the microstructure of the

copper thin film deposited on the Si wafer substrate were analyzed, using a copper target of

160 mm × 160 mm × 3 mm. Two magnetic arrangements were investigated to discuss the film

properties: case (a) without iron shielding and case (b) with iron shielding of 5 mm × 3 mm ×

1 mm, which is used for magnetic shielding. Copper films were prepared at an RF power of

100 W, an Ar pressure of 1.0 Pa, a processing time of 1.5 h, a gyratory speed of 40 rpm, and a

target– substrate distance of 45 mm. The magnetic shielding material reduces the horizontal

magnetic flux density, |Bx|, at x = 0 from ~1120–824 G and the axial component, |Bz|, from

~824–706 G at x = -15 and 15 mm. The RF discharge voltages, VRF, (peak-to-peak value) were

1080 V and 1160 V, and the absolute values of the self-bias dc voltages, Vsb, of the electrode

were 528 V and 573 V for cases (a) and (b), respectively. The estimated plasma densities were

8.4 × 109 cm-3 and 1.1 × 1010 cm-3 at r = 5 mm and z = 10 mm for cases (a) and (b), respectively.

A typical star-shaped plasma discharge has been observed. The thickness of the deposited

copper thin film for case (b) is around 0.6 m for 0 < r < 30 mm, which decreases at the edge

of the target, whereas the thickness profile fluctuates significantly for case (a). The roughnesses

of the thickness profile are approximately 24.45% and 8.06% for cases (a) and (b),

respectively. The resistivities are 7.89 × 10-8 m and 4.33 × 10-8 m at r = 30 mm for cases

(a) and (b), respectively. The Cu films for case (a) exhibit sharp needle-shaped grains vertical

to the substrate with a mean grain size of ~40 nm, and for case (b) the film surface shows an

improved smooth surface with a reduced mean grain size of ~20 nm. The roughnesses of the

copper thin films are ~3.73 nm and ~2.49 nm for case (a) and case (b), respectively. An XRD

investigation shows that the FWHM values of the diffraction peak were 0.42° and 0.66° for

cases (a) and (b), respectively and both samples exhibit a face-centered-cubic structure.

Therefore, the proposed gyratory square-shaped RF magnetized plasma sputtering source has

an advantage for making the functional films. The prepared AZO films on the glass substrate

for case (a) stationary and case (b) rotational speed of 45 rpm were not transparent because of

high input RF power and long processing time.

68

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70

Chapter 5. Ring-Shaped Plasma for Target Utilization Obtained with

Circular Magnets Monopole Setups

5.1. Introduction

CCP sources [1-15] driven by radio frequency (RF) power supply at 13.56 MHz are

widely used functional film preparation and material processing due to its simple experimental

arrangement, an easy maintenance as well as large-diameter substrate processing. However, it

is difficult to produce high-density plasma by CCP source. In particular, the CCP has some

limitations such as (1) low plasma density of less than 109 – 1010 cm-3, (2) low deposition rate,

(3) standing wave effect [16] on the powered electrode (target), and (4) difficulty of controlling

external parameters as well as energized ions at the target independently [3]. Thus, the

conventional CCP is not a good tool for plasma processing. A lot of theoretical [1,6,7,11] and

experimental [9, 10, 17-19] analysis have been studied from the viewpoint of the very high

frequency (VHF) operation. Experimentally [17-19], in order to obtain higher densities than

1010 cm-3 the VHF capacitive discharge is used. The other problem of the standing wave effect

[16] on the powered electrode has been reported in the numerical and the experimental studies

[1, 6, 7, 9-11, 17-20], so that there is considerable plasma non- uniformity. To improve the

plasma uniformity, some methods such as the segmented electrode [21], phase-shift control

[22] have been analyzed. However, the problem is not solved completely as far as the operation

of VHF introduction. In general, it is well known that the hollow-cathode discharges [23–32]

can produce a high-density plasma with only a simple structure of the hollow electrode, because

hollow trench can confine electrons effectively.

Recently, the RF magnetized plasma sources have been widely used in the

microelectronics [33–35] such as magnetic films [36], surface treatment and cleaning [30],

diamond-like carbon [37], biomaterial thin films, flat panel display fabrication; transparent

conductive oxides film preparation [33-35] for solar cells and mobile phones and many other

rapidly growing areas [1-2]. A target utilization of 88% rotational square-shaped arrangement

of rod magnets in RF magnetized plasma [38], 60-70% for planar magnetron sputtering using

a rotating tilted unbalanced and asymmetrical yoke magnet [39] and 59% for a magnet spacing

of 10 mm is obtained in the RF magnetron ring-shaped plasmas based on stationary monopole

arrangements [40]. It is also reported that the target erosion is very low near the chamber wall

[38-41]. S. Wickramanayaka et al reported that the development of a large area high-density

71

plasma source with higher radial plasma uniformity for large area wafer processing was

investigated with multipole arrangement of the magnets [42].

In general, plasma does not exist in outer region because of the presence of the ion

sheath near the chamber wall. It is required to produce (1) outer ring-shaped and (2) specific

area plasmas for obtaining (1) uniform, (2) high-density plasma as well as (3) convenient outer

target area erosion profile near the chamber wall. So, the high-density plasma is necessary an

outer region near the chamber wall for specific target area application from the viewpoint of

target utilization. Our idea is to use the mono-pole magnet arrangement in order to overcome

this problem.

The objective of this chapter is to make an RF magnetized outer ring-shaped plasma

sputtering source with a concentrically monopole arrangement of magnets at various different

magnet-gap distances for the specific area target utilization. The discharge characteristics,

electron temperature, plasma density and electron collision frequency in the monopole

arrangements are considered. In Section 6.2, the experimental setup is explained in detail. In

Section 6.3, results and discussions on the effects of monopole schemes, ring-shaped plasma

discharge, and the plasma parameters are described. In Section 6.4, the results obtained in this

chapter are summarized.

5.2. Experimental Methods

The schematic diagram of the experimental setup is shown in Fig. 5.1. A stainless-

steel cylindrical vacuum chamber with an outer diameter of 235 mm, an inner diameter of 160

mm and a height of 195 mm is used to perform experiments. The vacuum chamber was

evacuated to a base pressure of 2.0 10-5 Pa using an oil rotary pumps and a turbo molecular

before starting the experiment. Ar of 1.0 Pa was introduced into the vacuum chamber at a

working gas pressure through a regulating valve. An iron yoke is used to hold magnets as

monopole arrangement. The spacings between the magnets of different circles for monopole

arrangement were chosen by R= 5, 20 and 35 mm for case (a), (b) and (c) as will be mentioned

later, respectively. Figs. 5.2(a)–(c) show cross sections of magnet arrangements under the

target at R = 5, 20 and 35 mm, respectively.

The magnets are holding by iron yoke disk of 140 mm in diameter are put on the target

plate. A cooling fan was used to cool the magnets and the target. An aluminum plate of 140 ×

140 × 3 mm was used as the target to investigate the discharge and the plasma characteristics

as well as to avoid thin film deposition to a glass window at the bottom for the proposed

72

monopole magnet arrangements in the RF magnetized plasma sputtering source. The target is

input by an RF power of 40 W at the driving frequency of 13.56 MHz through an impedance

matching network and a blocking capacitor, while the vacuum chamber wall acts as a grounded

electrode. During experiments, the RF discharge voltage between the RF powered electrode

and the grounded wall and the DC self-biased voltage were measured by a high-voltage probe

with attenuator of high input impedance and digital oscilloscope with a sampling frequency of

2 G samplings/sec. The plasma densities at various positions are obtained by the measurements

of current-voltage characteristics curve. The current-voltage characteristics curve is measured

by a small cylindrical tungsten probe of 1.0 mm in diameter and 10 mm in length.

Figure 5.1. Schematic diagram of the proposed circular ring-shaped RF

magnetized plasma source by monopole magnet arrangement.

73

The biasing voltage was changed from -70 V to +70 V. The Langmuir probe was

compensated with an LC parallel filter circuit (choke) [3,4,13,15,19,23,40] to avoid the

influence of the RF plasma potential fluctuations on the probe current-voltage characteristics.

The probe measurements were carefully performed so as to minimize the disturbance to the

stable plasma. Moreover, the measurement position was fixed at an axial distance, z = 10 mm

from the target surface. In this position, the magnetic field is perpendicular to the probe surface.

So that electrons are easily collected by the Langmuir probe. In fact, as shown in Fig. 5.7,

electron saturation current is not decreased. Moreover, the probe can collect electrons without

an influence of magnetic field.

(a) (b)

74

(c)

Figure 5.2(a)-(c). Proposed circular monopole magnet arrangement for (a) R = 5 mm, (b) R

= 20 mm, and (c) R = 35 mm.

The measurement positions were selected in the radial positions of r = 45, 42 and 38

mm, where high luminous ring-shaped plasmas discharges are observed in the outer region in

the chamber for the three cases (a): R = 5, (b): R =20 and (c): R = 35 mm, respectively. The

plasma density and electron temperature were estimated from the current-voltage

characteristics curve [1-3] and their values at various radial positions were measured by moving

the L-shaped probe to the radial direction as shown in Fig. 5.1. The measurement positions

were selected in the radial positions of r = 45, 42 and 38 mm, where high luminous ring-shaped

plasmas discharge are observed in the outer region in the chamber for the three cases (a): R =

5, (b): R =20 and (c): R = 35 mm, respectively The origins of axial distance z and radial r

positions are defined at the surface and at the center of the RF powered target, respectively as

shown in Fig. 5.1.

75

5.3. Results and Discussions

5.3.1. Magnetic Field Profile Analysis

The proposed circular monopole magnet arrangements for (a) R = 5 mm, (b) R = 20

mm and (c) R = 35 mm are shown in Figs. 5.2(a)–(c), respectively. Here, “R” is the gap distance

between magnets in consecutive circles. The number of magnets positioned at the center, first

circle, second circle and third circle are 1, 5, 12 and 16, respectively for (a) R = 5 mm set up.

For (b) R = 20 mm set up, 1 and 15 magnets are placed at the center and first circle, respectively,

while for (c) R = 35 mm, 1 and 22 magnets are kept at the center and first circle, respectively.

Second circle or more circle is not used for (b) R = 20 mm and (c) R = 35 mm setups, because

the target effectively covers radial area from r = -50 mm to r = + 50 mm. The neodymium

cylindrical magnets with a surface magnetic flux density of 4120 Gauss, diameter of 10 mm,

and height of 5 mm are positioned on circular iron yoke disk 140 mm in diameter. In order to

investigate the effect of magnetic field pattern in the proposed monopole magnet schemes on

plasma production, 2D magnetic flux lines for three magnets arrangements such as (a) R = 5

mm, (b) R = 20 mm and (c) R = 35 mm are as shown in Figs. 5.3(a)–(c), respectively.

The 2D magnetic flux lines and their radial profiles are analyzed by conventional

magnetic-field analysis software poisson superfish [43] developed in Los Alamos National

Laboratory. As seen in Figs. 5.3(a)–(c), the magnetic field lines generated from N-pole of

magnets pass through the target, airspace, iron yoke and then return to the S-pole for all

monopole magnet arrangement. It is found that the axis symmetry property is satisfied for

monopole arrangement because the flux lines profiles for a half region from r = -80 to 0 mm

have a similar profile as for half region r = 0 to 80 mm [see Figs. 5.3(a)–(c)]. It is important

for generating the high-density magnetized discharge plasma at the outer and specific areas to

use effectively the E×B drift motion, where E and B are the electric field perpendicular to the

target and the magnetic flux density parallel to the target, respectively. In especial, the magnetic

flux density component parallel to the target is significant for the E×B drift motion.

76

(a)

(b)

(c)

Figure 5.3(a)-(c). 2D magnetic flux lines distribution in the proposed RF magnetized ring-

shaped plasma near the target surface for (a) R = 5 mm, (b) R = 20 mm and (c) R = 35 mm.

77

Figure 5.4 (a) and (b) shows the radial profile of absolute value of horizontal magnetic

flux density Bx and vertical magnetic flux density Bz at three magnet arrangements of (a) R

= 5 mm (b) R = 20 mm (c) R = 35 mm at the axial distance of z = 5 mm from the target surface,

respectively. It is found that multi-peaks are obtained at all setups. The pattern of their peaks

for the set up (b) is the same as that for the setup (c) and their magnetic flux density profiles

have six peaks. The setup (a) of R = 5 mm has two large peaks of 590 G at a radial position of

r= 52 mm. The second peaks with a weak value of 168 G are also observed at r = 35 mm.

However, other peak amplitudes are less than 100 G at r = 0 to 35 mm. The three highest

peaks are of 504 G, 370 G, and 371 G at a radial position of r= 38, 23 and 7 mm,

respectively for the set up (b). In the set up (c), the three highest peaks of 491 G, 475 G, and

473 G are also observed at a radial position of r= 53, 37 and 8 mm, respectively. Moreover,

the other magnetic flux peaks from r = 0 to 40 mm for setup (a) are the lowest among three

setups. Their high-value peaks will give some ring-shaped plasma discharges due to the E B

drift effect.

78

(a)

(b)

Figure 5.4. Distributions of the absolute value of the (a) horizontal magnetic flux density,

Bx and (b) vertical magnetic flux density Bz for (a) R = 5 mm, (b) R = 20 mm and (c) R =

35 mm at the axial distance of z = 5 mm from the target surface.

79

5.3.2. Discharge Characteristics

Figures 5.5(a)–(c) show typical images of RF magnetized plasma discharge at an RF

power of 40 W, argon gas pressure of 1 Pa for (a) R = 5 mm, (b) R = 20 mm and (c) R = 35

mm. The RF magnetized plasma discharge image is taken by a low-resolution digital camera.

Here, dashed circles denote the insulated wall positions. Multiple ring-shaped plasma

discharges are not seen for the case (a): R = 5 mm, whereas for the cases (b): R = 20 mm, and

(c): R = 35 mm, multiple ring plasma discharges are observed throughout the center to outer

target area [see Fig. 5.5(a)-(c)]. Moreover, drift motion of electrons has an important effect on

the distribution of plasma discharge. This is because charged particles are strongly confined by

the E B drift effect.

(a)

80

(b)

(c)

Figures 5.5. Typical images of RF magnetized plasma discharge at an RF power of 40 W

and an argon gas pressure of 1 Pa for (a) R = 5, (b) 20 and (c) 35 mm.

81

Figures 5.6 (a) and (b) show the RF discharge voltage, VRF (peak to peak) and absolute

value of the DC self-biased voltage, |Vsb| of the cathode as a function of argon gas pressure for

the three proposed monopole magnet arrangements such as (a) R = 5 mm, (b) R = 20 mm, (c)

R = 35 mm. The argon gas pressure flowing into the chamber was changing from 1 to 6 Pa and

the RF power was fixed at 40 W. The RF discharge voltage decreases gradually from

approximately 420 to 340 Vp-p with increasing argon gas pressure for case (a), because electron

collision frequency increases with increasing Ar gas pressure so that ionization rate increases.

It is seen that the RF discharge voltage does not almost depend on Ar gas pressure for case (b).

This is ascribed from the hall parameter effect, which represents the degree of magnetization

[1-3]. However, for case (c) discharge voltage, VRF falls down up to 4 Pa and then saturates

with rising Ar gas pressure. This saturation of discharge voltage might be predicted from

sustaining the magnetized plasma owing to the magnetic confinement. These results tendency

is almost similar to the tendency in the discharge voltage corresponding to all cases.

(a)

82

(b)

Figures 5.6. (a) Discharge voltage and (b) DC self-biased voltage as a function of argon gas

pressure for (a): R = 5, (b): R =20 and (c): R = 35 mm at a constant RF power of 40 W.

5.3.3. Plasma Characteristics

Figure 5.7 shows typical probe current-voltage characteristics for (c) R = 35 mm

measured at axial distance z = 13 mm from target surface, radial position r = 38 mm, where a

high luminous ring-shaped plasma is observed, and RF power of 40 W. Electron temperature

and plasma density are estimated by this probe current-voltage curve. Figure 5.8 (a) shows

electron temperature as a function of argon gas pressure for setup (a) R = 5, (b) 20 and (c) R=

35 mm at a constant RF power of 40 W and z = 13 mm away from the target surface. Here, the

probe current-voltage curves are measured at radial positions of r = 45, 42 and 38 mm where

the high luminous ring-shaped plasma are observed for R = 5, 20 and 35 mm as shown in Figs.

5.5(a)–(c), respectively. It is found that the electron temperature decreases with increasing gas

pressure for all cases. The electron temperatures are 2.42, 1.71 and 1.15 eV at Ar gas pressure

of 4 Pa for the setup (a), (b) and (c), respectively. However, the setup (a) has the highest

electron temperature, whereas the setup (c) has the lowest electron temperature for all argon

gas pressures.

83

Figure 5.7. Typical I-V Langmuir probe characteristics measured at axial distance z = 13 mm

from target surface for (a): R = 5, (b): R =20 and (c): R = 35 mm, and an RF power of 40 W

and argon gas pressure of 3 Pa.

Plasma density as a function of argon gas pressure for (a) R = 5, (b) 20 and (c) 35 mm

at a constant RF power of 40 W and z = 13 mm is shown in Fig. 6.8 (b). It is seen that the

plasma density increases with increasing gas pressure for all setups. The plasma density is

approximately same for set up (b) and (c) at all gas pressure. The highest plasma densities were

6.26×1015, 1.06×1016 and 1.11×1016 m-3 at 5 Pa for setups (a), (b) and (c), respectively. This is

reasonable because electron temperature is dropping with rising gas pressure. The electron

mean free path and electron-neutral collision frequency are estimated from electron

temperature, gas density, and electron neutral cross-section.

84

(a)

(b)

Figures 5.8. (a) Electron temperature and (b) plasma density as a function of argon gas

pressure for (a): R = 5, (b): R =20 and (c): R = 35 mm at a constant RF power of 40 W and z

= 13 mm away from the target surface.

85

Figures 5.9 (a) and (b) depict the electron mean free path and electron-neutral collision

frequency as a function of argon gas pressure for R = 5, 20, 35 mm at a constant RF power of

40 W and z = 13 mm away from the target surface, respectively. It is seen that the electron

mean free path decreases for Par ≤ 3.0 Pa and then increases with increasing gas pressure for

Par ≤ 5.0 Pa for setups (a), and (b). However, for setup (c), the electron mean free path

decreases abruptly for Par ≤ 2.0 Pa then increases with increasing gas pressure for Par ≤ 5.0

Pa. It is found that the electron mean free path is 41.4, 63.17 and 84.66 mm at Ar gas pressure

of 5 Pa for the setup (a), (b) and (c), respectively.

It is found that the electron neutral collision frequency increases for Par ≤ 2.0 Pa and

then decreases slowly with increasing gas pressure from 3.0 to 5.0 Pa. for set up (b) and (c).

However, for case (a) R = 5 mm set up, the electron collision frequency increases abruptly for

Par ≤ 2.0 Pa then remain constant for Par ≤ 4.0 Pa as well as decreases sharply for Par ≤5.0

Pa. These phenomena are caused by Ramsauer effect [1]. Electron neutral collision frequency

contributes to the generation of plasma charged particles. Electron neutral collision frequency

for case (a) R = 5 mm set up is higher than case (b) R = 20 mm and case (c) R = 35 mm at a

constant RF power of 40 W and z = 13 mm axial distance from the target surface. This means

the formation of outer ring-shaped plasma near the chamber wall for case (a) R = 5 mm is better

than for case (b) R = 20 mm and case (c) R = 35 mm set up. However, outer ring-shaped plasma

for case (b) and case (c) also produced in specific regions. Ion mean free path as a function of

argon gas pressure for R = 5, 20, 35 mm at a constant RF power of 40 W is also investigated.

It is also found that the ion mean free path is independent of argon gas pressure for all cases

with decreasing tendency.

86

(a)

(b)

Figures 5.9. (a) Electron mean free path and (b) electron-neutral collision frequency as a

function of argon gas pressure for (a): R = 5, (b): R =20 and (c): R = 35 mm at a constant RF

power of 40 W and z = 13 mm away from the target surface.

87

5.3.4. Ion Saturation Currents Profile

In the previous work [42], optimization of plasma density and radial uniformity using

multiple arrangements of magnets, there was a marked difference in the normalized ion density

measured at z = 10 mm and z = 58 mm, which decrease sharply during the period 100 to 200

mm. Moreover, the normalized ion density measured at z = 10 mm has more fluctuations with

a lower value than z = 58 mm data. Overall, the ion density radial profile declines abruptly for

all configurations throughout from the center to an outer wall of the chamber [42]. However,

in our works radial profile of ion saturation current has been improved by the monopole

arrangement [see Fig. 5.10]. Figure 5.10 shows that radial profiles of ion saturation current for

the three cases (a): R = 5, (b): R =20 and (c): R = 35 mm at argon gas pressure of 1.0 Pa, a

constant RF power of 40 W by Langmuir probe biased at Vp = -68V and axial distance z = 10

mm away from the target surface. According to probe theory, the ion saturation current is

proportional to the ion flux to the target. It is shown that for the case (b): R =20 mm, the ion

saturation current is almost constant of 50×10-4 A for 0 < r < 45 mm. In contrast, in the case of

case (c): R =35 mm, the ion saturation current has a maximum of 66×10-4 A at r = 5 mm with

keeping almost constant value for 0 < r < 35 mm then decreases gradually with radial position.

On the other hand, for case (a): R = 5 mm, the ion saturation current is the smallest among the

other cases, however, increases significantly for 30 < r < 48 mm. That is, the case (b) is the

best monopole arrangement of magnets from the viewpoint of the target utilization from center

to outer regions.

88

Figure 5.10. Ion saturation current as a function of radial distance for the three cases (a): R

= 5, (b): R =20 and (c): R = 35 mm at argon gas pressure of 1.0 Pa, a constant RF power of

40 W by a Langmuir probe biased at Vp = 68V and axial distance z = 10 mm away from

the target surface.

89

5.4. Conclusions

The outer ring-shaped RF magnetized plasma has been proposed by three circular

monopole magnet arrangement such for (a) R = 5 mm, (b) R = 20 mm and (c) R = 35 mm. It is

found that the axis symmetry property in monopole arrangement because the flux lines profiles

for half region from r = -80 to 0 mm have a similar profile as for a half region r = 0 to 80 mm.

It is seen that a high luminous ring-shaped plasma is observed in contiguity with the wall

position of r = 47-50 mm for (a) R = 5 mm, whereas multi-ring discharges are observed for (b)

R = 20 and (c) 35 mm. The tendency of the absolute value of the DC self-biased voltage |Vsb|

of the cathode is almost similar to the pattern in the discharge voltage |Vrf| corresponding to the

three monopole schemes. It is found that the electron temperature decreases with increasing

gas pressure for all cases. However, it is seen that the plasma density increases with increasing

gas pressure for all setups. The smaller gap distance, the fluctuations of the plasma density and

ion saturation current are more. However, a big gap distance as (c): R = 35 mm, has high ion

saturation current with more fluctuations comparison with case (b): R = 20 mm. So that case

(b): R = 20 mm is effective to improve target utilization from center to the outer area. From a

viewpoint of the center to outer target utilization, case (b): R =20 mm is more effective than

other two cases. However, case (a): R = 5 mm can be used to improve target utilization only in

the outer area, whereas (c): R = 35 mm has the highest ion saturation current and lowest electron

temperature among all cases.

90

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92

Chapter 6. Outer Ring-Shaped Radio Frequency Magnetized Plasma

Source for Target Utilization in Specific Area

6.1 Introduction

The development and application of magnetron sputtering systems have found

widespread use in many industrial manufacturing sectors for physical vapor deposition

[1–2]. Magnetron sputtering techniques use mutually perpendicular magnetic and electric

fields, which confine the plasma close to the deposition source, so increasing the

probability of gas ionization and therefore the plasma density and sputter rates [3–4]. Due

to an ion bombardment, the deposition species usually in metals, alloys, ceramics and

polymers are atomized significantly by sputtering from the target (cathode). On the other

hand, CCP discharges functioned by radio frequency (RF) power supply are widely used

for a great variety of technological applications ranging from plasma dry etching [5] and

medical applications such as sterilization or wound healing [6–7], surface treatment and

cleaning [8], microelectronics [9–11] such as magnetic films [12], diamond like carbon

[13], biomaterial thin films, flat panel display fabrication; transparent conductive oxides

film preparation for solar cells [9], [11], [14], mobile phones and many other rapidly

growing areas in material processing [3–4], [7]. Because the CCP discharges are in

principle of the simplest setup and easily maintenance capable techniques for large-

diameter substrate processing in microelectronic device fabrication, work in various

gases [15–24].

Generally, near the chamber wall, the plasma potential and the plasma density is

very small because an ion sheath exists near the chamber wall [3], [24]. The plasma

potential varies slowly in the plasma, but rapidly in the sheath region. Only in the sheath

region, the quasineutrality property cannot be satisfied. The plasma density profile is

relatively flat in the center and falls sharply near the sheath edge [4], [24]. Moreover, in

practical industrial application, the outer width of the target is large. Therefore, it is

required a high-density plasma discharge in a specific area and also in the outer region of

the chamber to obtain convenient outer target area erosion profile near the chamber wall.

Moreover, to deposit a functional thin film in a specific area and near the chamber wall,

the target utilization in a specific area is required.

The main idea is to use a CCP discharges by the mono-pole magnet arrangement

at a fixed magnet-gap distance in order to get high density plasma in the outer region. In

our previous work [25], we have developed RF ring-shaped plasma obtained with circular

93

magnet monopole arrangement. In the conventional magnetron system, target material is

not effectively used because the high-density plasma is localized on the target surface.

The target utilization is very low approximately at 20-30%. From the practical viewpoint

of the limited resources, the utilizations of the target material are necessary. The

symmetrical magnets magnetron sputtering method with one inner magnet and two outer

annular magnets facing each other was investigated [26]. The maximum target erosion

rate was 57%. The rotating magnet sputtering has also been proposed by rotating helical

magnets for increasing the target utilization efficiency [27]. Using a rotating unbalanced

and asymmetrical magnet, a flat erosion-sputtering method has been developed [28]. The

estimated target utilization had a value of 80% and 77% for 5-in, 4-in aluminum target

material, respectively. The target utilization efficiency was increased from 73.6% to

86.3% when iron pole pieces were used in the rotating cruciform arrangement of

neodymium magnets. However, effect of outer ring-shaped plasma discharge near the

chamber wall in a specific area had not been investigated in detail.

The objective of this work is to produce a outer ring-shaped and specific area

plasmas for obtaining high-density plasma and outer target erosion profile near the

chamber wall. In Section 6.2, the proposed experimental setup of CCP discharges is

explained in detail. In Section 6.3, the results and discussions on the effects of the three

setups are investigated in a monopole circular magnet arrangement of the magnetic flux

lines and their profiles, discharge characteristics, plasma discharge luminescence, plasma

density, and ion saturation currents are explained. In Section 6.4, the results obtained in

this work are summarized.

94

6.2 Experimental Setup Details

The walls of the vacuum chamber are an outer diameter of 235 mm, inner

diameter of 160 mm and a height of 195 mm, which was used to carry out the experiments.

The vacuum chamber was evacuated by a combination of an oil rotary pump and a turbo

molecular pump to a very low pressure. Before starting the experiment, the base pressure

in the chamber of 1.6 10-5 Pa is attained. Ar gas was fed into the chamber through a

regulating flow meter at an operating gas pressure of 1.5 Pa. The schematic illustrations

accompanying diagnostics of the proposed outer ring-shaped CCP source for target

utilization in specific areas using magnets monopole arrangement is shown in Fig. 6.1.

The neodymium cylindrical magnets with surface magnetic flux density of 4120 Gauss,

diameter of 10 mm, and height of 5 mm are used. The gap distance between magnets was

fixed at 5 mm in consecutive circles shown in Fig. 6.1. We selected a fixed gap distance

of 5 mm because high luminescent ring-shaped plasma discharge obtained by circular

magnet monopole arrangement [25].

In especial, R = 5 mm is chosen due to the ion saturation current increases

significantly for 30 < r < 49 mm. The number of magnets positioned in setups (a): 1, 5,

11, and 14, (b): 1, 5, and 11, and (c): 1, and 5. An iron yoke was used to hold the magnets

and positioned on the target surface. Fig. 6.2 shows the cross sections of the magnet

monopole arrangements of fixed gap distance of 5 mm, including a center magnet, with

magnets in setups (a): three circles, (b): two circles, and (c): one circle, respectively. To

cool the magnets and target, a cooling fan was used. An RF power of 50 W at 13.56 MHz

was given as input power to the target, where the chamber wall was acted as a grounded

electrode. An impedance-matching network and a blocking capacitor were also used to

connect RF power supply with the target. To investigate the discharge and the plasma

characteristics, an aluminum target of 140 mm × 140 mm × 3 mm was used, in order to

avoid film deposition on the glass window at the bottom in the chamber.

A high-voltage probe with an attenuator of high input impedance and a digital

oscilloscope with a sampling frequency of 2 giga samplings/sec were used to measure the

RF discharge voltage between the RF powered electrode (target) and the grounded

vacuum wall, as well as a DC self-biased voltage. The sputtering rate is directly

proportional to the ion flux incident to the target. The ion saturation currents, proportional

to the ion flux to the target were measured by a negatively biased cylindrical tungsten

probe of 1.0 mm in diameter and 10 mm in length [4]. The measurements of the ion

95

saturation currents and current-voltage (I-V) characteristics were done by moving the L-

shaped probe to the radial direction, as shown in Fig. 1, and the position were set at 0 < r

< 49 mm. The biasing voltage was changed from –65 V to +65 V. The probe wire was

compensated with an LC filter circuit (choke) in order to reduce the influence of the RF

plasma potential fluctuations on the probe current-voltage characteristics [3], [7]. The

probe measurements were carefully performed so as to minimize the disturbance to the

stable plasma and also taking care of the fluctuation of the plasma potential at the

excitation frequency and its higher harmonics. Moreover, the measurement position was

fixed at axial distance, z = 9 mm from the target surface. The magnetic field is

perpendicular to the probe surface in this position, so that electrons are easily collected

by the Langmuir probe. A Cartesian coordinate system is used. As shown in Fig. 6.1, the

origins of the axial distance z and the radial positions r are defined at the surface and at

the center of the RF powered target, respectively.

Figure 6.1. A schematic illustrations of the proposed outer ring-shaped CCP source

for target utilization in specific area using magnets monopole arrangement.

96

Figure 6.2. Cross sections of the magnet monopole arrangements, with a center

magnet, and magnets in setups (a): three circles, (b): two circles, and (c): one circle.

6.3. Results and Discussions

6.3.1. Profiles of Magnetic Field of the Monopole Setups

The absolute value of the horizontal, Bx magnetic flux densities distributions

were analyzed by Poisson Superfish software developed in Los Alamos National

Laboratory. Fig. 6.3 demonstrates magnetic flux density profile in the proposed outer

ring-shaped CCP source for target utilization in specific areas at the axial position, z = 7

mm from the target surface. It is seen that, Bx has only two peaks and the patterns of

their peaks are almost same for setups (b) and (c). The three highest peaks of Bx are (a):

356 G, (b): 486 G, (c): 495 G, at a radial position of r= 49, 37, and 22 mm,

97

respectively. The radial profiles of magnetic flux in additional two cases of 1,11 (center

and 2nd circle: setup (d)) magnets, and 1, 14 (center and 3rd circle: setup (e)) magnets

were also investigated. To discuss the role of the inside magnets for the production of the

plasma, 1 and 11 magnets were placed at the center and in the second circle, 1 and 14

magnets were placed at the center and in the third circle, respectively. Multiple peaks

were obtained in setups (d) and (e). The three highest peaks had values of 386, 222, and

245 G at radial positions of r = ±39, ±23, and ±7 mm, respectively, for setup (d). In the

setup (e), the three highest peaks with amplitudes of 226, 355, and 342 G were also

observed at radial positions of r = ±52, ±36, and ±8 mm, respectively. Moreover, the

other magnetic flux peaks from r = 0 to ±40 mm for setup (a) and r = 0 to ±35 mm for

setup (b) had the lowest amplitudes among the five setups. Moreover, high-amplitude

peaks are observed in the outer area for setups (a) and (b). To discuss the effect of the

outer ring-shape structure on axial profile of magnetic flux at some radial positions, two

dimensional magnetic field lines distributions for setups (a), (b), and (c) were investigated.

The magnetic channeling increases with increasing the magnets in the inside circle.

Moreover, magnetic flux distributions of high-amplitude are restricted at the portion of

the outer area so that inside magnet arrangements for setup (a) and (b) produce high

luminescence plasma near the chamber wall [see Fig. 6.3 and 6.4].

High luminescent ring-shaped plasma discharges were produced by high-value of

Bx due to the E B drift effect at their corresponding position. The magnetic flux density,

Bx for setup (a) have the lowest among three setups from r = 0 to 50 mm. The larmor

radius is known as gyro radius. The gyro radius of electron, rLe will be smaller in

accordance with, , and the cyclotron frequencies, increase in a rising

magnetic field by according to the equation, , where e, m, and ve are electronic

charge, masses of charged particles, and velocity of electron is the speed perpendicular

to the magnetic field, Bx. The gyro radius and cyclotron frequency are calculated with

an assumption of Te = 2 eV at the highest magnetic flux position. It was found that gyro

radius of the electron, re are (a): 0.1519 mm, (b): 0.1111 mm, and (c): 0.1093 mm at r=

49, 37, and 22 mm, respectively. Moreover, the cyclotron angular frequencies of

the electron, were (a): 6.25×109 Hz, (b): 8.54×109 Hz, (c): 8.68×109 Hz, at r= 49,

37, and 22 mm, respectively. It is important to investigate the electron motion in the

magnetic field because the electrons produce plasma. In order to analyze the

magnetization degree of the charged particle, the electron hall parameter was calculated

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from c/νm, where c and νm are the charged particle cyclotron frequency and the charged

particle-neutral collision frequency, respectively. The neutral collision frequency of

electrons was calculated from the relation of nArve, where , nAr, and ve are the electron

neutral collision cross section, the Argon gas density and the electron velocity,

respectively. The electron hall parameter was 245, 395, and 393 at high luminescent

plasma discharge position for setups (a), (b), and (c), respectively. Thus, electrons are

strongly magnetized in the high luminescent plasma discharge position, i.e. near the

chamber wall.

Figure 6.3. Absolute value of the horizontal magnetic flux densities, Bx in the outer

ring-shaped CCP source with a center magnet, and magnets in setups (a) three circles,

(b) two circles, (c) one circle, (d): center and 2nd circle, and (e): center and 3rd circle

magnets at z = 7 mm from the target surface.

6.3.2. Emission of Outer Ring–Shaped Plasma

Fig. 6.4(a)–(c) show a typical RF magnetized plasma discharge at an RF power

of 50 W, Ar gas pressure of 1.5 Pa, with a center magnet, and magnets in setups (a): three

circles, (b): two circles, and (c): one circle. Ring-shaped plasma in the specific outer area

is observed at the position with the peak magnetic flux density and its diameter depends

on a number of magnet circles. This is because, electrons strongly are affected based on

the E×B drift motion, where the electric field, E, and the magnetic flux density, B, are

99

perpendicular to the target and parallel to the target, respectively. At the high magnetic

flux density position, which produce high luminescent plasma discharge and E×B drift

motion considered to play a significant role in the magnetized plasma discharge. However,

high luminescent plasma discharge was observed in the outer area.

(a)

(b)

100

(c)

Figure 6.4. Typical RF outer ring-shaped RF magnetized plasma discharge at an RF

power of 50 W, Argon gas pressure of 1.5 Pa with a center magnet, and magnets in

setups (a): three circles, (b): two circles, and (c): one circle.

6.3.3. Outer Ring–Shaped Plasma Characteristics

Fig. 6.5 (a) and (b) show the discharge voltage, VRF, and the absolute value of the

DC self-biased voltage, |Vsb|, respectively, in the outer ring-shaped RF magnetized plasma

sputtering source as a function of the Ar gas pressure at an RF power of 50 W, for setups

(a), (b), and (c). The discharge voltage is defined as the peak to peak value of RF voltage

when RF plasma is sustained at some condition. Discharge voltage denotes a voltage level

at which the charged particle collision rate with neutral particles occurred and ionization

of neutrals sustains the plasma in the steady state. The RF discharge voltages, VRF, were

460, 340, and 440 Vp-p and the absolute value self-bias dc voltages, |Vsb|, were 154, 83,

and 143 V at Ar gas pressures of 1.5 Pa, for setups (a), (b), and (c), respectively, at an RF

power of 50 W.

The discharge voltage, VRF, and the self-bias dc voltage, |VSB|, for (b) is the lowest

among the other setups, where setups (a) and (b) have approximately the same value. This

means RF discharge for setup (b) is easier than the other setups because of lower

discharge and self-biased voltage among three cases. In general, the discharge voltage,

VRF, decreases with increasing Ar gas pressure, this is because the electron collision

101

frequency and the ionization rate, increases with an increasing Ar gas pressure, and the

discharge voltage is approximately inversely proportional to the electron collision

frequency at a fixed absorbed power [1–2]. However, in this proposed outer ring-shaped

RF magnetized plasma, the discharge voltage, VRF, is almost constant due to stable plasma

in all Ar gas pressure at an RF power of 50W.

Fig. 6.6 shows typical probe current-voltage characteristics for setups (a), (b), and

(c), respectively. The measurements position was at axial distance z = 10 mm from target

surface, radial position (a): at r = 45 mm, (b): at r = 40 mm, and (c): at r = 33 mm, where

a high glowing ring-shaped plasma is observed, and an RF power of 50 W.

(a)

102

(b)

Figure 6.5. (a) RF discharge voltage, VRF (peak to peak) and (b) absolute value of the

DC self-biased voltage, |VSB| of the target as a function of Argon gas pressure in the

outer ring-shaped RF magnetized plasma at an RF power of 50 W for all setups.

Figure 6.6. A typical current-voltage probe characteristics for monopole arrangements

with a center magnet, and magnets in setups (a): three circles, (b): two circles, and (c):

one circle.

103

6.3.4. Ion Saturation Currents in Outer Ring–Shaped Plasma

In general, the electrons have much smaller gyro radius than ions because ions

mass is more that electrons. In a relatively weak magnetic field, electron gyro radius can

be quite large. The ion current collection will not be affected by stronger fields are

existing. All the electron Hall parameters are much larger than one. Thus, it is suggested

that the electrons are intensely magnetized in the high luminescent plasma discharge

position, i.e. near the chamber wall. In contrast, all the ion Hall parameters are much less

than one, so that, ions are not almost magnetized. In particular, the strong magnetic fields

make the electron gyro radius smaller than the probe radius, which will lower the

magnitude of electron saturation currents, Ies. This is because the probe depletes the field

lines that it intercepts, and further electrons can be collected only if they diffuse across

the magnetic fields. Moreover, the probe cannot collect electron when the magnetic fields

are parallel to the probe.

Fig. 6.7 shows the ion saturation currents, Iisat as a function of the radial distance

from the center of the target in the proposed outer ring-shaped CCP source for obtaining

specific area target erosion profile near the chamber wall by magnetic monopole

arrangements for setups (a), (b), and (c), respectively. The ion saturation currents, Iisat

were measured by an L-shaped Langmuir probe, negatively biased at Vp = -65 V and

positioned in the at an axial distance z = 9 mm away from the target surface. The Iisat are

0.6 mA, 0.79 mA, and 0.46 mA, for setups (a), (b), and (c), respectively at a radial

position of r = 47 mm. The Iisat for setup (a) is the lowest for 0 < r < 35 mm, whereas for

setup (c), Iisat has the highest value among three setups. It is seen that, Iisat is very high in

the outer target region near the chamber wall for setups (a) and (b), where Iisat for setup

(c) decreases slowly. Comparison of the positions of the high luminescent plasma

discharge, peak ion saturation current and peak of the horizontal, Bx magnetic flux

position at the various setups are listed in table 6.1.

104

Figure 6.7. Radial profiles of ion saturation currents in the proposed outer ring-shaped

RF plasma using magnetic monopole arrangements, with a center magnet, and magnets

in setups (a) three circles, (b) with two circles, and (c) one circle.

It is defined that the effective radial width (ra, rb, and rc) as a distance between

the positions, where a level of 0.707 of the maximum value of ion the saturation currents,

Iisat and maximum of Iisat. It was found that ra = 10 mm (39 to 49 mm), rb = 8 mm (37 to

45 mm), and rc = 10 mm (25 to 35 mm). Moreover, the effective radial width position is

shifted from the chamber wall to center with removing the magnets in the outer circles.

The roughnesses of the ion saturation current for setups (a), (b), and (c) were also

calculated using, (Iisat, max - Iisat, min)/(Iisat, max + Iisat, min). The roughnesses of Iisat are 51.3%,

42.6%, and 20.3% for setups (a), (b), and (c), respectively. The ion saturation current is

directly proportional to the ion flux incidence to the target. The measurement of Iisat is

the simplest and the best technique to estimate the plasma density, n. Plasma density is

estimated from ion saturation current using Bohm sheath criteria [3-4]. The calculated

plasma densities were 6.78×1016 m-3, 8.98×1016 m-3, and 5.19×1016 m-3 in the proposed

outer ring plasma for setups (a), (b), and (c), respectively at a radial position of r = 47

mm and axial position of z =9 mm from the target surface.

105

Table 6.1

Comparison of the various positions

Monopole magnet

setups [mm]

Setup (a) Setup (b) Setup (c)

High luminescence plasma

position [mm]

48-50 42-46 22-35

Peak ion saturation current

position [mm]

49 45 35

Peak horizontal magnetic

flux, Bx position [mm]

50 43 23

6.4. Conclusion

Ring-shaped plasma in the specific outer area was observed at the position of the

peak magnetic flux density and its diameter depends on a number of magnet circles. The

high-density plasma was found in the outer target region near the chamber wall for all

type arrangements. The plasma discharge results that the target utilization can be

controlled in the outer specific area near the wall. The electron hall parameter was (a):

245, (b): 395, and (c): 393 at high luminescent plasma discharge position. It was observed

that the ion saturation currents, Iisat are 0.6 mA, 0.79 mA, and 0.46 mA, for setups (a),

(b), and (c), respectively at a radial position of r = 47 mm. The Iisat for setup (a) is the

lowest for 0 < r < 35 mm, whereas for setup (c), Iisat has the highest value among three

setups. It is seen that, Iisat is very high in the outer target region near the chamber wall for

setups (a) and (b), where Iisat for setup (c) decreases slowly. Thus, the proposed CCP

plasma source has an advantage of the outer target erosion profile and target utilization

in the specific area.

106

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108

Chapter 7. Characteristics of a Ring–Shaped Pulsed DC Discharge

Plasma Source Using Single Pole Magnet Setups

7.1. Introduction

Magnetron sputtering (MS) technologies have become the worldwide standard for

most of the physical vapor deposition (PVD) since the invention of the planar cathode

arrangement credited to J. S. Chapin with a patent filed in 1974 [1]. Most of the metallic

materials can be sputtered where oxides and nitrides can be also synthesized. The

depositions show excellent uniformity and reproducibility, smoothed the film surface (no

droplets), and the target (up to a few meters long) can be made with different geometries

in order to coat various shapes of the substrate [2–3]. All these advantages make MS a

highly flexible deposition process and its promising applications are commonly used in

many research, industrial manufacturing sectors and functional coatings preparation [1–

16]. MS glows are, in principle, the sputtering resulting from the target by bombarding

energetic gas ions, where magnetic fields are used to concentrate and confine the plasma

closer to the deposition source [3, 14].

In general, various types of power schemes, such as direct current (DC), radio

frequency (RF), microwave and pulsed DC, have been developed for MS system. Among

these, RF power is mostly used in Plasma Enhanced Chemical Vapor Deposition

(PECVD) and MS system particularly depositing functional films of highly insulating

properties, because the substrates were required to be powered with an alternating pulse

[17-19]. The RFMS method is advantageous to repeat reproducible deposition under an

appropriate impedance matching network between RF power source and powered

electrode load with a matching circuit. For independent control of the RFMS condition,

the substrate bias voltage is not independent of other conditions and depend on the applied

RF power density because of the substrate bias voltage influences incident ion energy on

the deposited functional film [20].

Pulsed DC discharge plasma is considered as an alternative to the RFMS method

and supposed to be a simple and cost-effective deposition system that can be used to

improve film properties as compared to widely used RFMS method [21-23]. In a pulsed

DC discharge plasma source, pulsed DC power supply is a significant component for the

reason that it may possible to change the gas phase chemistry, energy of the incident ions

bombardment on substrate, electron density, electron temperature to meet the

requirements of highly selective deposition method by changing the pulse duration and

109

frequency [24-26]. However, most of the models of commercially available pulsed DC

discharge power supplies are expensive and custom-made which led to less flexibility to

control pulse conditions. A number of researchers reported the pulsed DC discharge

power supplies system mentioning different techniques [27-28]. In comparison with RF

power supplies pulsed DC discharge sources possess a considerably higher efficiency, a

lower cost, a considerably simplified tuning and controlling [20-21]. However, until now

the medium-frequency pulsed discharges are much less studied than radio-frequency or

direct current ones that impede essentially the progress in this area [21, 28].

A large number of theoretical and experimental works have been investigated to

study the pulsed dc discharges plasma [29-41]. However, the problem that the target

utilization rate is not uniform, has been not yet investigated in pulsed DC discharged

plasma. Recently, we have developed radio–frequency ring–shaped magnetized plasma

achieved with magnetic single pole arrangement for uniform target utilization [42]. The

current-voltage characteristics, the electron temperature, the plasma density, the ion and

electron mean free path were analyzed. Moreover, we proposed the outer ring-shaped

radio frequency magnetized plasma source for target utilization in a specific area using

single-pole magnet setups [43]. The relations among various positions of the high-

luminous plasma discharge, peak-magnetic densities and effective ion saturation currents

were discussed in detail for RF power source. The main idea is to analyze the pulsed DC

discharged plasma by the single pole magnet setups at a fixed magnet-gap distance in

order to get high–density ring–shaped plasma. However, the effect of ring–shaped plasma

with single pole magnet setups produced by pulsed DC discharged source had not been

investigated yet in details. Moreover, the differences between RF and pulsed DC

discharges for single-pole magnet setups were not investigated in details.

The objective of this work is to produce a ring–shaped plasma in a specific area

for obtaining outer target erosion profile near the chamber wall. In Section 7.2, the

proposed experimental setup of pulsed DC discharged plasma is explained in detail. In

Section 7.3, the results and discussions on the effects of the three setups are investigated

in a single pole magnet arrangement, three-dimensional magnetic flux lines and their

profiles, gyro–radius of the electrons and ions, Hall parameters of the electrons and ions,

discharge characteristics, ion saturation current profiles are explained. The results

obtained in this work are summarized in section 7.4.

110

7.2. Experimental Arrangements

Figure 7.1 shows a schematic diagram of the proposed pulsed DC discharge

plasma using the single pole magnet setups. The experiments were carried out in a

stainless–steel cylindrical vacuum chamber with an outer diameter of 235 mm, an inner

diameter of 160 mm and a height of 195 mm. The cross-sections of the proposed magnetic

single pole setups of fixed gap distance of 5 mm, including a center magnet, with magnets

in setups (a): one circle, (b): two circles, and (c): three circles are shown in Figs. 7.2(a)–

(c). The pulses were supplied from a pulsed power supply, which is known as pulse

modulator (PuM) to the target. The PuM was powered by three-phase power supply. The

pulse duration of 20μs was set in PuM. The pulse repetition rate was 2.5 kHz. The duty

cycle was 5%. Argon (Ar) gas was introduced into the chamber through a regulating flow

meter at an operating gas pressure of 12 Pa for setups. During the experiments, the target

currents I(t) through the circuit and the target voltage V(t) were measured using a current

monitor and a high–voltage probe, respectively. A current transformer has been used to

measure the instantaneous current at the target. The target was connected to the earth

directly by grounding cable. The consumed electrical power P is calculated by P = V(t)

I(t). A magnetron system equipped with the neodymium cylindrical magnets with a

surface magnetic flux density of 4120 Gauss, diameter of 10 mm, and height of 5 mm

were used. No arc was observed during the experiment. The gap distance between

magnets was fixed at 5 mm in consecutive circles, which are shown in Fig. 7.1. The

number of magnets positioned in setups (a): 1, and 5, (b): 1, 5, and 11, and (c): 1, 5, 11,

and 14. An iron yoke is used to hold the magnets and positioned on the target surface.

The evacuation system consists of a turbo molecular pump (TMP) as a main Evacuator

and a rotary pump (RP) as a mechanical booster pump. Before starting the experiment,

the base pressure in the vacuum chamber of 0.710–5 Pa is attained. The inlet gas pressure

is controlled by a mass flow controller. We selected a fixed gap distance of 5 mm because

high luminescent radio–frequency (RF) magnetized ring–shaped plasmas were attained

with circular magnet monopole setups [42]. Moreover, the ion saturation current

increased significantly for 30 < r < 48 mm [42]. A cooling fan was used to cool the

magnets and the target.

To investigate the pulsed discharge, target voltage and the ion saturation current,

an aluminum target of 140 mm × 140 mm × 3 mm was used, in order to avoid film

deposition on the glass window at the bottom in the chamber. The ion saturation currents,

111

proportional to the ion flux to the target were measured by a negatively biased cylindrical

tungsten Langmuir probe of 1.0 mm in diameter and 10 mm in length [42-43]. The ion

saturation current measurements were done by moving a Langmuir probe of L–shaped in

the radial direction, as shown in Fig. 7.1. The measurement radial positions were set at 0

< r < 48 mm. The biasing voltage was fixed at – 65 V and the voltage drop of the

resistance of 1 k was monitored with a digital oscilloscope. The probe measurements

were carefully performed so as to minimize the disturbance to the stable plasma.

Moreover, the measurement position was fixed at an axial distance, z = 8 mm from the

target surface. The magnetic field is perpendicular to the probe surface in this position so

that electrons are easily collected by the Langmuir probe. As shown in Fig. 7.1, the

origins of the axial distance z and the radial positions, r are defined at the surface and at

the center of the pulsed discharge powered target, respectively.

Figure 7.1. Schematic diagram of the experimental setup used for the proposed pulsed

DC discharge plasma using a single pole magnet setup.

112

113

Figure 7.2 (a)–(c). Proposed single pole magnet arrangements of the ring–shaped

pulsed discharge plasma source including a center magnet, with magnets in setups (a):

one circle, (b): two circles, and (c): three circles for material processing.

7.3. Results and Discussions

7.3.1. Simulation of Single Pole Magnet Setups for Ring-Shaped Pulsed Discharge

The three-dimensional (3D) magnetic flux lines for the three magnet setups (a),

(b), and (c) are analyzed using Femtet software in order to discuss the effect of the

magnetic field profiles in the proposed single pole magnet setups on the pulsed discharge.

Figure 7. 3 shows a map of the three-dimensional distribution of the magnetic flux lines

generated by the permanent magnets on the target surface, z = 0 mm for setups (a), (b),

and (c). A circular iron disk, that is, magnet holder of 140 mm of diameter and 1 mm of

thickness diameter was used. The magnetic field lines generated from the N-pole of the

magnets passed through the target, the airspace, the magnet holder and then returned to

the S–pole for all single–pole magnet arrangements as shown in Fig. 7. 3(a)–(c). The

axisymmetric property for the single–pole magnet setup is satisfied because the flux lines

profile for the half region from r = −70 to 0 mm is similar to that for the half region from

114

r = 0 to +70 mm [see Figs 7.3 and 7.4]. It is important to effectively use the E × Bx drift

motion, to generate ring–shaped pulsed discharge at the specific areas, where E and Bx

are the electric field perpendicular to the target and the magnetic flux density parallel to

the target, respectively. Moreover, the simulated magnetic flux density component

parallel to the target has a significant effect on the E × Bx drift motion, the gyro–radius,

and the Hall parameter. Setup (c) has high-density magnetic flux lines distributions in the

outer area than setups (a) and (b), where setup (a) has the lowest among three setups from

approximately r = 20 to 50 mm [Fig. 7. 3(a)–(c)]. The highest 2D magnetic flux lines

concentrations were found approximately at 18 to 23 mm, 33 to 38 mm, and 48 to 53 mm

for setups (a), (b), and (c), respectively.

115

Figure 7. 3 (a)–(c). Maps of the two-dimensional distribution of the magnetic flux lines

generated by the permanent magnets on the target surface for setups (a), (b), and (c).

116

Figure 7.4 shows the radial profile of the absolute value of (a) the horizontal

magnetic flux density, |Bx| and (b) the vertical magnetic flux density |Bz| for the setups (a),

(b), and (c) at an axial distance of z = 0 mm from the target surface. Multiple peaks are

obtained in all magnet arrangements. The patterns of the |Bx| for setup (b) is almost same

as setup (c) from r = −23 to 23 mm. The peaks of the horizontal magnetic flux density,

|Bx| profiles are 6, 10, and 14 for the setups (a), (b), and (c), respectively. The magnetic

flux density from r = 0 to ±45 mm for setup (c) had the lowest magnitudes among the

three setups. A maximum flux density of approximately 521 G at r = 0 to ±16 mm for

setup (a), 496 G at r = 0 to ±31 mm for setup (b), and 504 G at r = 0 to ±46 mm for setup

(c) were observed with a fluctuation from zero. Moreover, the three highest peak values

were 1080 G at r = ±19 mm, 1166 G at r = ±36 mm, 1158 G at r = ±50 mm for the setups

(a), (b), and (c), respectively. The peak amplitude of |Bz| for setup (a) is lower than that

for setups (b) and (c) from r = 0 to ±20 mm. The peak amplitudes of |Bz| approximately

846 G, 1000 G, and 373 G at r = 0, ±15, and ±30 mm for setup (a), approximately 588 G,

642 G, 884 G, and 444 G at r = 0, ±15, ±30, and ±45 mm for setup (b), approximately

474 G, 493 G, 574 G, and 816 G at r = 0, ±15, ±30, and ±45 mm for setup (c) were found,

respectively. It is noted that the vertical magnetic flux density components do not

contribute to the E × B drift motion of the charged particles.

(a)

117

(b)

Figure 7.4. Radial profiles of the absolute value of (a) the horizontal, |Bx| and (b) the

vertical, |Bz| magnetic flux density at the target surface for setups (a), (b), and (c).

The cyclotron angular frequency of the charged particles such as electrons and

ions are important parameters for magnetized pulsed discharge plasma. These calculated

cyclotron frequencies based on the magnetic field density, Bx, the electronic charges, e,

and the masses of charged particles, m, were 1.9×1010, 2.1×1010, and 2.0×1010 Hz at a

high flux density position for setups (a), (b), and (c), respectively. These high–magnitudes

flux density will provide some ring–shaped discharges due to the E×B drift effect in the

specific areas for all setups. The larmor radius is also known as gyro radius. The larmor

radius is investigated to discuss the magnetization of the charged particles in the

magnetized pulsed discharge plasma. The larmor radius of the electrons is calculated

from , where m, v, e, and Bx denote the charged particle mass, the velocity of

the charged particles, the electronic charge, and the magnetic flux density in the radial

direction, respectively.

Figure 7.5 shows radial profiles of the larmor radius of the (a) electrons and (b)

ions, respectively. The larmor radius is also known as gyro radius. The larmor–radii are

calculated by the magnetic flux density in the radial direction at an axial position of z =

0 mm with the assumption of electron temperature, Te = 2 eV, ion temperature of Ti = 0.2

eV, and the charged particle velocity of 9.5×105 ms-1 [43]. It is seen that the electron

larmor radius, rLe are less than 4 mm except at r = 0 mm position for all setups. For

example, the electron velocity, ve = 9.5 × 105 ms-1 assumed by the electron temperature

118

Te = 2 eV, at r = 40 mm, z = 0 mm, and the magnetic flux density in the radial component

(a): Bx = 158 G, (b): Bx = 803 G, (c): Bx = 295 G, the estimated electron larmor radius, rLe

is approximately (a): 0.34 mm, (b): 0.06 mm, and (c): 0.18 mm, respectively. Moreover,

the electron larmor radius rLe are 0.17 mm, 1.64 mm, 5.82 mm for setups (a), (b), and (c),

respectively. In single pole magnet arrangements, the highest value for the electron gyro–

radius is approximately (a): 17.23 mm, (b): 9.67 mm, and (c): 9.40 mm, at r = 0 mm,

where the magnetic flux density in the radial direction has the lowest value. It is also seen

that the electron gyro–radius is much smaller than the size of the chamber radius. On the

other hand, the argon ion larmor radius, rLi has the maximum value of 108m, 17.76m,

17.27m at r = 0 mm for setups (a), (b), and (c), respectively. The radial profile of argon

ion larmor radius is the same as the profile of the electron larmor radius. Moreover, argon

ion larmor radius is higher than the electron larmor radius corresponding to each point.

(a)

119

(b)

Figure 7.5. Radial profiles of the larmor radius of the (a) electrons and (b) ions for

setups (a), (b), and (c).

Figures 7.6(a)–(b) show the Hall parameters (HP) of electrons and ions at z = 0

mm for setups (a), (b), and (c) in a logarithmic scale. The Hall parameter for electrons

indicates the magnetization degree, which represents a rotating number of electrons

around the magnetic field in their collision times. The HP is estimated by the product

between the electron cyclotron angular frequency, ωc and the electron collision time, τce

which is inversely proportional to the electron collision frequency, ʋme [17, 43]. The

electron Hall parameter, he and the ion Hall parameter, hi of electrons are expressed as he

= ωce/ʋmi and hi = ωci/ʋmi where ωce, ωci, ʋme, and ʋmi are the cyclotron angular frequencies

of electrons and ions, and collision times of electrons and ions, respectively.

It is seen that the radial profiles of the Hall parameters are inversely proportional

to the radial profiles of the gyro-radii [see Figs. 7.5 and 7.6]. This is reasonable because

the cyclotron frequency is the reciprocal of the gyro–radius. The highest electron Hall

parameters of approximately 561 at r = ±21 mm for setup (a), 544 at r = ±36 mm for

setup (b), and 297 at r = ±50 mm for setup (c) were found. The electron Hall parameters

120

are much larger than one for all magnet setups except r = 0 and ±16 mm for setup (a), r

= 0 and ±31 mm for setup (b), and r = 0 and ±46 mm for setup (c), respectively. Thus,

the electrons in the pulsed discharges are strongly magnetized at the regions with high

electron HP. Thus, the energetic beam electrons are accelerated in the pulsed discharges

by single–pole magnet setups. So that along the magnetic field line, the bounce resonance

heating happens to produce the ring–shaped pulsed discharges. The highest ion Hall

parameters of approximately 0.3 at r = ±21 mm, 0.29 at r = ±36 mm, and 0.16 at r = ±50

mm were found for setups (a), (b), and (c), respectively as shown in Fig. 7.6 (b).

Moreover, all the ions Hall parameters are much less than one, whereas the radial profile

has the same pattern as the electron Hall parameter. Therefore, all ions are not magnetized

because all the ion gyro–radius in the pulsed discharges are more than the size of the

vacuum chamber radius.

(a)

121

(b)

Figure 7.6. Radial profiles of the Hall parameters of the (a) electrons and (b) ions for

setups (a), (b), and (c).

7.3.2. Electrical Characteristics of the Ring–Shaped Pulsed Discharge Plasma

Figure 7.7 shows the temporal behavior waveforms of (a) target voltage, (b)

current through the target (the glow current), and (c) power consumed in the plasma, at

Ar gas pressure of 12 Pa, the pulse repetition rate of 2.5 kHz for setups (a), (b), and (c),

respectively. The substrate voltage was fixed at –65V. The source voltage applied to the

target is 0.6 kV, 0.6 kV, and 1.1 kV for setups (a), (b), and (c), respectively and its pulse

has a duration of 20μs. Longer pulses were not applied due to severe arcing. The voltage

applied after 10 μs have passed and then the ionization started. The peak currents were

approximately 0.14 A, 0.38 A, and 0.28 A for setups (a), (b), and (c), respectively. It

seems that the current is very small. This is reasonable because the output impedance of

the pulse modulator was 1.5 kΩ. At the end of the pulse, the current through the target

becomes lower (almost zero). Moreover, the current through the target for setup (c) is

about –0.26 A due to the displacement current and voltage changes to 0 V very rapidly

after the end of the pulse. The peak powers consumed in the plasma for pulsed discharges

were approximately 72.8 W, 187.2 W, and 171.6 W for setups (a), (b), and (c),

122

respectively. The consumed power in the plasma is also low because the current through

the target is limited by the output impedance of 1.5 kΩ of the pulse modulator.

(a)

(b)

123

(c)

Figure 7.7. Waveforms of (a) target voltage, (b) current through the target, and (c)

power consumed in the plasma at Ar gas pressure of 12 Pa, pulse repetition rate of 2.5

kHz for setups (a), (b), and (c). The output impedance of pulse modulator was 1.5 kΩ.

7.3.3. Discharge Characteristics of the Ring–shaped Pulsed Discharge Plasma

Figures 7.8(a)–(c) show the typical photographs of the pulsed plasma DC

discharge plasma at Ar gas pressure of 12 Pa, pulse duration of 20μs, and a pulse

repetition rate of 2.5 kHz for setups (a), (b), and (c), respectively. The white dashed circle

denotes the position of the outer wall of the chamber. The pulsed DC discharge

magnetized plasma photographs were taken using a low-resolution digital camera. The E

B drift motion of the electrons plays a significant effect on the distribution of plasma

discharge as mentioned in the above section. The effect of high plasma density and the

superposition of the electric and the magnetic field results in the reformation of the dense

ionization regions of the target in pulsed DC discharge plasma, which is good agreement

with Y. Ohtsu et al [44]. These ionization regions were captured on the target surface and

also observed to intensify and extend along the target racetrack with increasing Ar

pressure from 6 Pa to 12 Pa. The strong ring–shaped plasma discharge is observed for all

setups. Multiple ring–shaped plasma discharges were not found for all setups. The high

luminous plasma positions were approximately at 20 mm, 40 mm, and 48 mm, where

almost correspond to the positions of the highest electron HP, for setups (a), (b), and (c),

respectively. The typical discharge voltages were 0.6 kV, 0.6 kV, and 1.0 kV for setups

(a), (b), and (c), respectively.

124

(a)

(b)

125

(c)

Figure 7.8. Typical photographs of the pulsed DC discharge plasma at Ar gas pressure

of 12 Pa for setups (a), (b), and (c). The dashed circle denotes the outer wall of the

chamber.

Figure 7.9 shows the target voltage and duty cycle as a function of pulse

repetition rate for setup (b). It was difficult to do experiments for setups (a) at 0.7 kV or

more, (b) at 0.7 kV or more, and (c) at 1.1 kV or more because the overloading currents

trip the circuit breaker and shut down the pulsed power source. The setups (a) and (b)

were easy for generating pulsed discharge plasma compared to setup (c). Moreover, the

critical discharge point for the proposed pulsed discharge was also observed. There was

no pulsed discharge plasma at ≤ 2 kHz. . Moreover, plasma discharges were not observed

at low gas pressure less than 6.0, 6.0, and 12.0 Pa for setups (a), (b), and (c), respectively.

126

Figure 7.9. Target voltage and duty cycle as a function of pulse repetition rate for setups

(b) to investigate the critical point for the proposed pulsed discharge plasma.

7.3.4. Ion Saturation Currents Profiles

Figure 7.10 shows the temporal waveforms of the ion saturation currents

detected by a tiny Langmuir probe positioned at z = 8 mm from the target surface, the

radial position of r = 40 mm, Ar gas pressure of 12 Pa current, a fixed bias voltage of –

65 V, pulse duration of 20 μs and the pulse repetition rate of 2.5 kHz for setups (a), (b),

and (c). Ion saturation current, Iisat can be used as a proxy for the incoming ion flux to the

target, which in turns has a significant effect on the film properties. The ion saturation

current, Iisat for setup (c) dominates for the first 20 μs of the pulse, while later in the pulse

the Iisat for setup (b) dominates. This is because the application voltage was high for t <

20 μs for setup (c). At the end (0 μs) of the pulse, the target voltage drops instantaneously

to zero [see figure 7(a)], and the ions are not returned immediately by the high target

voltage after the pulse. As a result, Iisat for all setups gradually decreases. The Iisat were

3.2 mA, 2.8 mA, and 4.3 mA at t = –10μs for setup (a), (b), and (c), respectively. A

resistance of 1.5 k was used as the output impedance of the pulse modulator.

127

Figure 7.10. Temporal behavior of the ion saturation currents as a function of time at

the radial position of r = 40 mm, Ar pressure of 12 Pa current, and z = 8 mm for setups

(a), (b), and (c). Here, the lower value of peak Iisat observed because the output

impedance of pulse modulator was 1.5 kΩ.

Figure 7.11 shows the ion saturation current, Iisat of the pulsed DC discharge as

a function of radial position at a fixed time (t = –12 μs, –5 μs, +10 μs, +40 μs), at Ar gas

pressure of 12 Pa for setups (a), (b), and (c). The ion saturation currents for setups (a),

(b), and (c) were approximately same for the radial position of r = 0 to 20 mm at t = –12

μs, –5 μs, +10 μs. The highest ion saturation currents were found at t = –5 μs for all setups.

The Iisat of setup (b) is less fluctuating, whereas Iisat of setup (c) is highly variable in all

radial position at t = –12 μs, –5 μs, +10 μs. It is important to note that, the highest ion

saturation currents were found for setup (b) at a radial position of r = 45 mm at all fixed

time. The Iisat of 1.44, 2.88, and 2.2 mA for setups (a), (b), and (c), respectively at r = 45

mm and t = +10 μs were found. It is seen that, as the time passes, the ion saturation

magnitudes are decreasing gradually [see Figs. 7.11 (a)–(d)]. This effect is reasonable

due to plasma diffusion. The decay of the ionization is occurred by electron localization

in a ring–shaped pulsed plasma discharge using single pole magnet setups. The lowest

Iisat is observed at t = +40 μs for all setups because the target voltage and current through

the plasma are returned to zero. Moreover, ion saturation current decreases very slowly

near the chamber wall for setups (b) and (c).

128

(a)

(b)

(c)

129

(d)

Figure 7.11. Radial profiles of the ion saturation currents of the pulsed discharge at Ar

pressure of 12 Pa at t = –12 μs, –5 μs, +10 μs, and +40 μs for setups (a), (b), and (c).

7.4. Conclusion

The proposed single pole setups of fixed gap distance of 5 mm, including a

center magnet, with magnets in setups (a): one circle, (b): two circles, and (c): three

circles were used. The number of magnets placed in setups (a): 1, and 5, (b): 1, 5, and 11,

and (c): 1, 5, 11, and 14. The pulse duration of 20μs, the pulse repetition rate was 2.5 kHz,

Ar pressure of 12 Pa current, and the source voltage to the target is (a): 0.6 kV, (b): 0.6

kV, and (c): 1.1 kV were applied to do pulsed discharge experiments. The highest two

dimensional magnetic flux lines concentrations were found approximately at (a): 18 to

23 mm, (b): 33 to 38 mm, and (c): 48 to 53 mm. The peaks of the horizontal magnetic

flux density, |Bx| profiles were 6, 10, and 14 for the setups (a), (b), and (c), respectively.

The three highest peak values of |Bx| were 1080 G at r = ±19 mm, 1166 G at r = ±36 mm,

1158 G at r = ±50 mm for the setups (a), (b), and (c), respectively. It is also seen that the

electron larmor radius is much smaller than the size of the chamber radius. The argon ion

larmor radius has the maximum value of 108 m, 17.76 m, 17.27 m at r = 0 mm for setups

(a), (b), and (c), respectively. The electron Hall parameters are much larger than one for

all magnet setups except r = 0 and ±16 mm for setup (a), r = 0 and ±31 mm for setup (b),

and r = 0 and ±46 mm for setup (c), respectively, whereas all the ions Hall parameters

are much less than one. The peak currents in the plasma were approximately 0.14 A, 0.38

A, and 0.28 A for setups (a), (b), and (c), respectively, whereas the output impedance of

the pulse modulator was 1.5 kΩ. The high luminous plasma discharge positions were

130

approximately at 20 mm, 40 mm, and 48 mm for setups (a), (b), and (c), respectively.

The ion saturation current, Iisat for setup (c) dominates for the first 20 μs of the pulse and

the Iisat for setup (b) dominates at t >20 μs. The highest ion saturation currents were found

for setup (b) at a radial position of r = 45 mm at all fixed time. Later, functional films

will be prepared using this plasma source.

131

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134

Chapter 8. Summary and Suggestions for Future Plan

8.1. Summary and Conclusions

Chapter 1 discussed the plasma and its typical features, thin film deposition by

plasma sputtering background research on plasma sputtering problems of the plasma

sputtering, objectives of this Thesis, and structure of this thesis. Different measurement

techniques and methods of the ion saturation currents, plasma density, and electron

temperature, eroded target profile and target utilization rate calculation, resistivity and

thickness of the film were explained. In the chapter 3, a square-shaped magnet

arrangement consisting of eight neodymium rod magnets of 30 × 5 × 3 mm has been

investigated in order to realize a uniform utilization of a copper target in an RF

magnetized sputtering plasma. A copper plate of 160 × 160 × 3 mm and an aluminum

disc of 160 mm in diameter were used as the target to measure the sputtering

characteristics based on rotational square-shaped magnet schemes in the high-density RF

magnetized plasma sputtering source. The magnetic shielding material, iron (Fe) cover,

suppresses the horizontal magnetic flux density |Bx| at x = 0 from approximately 65 to 50

mT. Strong plasma emission in an outward direction is observed at four intersections

between the four vertically placed magnets. It is seen that the RF discharge voltage and

the absolute value of the dc self-bias voltage decrease gradually from approximately 760

to 600 Vp-p and approximately 400 to 270 V, respectively, with increasing argon gas

pressure for all cases. The uniformity of the target erosion depth and ion flux profile has

been improved by using the iron (Fe) cover and decreasing the air gap between the

shielded magnets. The iron (Fe) cover is effective to prohibit the inward E×B drift motion

to the center of the target so that the uniformity of the copper erosion profile is improved

by the addition of the iron cover and reducing the air gap between the one–pair shielded

magnet and the target utilization reaches the highest value of 87.49%.

In the chapter 4, a copper film was deposited using a gyratory square-shaped

magnet arrangement consisting of eight neodymium bar magnets of 30 mm × 5 mm × 3

mm. Two magnetic arrangements were investigated to discuss the film properties: case

(a) without iron shielding and case (b) with iron shielding of 5 mm × 3 mm × 1 mm,

which is used for magnetic shielding. Copper films were prepared at an RF power of 100

W, an Ar pressure of 1.0 Pa, a processing time of 1.5 h, a gyratory speed of 40 rpm, and

a target– substrate distance of 45 mm. The magnetic shielding material reduces the

horizontal magnetic flux density, |Bx|, at x = 0 from ~1120–824 G and the axial

135

component, |Bz|, from ~824–706 G at x = -15 and 15 mm. The RF discharge voltages,

VRF, (peak-to-peak value) were 1080 V and 1160 V, and the absolute values of the self-

bias dc voltages, VSB, of the electrode were 528 V and 573 V for cases (a) and (b),

respectively. A typical star-shaped plasma discharge has been observed. The Cu films for

case (a) exhibit sharp needle-shaped grains vertical to the substrate with a mean grain size

of ~40 nm, and for case (b) the film surface shows an improved smooth surface with a

reduced mean grain size of ~20 nm. The roughnesses of the copper thin films are ~3.73

nm and ~2.49 nm for case (a) and case (b), respectively. An XRD investigation shows

that the FWHM values of the diffraction peak were 0.42° and 0.66° for cases (a) and (b),

respectively and both samples exhibit a face-centered-cubic structure.

In the chapter 5, an outer ring-shaped RF magnetized plasma has been proposed

by three circular monopole magnet arrangement such for (a) R = 5 mm, (b) R = 20 mm

and (c) R = 35 mm It is seen that a high luminous ring-shaped plasma is observed in

contiguity with the wall position of r = 47-50 mm for (a) R = 5 mm, whereas multi-ring

discharges are observed for (b) R = 20 and (c) 35 mm. The tendency of the absolute value

of the DC self-biased voltage |Vsb| of the cathode is almost similar to the pattern in the

discharge voltage |Vrf| corresponding to the three monopole schemes. It is found that the

electron temperature decreases with increasing gas pressure for all cases. However, it is

seen that the plasma density increases with increasing gas pressure for all setups. The

smaller gap distance, the fluctuations of the plasma density and ion saturation current are

more. However, a big gap distance as (c): R = 35 mm, has high ion saturation current with

more fluctuations comparison with case (b): R = 20 mm. So that case (b): R = 20 mm is

effective to improve target utilization from center to the outer area. From a viewpoint of

the center to outer target utilization, case (b): R =20 mm is more effective than other two

cases. However, case (a): R = 5 mm can be used to improve target utilization only in the

outer area, whereas (c): R = 35 mm has the highest ion saturation current and lowest

electron temperature among all cases.

In the chapter 6, a ring-shaped plasma in the outer area is observed at the position

of the peak magnetic flux density found in monopole magnet arrangement and its

diameter depends on a number of magnet circles. The high-density plasma was found in

the outer target region near the chamber wall for all type monopole arrangements. The

plasma discharge results that the target utilization can be controlled effectively in the

outer specific area near the chamber wall. The electron hall parameter was (a): 245, (b):

136

395, and (c): 393 at high luminescent plasma discharge position. It was observed that the

ion saturation current, Iisat are 0.6 mA, 0.79 mA, and 0.46 mA, for setups (a), (b), and (c),

respectively at a radial position of r = 47 mm. The Iisat for setup (a) is the lowest for 0 <

r < 35 mm, whereas for setup (c), Iisat has the highest value among three setups. It is seen

that, Iisat is very high in the outer target region near the chamber wall for setups (a) and

(b), where Iisat for setup (c) decreases slowly. Thus, the proposed CCP plasma source can

be used for the outer target erosion and target utilization in the specific area.

In the chapter 7, A pulsed DC plasma discharge is proposed. The pulse duration

of 20μs, the pulse repetition rate was 2.5 kHz, Ar pressure of 12 Pa current, and the source

voltage to the target is (a): 0.6 kV, (b): 0.6 kV, and (c): 1.1 kV were applied in pulsed DC

discharge plasma source experiments. It was found that the three highest peak values of

|Bx| were 1080 G at r = ±19 mm, 1166 G at r = ±36 mm, 1158 G at r = ±50 mm for the

setups (a), (b), and (c), respectively. It is also seen that the electron larmor radius is much

smaller than the size of the chamber radius, while the argon ion larmor radius has the

maximum value of 108 m, 17.76 m, 17.27 m at r = 0 mm for setups (a), (b), and (c),

respectively. The electron Hall parameters are much larger than one for all magnet setups

except r = 0 and ±16 mm for setup (a), r = 0 and ±31 mm for setup (b), and r = 0 and ±46

mm for setup (c), respectively, whereas all the ions Hall parameters are much less than

one. The peak currents in the plasma were approximately 0.14 A, 0.38 A, and 0.28 A for

setups (a), (b), and (c), respectively, whereas the output impedance of the pulse modulator

was 1.5 kΩ. The high luminous plasma discharge positions were approximately at 20 mm,

40 mm, and 48 mm for setups (a), (b), and (c), respectively. The ion saturation current,

Iisat for setup (c) dominates for the first 20 μs of the pulse and the Iisat for setup (b)

dominates at t >20 μs. The highest ion saturation currents were found for setup (b) at a

radial position of r = 45 mm at all fixed time.

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8.2. Suggestions for Future Works

Based on the present study, the results could be improved via some suggestions.

A comparison may be summarized based on the experimental works between RF

magnetized plasma and Pulsed DC discharge using single-pole magnet setups. Few new

magnet setups can be investigated experimentally to get the target utilization of

approximately 9095% Moreover, Ring-shaped plasma for specific area can be used for

different kind of functional films preparation and the film characteristic may be

summarized. For next step, it is recommended that various functional films can be

deposited by the developed square-shaped and monopole setups plasma source. The

evaluation of the prepared functional film will be analyzed by some useful technical tool

such as surface profiler, four-point probe method, atomic force microscopy (AFM),

scanning force microscopy (SEM), raman spectroscopy, x-ray photoelectron

spectroscopy (SPS), x-ray diffraction (XRD), optical emission spectrometer (OES),

Ultraviolet Visible Near-Infra red (UV-Vis-NIR) spectroscopy and Hall measurement

techniques.

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List of Publications

International Journals

(1) M. A. Hossain, T. Ide, K. Ikari, and Y. Ohtsu, “High-density radio-frequency

magnetized plasma sputtering source with rotational square-shaped arrangement

of rod magnets for uniform target utilization,” Vacuum, vol. 128, pp. 219-225,

Jun. 2016.

(2) M. A. Hossain, Y. Ohtsu, and T. Tabaru, “Performance of a Gyratory Square-

Shaped Capacitive Radio Frequency Discharge Plasma Sputtering Source for

Materials Processing,” Plasma Chem. Plasma Process, vol. 37, pp. 1663–1677,

2017.

(3) Md. Amzad. Hossain and Yasunori Ohtsu, “RF magnetized ring-shaped plasma

for target utilization obtained with circular magnet monopole arrangement,” Jpn.

J. Appl. Phys., vol. 57, 01AA05, 2018.

(4) Md. Amzad. Hossain and Yasunori Ohtsu, “Outer Ring-Shaped Radio

Frequency Magnetized Plasma Source for Target Utilization in Specific Area”,

IEEE Transaction on Plasma Science, Manuscript accepted dated on 14 June,

2018.

(5) M. A. Hossain and Yasunori Ohtsu, Tentative title “Observation of Ring-shaped

magnetized pulsed DC discharge plasma source using single pole magnet setups”,

Prepared manuscript will be submitted in refereed international journal on middle

of August 2018.

139

Conference Proceedings

(1) M. A. Hossain and Y. Ohtsu, “Development of cross-shaped magnetized plasma

by square-shaped magnets and uniform utilization of Cu target”, JSAP annual

spring meeting, March 2016, Tokyo, Japan.

(2) M. A. Hossain and Y. Ohtsu, “Electrical and Structural Properties of Copper Thin

Films Deposited by Novel RF Magnetized Plasma Sputtering with Gyratory

Square-Shaped Arrangement by Bar Permanent Magnets”, 69th Gaseous

Electronics Conference (GEC 2016), October 2016, Bochum, Germany.

(3) M. A. Hossain and Y. Ohtsu, “RF Magnetized Ring-Shaped Plasma for Uniform

Target Utilization by Circular Magnets Monopole Arrangement”, accepted in 9th

International Symposium on Advanced Plasma Science and its Applications for

Nitrides and Nanomaterials/10th International Conference on Plasma-Nano

Technology & Science (ISPlasma2017/IC-PLANTS2017), Nagoya, Japan.

(4) M. A. Hossain and Y. Ohtsu, “Outer Circular Ring-Shaped RF Magnetized

Plasma for Specific Area Target Utilization by Magnetic Monopole

Arrangement”, 44th IEEE International Conference on Plasma Science (ICOPS

2017), May 2017, Atlantic City, New Jersey, USA.

(5) M. A. Hossain and Y. Ohtsu, “Outer Ring-Shaped Magnetized Plasma by RF and

HiPIMS source”, 70th Gaseous Electronics Conference (GEC 2017), November,

2017, Pittsburgh, Pennsylvania, USA.

(6) M. A. Hossain and Y. Ohtsu, “Ring-Shaped Plasma for Target Utilization in

Specific Area by HiPIMS Source”, 39th International Symposium on Dry Process

(DPS2017), November, 2017, Tokyo, Japan.

(7) M. A. Hossain, Y. Nakamura, A. Sugawara, and Y. Ohtsu, “Preparation of

aluminum doped zinc oxide film by radio-frequency magnetized plasma

sputtering source with square-shaped rod magnets”, 19th International Congress

on Plasma Physics (ICPP 2018), June 2018, Vancouver, Canada.