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Synthesis of Thin and Thick Ultra-nanocrystalline Diamond

Films by Microwave Plasma CVD System

by

Dzung Tri Tran

A THESIS

Submitted to
Michigan State University
In partial fulﬁllment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Electrical and Computer Engineering

2005

ABSTRACT

Synthesis of Thin, Thick Ultra-nanocrystalline Diamond Films
by Microwave Plasma CVD System

By

Dzung Tri Tran

Ultrananocrystalline diamond (UNCD) ﬁlms offer a number of valuable properties like
high Young’s modulus, chemical inertness, and low coefﬁcient of friction. These
properties combined with small crystal size and ﬁlm smoothness result in UNCD being
very promising for many applications such as surface acoustic wave (SAW) devices,

coatings for AFM tips, and ﬁlms for Micro-Electro-Mechanical System (MEMS) devices.

The process to grow a variety of thin, thick, or conductive UNCD ﬁlms using a
Microwave Plasma Assisted Chemical Vapor Deposition (MPACVD) System are
investigated. UNCD ﬁlms are deposited over a wide pressure range (60-180 Torr) and
temperature range (400-800 0C). UNCD ﬁlms were grown on Si (100), p-type boron
doped, substrates with thicknesses ranging from 58 nm to greater than 70 pm. The
highest grth rate of 1.12 um/h was achieved at 180 Torr, with gas mixtures of
szArzCH4 = 4:100:2 sccm and 3 kW microwave power. Film surface roughness, as low
as 10 nm, was obtained as measured by AFM Microscope. The conductivity of UNCD
diamond ﬁlms varied with nitrogen ﬂow rate. At 20 sccm ﬂow rate of nitrogen in the gas

mixture, the conductivity of UNCD ﬁlms was found to be 10.3 (Q.cm)".

Dedicated to my loving parents,

Hai V Tran and Muon T Nguyen.

ACKNOWLEDGEMENTS

The author wishes to express his sincere appreciation to Professor Dr. Timothy
Grotjohn for his guidance, encouragement and support for this thesis research. Thanks are
also due the other members of the author’s guidance committee: Professor Dr. J es
Asmussen, Professor Dr. Donnie Reinhard. The author would like to thank Dr. Thomas
Schuelke, Dr. Hans J Scheibe and Claire Rosser for their encouragement and support. In
addition, the author would like to thank Professor Dr. Stanley L. Flegler, Dr. Shirley A.
Owens, and Dr. Carol F legler for training the SEM and F ESEM. The author would like to
thank Dr. Ning Xi and his students: Guang Yong Li, J iangbo Zhang for help me with the
AFM images. The author would also like to thank Mr. Brian Wright and Mrs. Roxanne
Peacock for their technical support. Last the author would like to thank all friends and co-
worker: Stanley Zuo, K.W. Hemawan, Jeffri J Narendra, Jing Wang, Chandra Romel,
Muhammad Ajimal Khan, Muhammad Farhan, Mitchell Parr, Kagan Yaran, Michael

Becker for providing me with their knowledge.

My deepest thanks are extended to my family: Dad, Mom, Sisters and Brothers
for their loves which have brought me up to this point. I would also like to special thank
my wife and son for their loves and support throughout my graduate study at Michigan

State University.

iv

TABLE OF CONTENT

LISTOFTABLES................‘ ................................................................ vii
LIST OF FIGURES .............................................................................. viii
1 Introduction ................................................................................. l
l . 1 Introduction .................................................................................... 1
1 .2 Motivation ...................................................................................... 2
1.3 Research Objectives .......................................................................... 3
1.4 Thesis Outline ................................................................................. 4
2 Background ................................................................................. 6
2.1 General Information ........................................................................... 6 ‘
2.1.1 Historical ................................................................................... 6
2.1.2 Classiﬁcation .............................................................................. 7
2.1.3 Crystal Structure... .......9
2.1.4 Properties of d1amond ... ......10
2.2 Chemical Vapor Deposition of Diamond.. . ......12
221 Introduction... .. .........12
2. 2. 2 The gas phase chemical envnonment ......12
2. 2 2.1 Substrate Temperature... ...14

2.2.2.2 Atomic Hydrogen

.15

2.2.2. 3 Hydrocarbon Chemistry... ..16

2.23 Thegrowth Species... ......17

2. 2. 4 Diamond Surface Chem1stry . . .18

2.3 The Ultra-nanocrystalline Diamond Synthes1s ..... . 19
2.3.1 Carbon Dimer Growth Processes... ... ......19

2. 3. 2 Ultra-nanocrystalline Diamond Re-nucleation Growth ......21

2.4 Ultra-nano Crystalline Diamond Deposition Techniques... ...22

2.4.1 Hot Filament CVD
2.4.2 Microwave Plasma CVD... .
2.4.3 Radio Frequency Plasma CVD ................................................

.22
.24
.27

2.4.4 D.C Art Plasma CVD........... ...........29
2.5 Pre—nucleation Techniques ................................................................................. .30
2.6 Ultra-nano Crystalline Diamond Deposition at Low Temperature . . . . ....36
2.7 Conducting UNCD ﬁlms ................................................................................... .38
3 System Operation/Experimental Method ............................................ 40
3. 1 Introduction ........................................................................................................ .40
3.2 Experimental Systems ........................................................................................ .40
3.2.1 Microwave Power and Wave Guide System ................................................ .41
3.2.2 Transmission System .................................................................................... .44

3.2.3 Vacuum Pump and Gas Flow Control System ............................................. .46

3.2.3.1 Vacuum Pump System ............................................................................ .46
3.2.3.2 Gas Flow Control System ....................................................................... .47

3.2.4 Computer Control System ........................................................................... .51
3.2.5 Microwave Plasma Cavity Reactor .............................................................. .52
3.2.6 Operating Filed Map .................................................................................... .60

3.3 Experimental Procedures ................................................................................... .65
3.3.1 Prepared Sample .......................................................................................... .65
3.3.1.1 Scratch Seeding ..................................................................................... .66

3.3.1.2 Ultrasonic Seeding ................................................................................ .68

3.3.2 Experimental Set-up ...................................................................................... .69
3.3.3 Start-up and Shut-down ................................................................................. .70

4 Experimental Results72
4. 1 Introduction ........................................................................................................ .72
4.2 UNCD Films Growth by Ar/Hz/CH4 ................................................................... .72
4.2.1 Film Morphology .......................................................................................... .74
4.2.2 Film Growth Rates ........................................................................................ .81
4.2.3 Thin and Thick UNCD Films ....................................................................... .83
4.2.4 Young’s Modulus of UNCD Films ............................................................... .91
4.3 UNCD Films Growth by He/Hz/CH4 ................................................................... .93
4.3.1 Film Morphology .......................................................................................... .94
4.3.2 Film Growth Rates ....................................................................................... .102
4.4 UNCD Films Growth by Ar/Nz/CH4 ................................................................. .105
4.4.1 Film Morphology ........................................................................................ .106
4.4.2 Film Growth Rates ...................................................................................... .112
4.4.3 Film Conductivity ...................................................................................... .114

5 UNCD Film Applications...................................... . . . . . . . . . . . . . . ......1 17
5.1 Introduction ....................................................................................................... .117
5.2 UNCD Surface Acoustic Wave (SAW) Devices .............................................. .119
5.3 UNCD Resonators ............................................................................................ .121
5.4 UNCD Tips Coated ........................................................................................... .123
6 Conclusmns . 127
6.1 Introduction ....................................................................................................... .127
6.2 Summary ........................................................................................................... .128
6.2.1 UNCD Films Growth by Ar/Hz/CH4 ........................................................... .128
6.2.2 UNCD Films Growth by He/Hz/CH4 ........................................................... .129
6.2.3 UNCD Films Growth by Ar/Nz/CH4 ........................................................... .130
6.3 Discussion ......................................................................................................... . 13 1

7 Appendix . 133

8 References 137

vi

LIST OF TABLES

Table 2.]: Characteristics of Diamond compares with Silicon and Ga—As .................. 11
Table 2.2: Summary of results for C mole fraction (%) in the diamond ﬁlm . ..18

Table 2.3: Nucleation densities of diamond after various pre-treatment ............ . . . . . . . ..36

Table 3.1: The gas ﬂow range ..................................................................... 50
Table 4.1: Young’s modulus results of UNCD ﬁlm ............................................ 93
Table 5 .1: Characteristics of SAW ﬁlter comparisons (Fujimori 1998) .................... 120
Table A.1: Experiment data for Ar/Hz/CH4 ...................................................... 133

Table A2: Experiment data for He/Hz/CH4135

Table A.3: Experiment data for Ar/Nz/CH4 ...................................................... 136

vii

LIST OF FIGURES

FIGURE 2.1: Diamond Structure ....................................................................................... 9
FIGURE 2.2: Hexagonal Graphite................. .................................................................... 10
FIGURE 2.3: Schematic of processes occurring during growing CVD diamond

[Butler1993] ................................................................................................ 13
FIGURE 2.4: Film growth rate versus substrate temperature (COz/CH4, 50/50%)

[Pether 2002] ........................................................................ 14
FIGURE 2.5: Hot-ﬁlament system diagam ....................................................................... 23
FIGURE 2.6: Microwave plasma CVD diagram ............................................................... 24
FIGURE 2.7: Radio-frequency plasma CVD diagram ...................................................... 28
FIGURE 2.8: DC Art Jet plasma CVD diagram ............................................................... 29
FIGURE 2.9: SEM pictures of ﬁlm deposited after 01 hours using different powder ..... 32

FIGURE 2.10: SEM pictures of ﬁlm deposited after 10 hours using different powder....33

FIGURE 2.11: SEM morphology of ﬁlm using BEN seeding method ............................. 33
FIGURE 2.12: SEM morphology of ﬁlm using different seeding method ........................ 34
FIGURE 2.13: SEM morphology of ﬁlm using tungsten seeding method ....................... 35
FIGURE 2.14: SEM morphology of UNCD ﬁlms with substrate temperature at 400°C
and 800°C .................................................................................................. 37
FIGURE 2.15: Low temperature UNCD coating for bio-MEMS application ................... 38
FIGURE 2.16: Surface morphology of conducting diamond ﬁlm 1% and 20% N2 .......... 39
FIGURE 2.17: Surface morphology of conducting diamond ﬁlm .................................... 39
FIGURE 3.1: MSU-Microwave Plasma Assist CVD System ........................................... 41
FIGURE 3.2: Control Board of Microwave Generator model S6F .................................. 42

viii

FIGURE 3.3: Microwave Plasma Assisted CVD System Operating ............................... 42

 

 

 

 

 

FIGURE 3.4: Microwave power and Wave Guide System .............................................. 43
FIGURE 3.5: Transmission System ................................................................................... 45
FIGURE 3.6: The Vacuum and Nitrogen purge System ................................................... 46
FIGURE 3.7: The Gas Flow Control System .................................................................... 49
FIGURE 3.8: Computer Control System Diagram ........................................................... 51
FIGURE 3.9: Microwave Cavity Plasma Reactor ............................................................. 54
FIGURE 3.10: Cavity Applicator ..................................................................................... 55
FIGURE 3.11: Substrate holder assembly ......................................................................... 56
FIGURE 3.12 a: The screen viewing window. 57
FIGURE 3.12 b: Ar-HTCH4 gas mixture plasma ................................................... 57
FIGURE 3.12 c: He-Hz-CH4 gas mixture plasma ..... - - 60
FIGURE 3.12 d: Ar-Nz-CH4 gas mixture plasma 58
FIGURE 3.13: The chiller Neslab model CFT300 ............................................................ 59
FIGURE 3.14: The water ﬂow indicator ........................................................................... 60
FIGURE 3.15: The MPACVD operating ﬁeld map/Ar-Hz-CH4 = 100-4-1 (sccm) .......... 61

FIGURE 3.16: The MPACVD operating ﬁeld map/He-Hz-CH4 = 100-4-1 (sccm) .......... 62
FIGURE 3.17: The MPACVD operating ﬁeld map/Ar-Nz-CH4 = 100-4-1 (sccm) .......... 63
FIGURE 3.18: Substrate temperature ................................................................................ 64
FIGURE 3.19: Prepare for scratching method ...................................................................... 66
FIGURE 3.20: Substrate surface using scratch seeding method ...................................... 68
FIGURE 3.21: Substrate surface using Ultrasonic method .............................................. 69

FIGURE 4.1 a: Film Morphology (AFM)
Pressure =120 Torr, gas mixtures, Ar:CH4:H2 =100:l :lsccm , deposition time = 8 hrs...73

FIGURE 4.1 b: Film Morphology (AFM) _
Pressure =120 Torr, gas mixtures, ArzCH4zH2 =100:1223ccm , deposition time = 8 hrs...74

FIGURE 4.1 c: Film Morphology (AF M)
Pressure =120 Torr, gas mixtures, ArzCH4:H2 =100:1:4sccm , deposition time = 8 hrs...75

FIGURE 4.1 d: Surface Roughness (AFM)
Pressure =120 Torr, gas mixtures, ArzCH4zH2 =100:1213ccm , deposition time = 8 hrs...76

FIGURE 4.1 e: Surface Rouglmess (AFM)
Pressure =120 Torr, gas mixtures, ArzCH4zH2 =100:1223ccm , deposition time = 8 hrs...77

FIGURE 4.1 f: Surface Roughness (AFM)
Pressure =120 Torr, gas mixtures, ArzCH4zH2 =100:1z4sccm , deposition time = 8 hrs...78

FIGURE 4.1 g: Surface Roughness vs Hydrogen Flow Rate
Ar:CH4:H2 =100:1z1-43ccm , deposition time = 8 hrs ....................................................... 79

FIGURE 4.1 g: Surface Roughness vs Film Thickness
Ar:CI-I4:H2 =100:1-2zl-45ccm , deposition time = 8 hrs .................................................... 80

FIGURE 4.2 a: Grow Rate vs Pressure
ArzCH4zH2 =100:1:1-4sccm , deposition time = 8 hrs ....................................................... 81

FIGURE 4.2 b: Grow Rate vs Methane Flow Rate
Ar:CH4:H2 =100:1-224sccm .............................................................................................. 81

FIGURE 4.2 c: Grow Rate vs Substrate Temperature
Ar:CH4:H2 =100:1-221-48ccm ........................................................................................... 83

FIGURE 4.3 a: Thin Film Morphology (less than 50 nm)
Ar:CH4:H2 =100:111 sccm , deposition time = 75 minutes ................................................ 84

FIGURE 4.3 b: Thin Film Morphology (58 nm)
ArzCH4zH2 =100:1:1 sccm , deposition time = 1 hr ........................................................... 85

FIGURE 4.3 c: Thin Film Morphology (61.2 nm)
Ar:CH4:H2 =100:1 :1 sccm , deposition time = 1.25 hrs ................................................ 86

FIGURE 4.3 (1: Thick Film Morphology (56 um)
Ar:CH4:H2 =100:1.5:4 socm , deposition time = 52 hrs ................................................... 87

FIGURE 4.3 e: Thick Film Morphology (72.3 mm)
ArzCH4zH2 =100:2:4 sccm , deposition time = 65 hrs ...................................................... 88

FIGURE 4.3 f: Thin Film Morphology (331 nm)

ArzCH4zH2 =100:1:1 sccm , RMS = 12.12 nm ................................................................. 89

FIGURE 4.3 g: Thick Film Morphology (72.3 mm)

Ar:CH4:H2 =100:2:4 sccm , RMS = 60.88 nm ................................................................. 89
FIGURE 4.3 h: Thick Film Morphology (56 um)

Ar:CH4:H2 =100:1.5:4 sccm , RMS = 11.8 nm (in the back side ...................................... 90
FIGURE 4.3 k: Thick Film Morphology (56 um)

Ar:CH4:H2 =100:1.5:4 sccm , RMS = 50.46 nm .............................................................. 90
FIGURE 4.3 m: The UNCD Film Thickness = 72.3 mm (SEM image) ............................ 91
FIGURE 4.4: Fraunhofer’s LAwave Instrument ................................................ 92

FIGURE 4.5 3: Thin Film Morphology (SEM image)
He:CH4:H2 =100:1:1 sccm , deposition time = 8 hrs ......................................................... 94

FIGURE 4.5 b: Thin Film Morphology (SEM image)
HezCH4:H2 =100:1:2 sccm , deposition time = 8 hrs ......................................................... 95

FIGURE 4.5 c: Thin Film Morphology (SEM image)
HezCH4zH2 =100:] :4 seem , deposition time = 8 hrs ......................................................... 96

FIGURE 4.5 (1: Film Surface Morphology (AF M image)
He:CH4:H2 =100:1:1 sccm ................................................................................................ 97

FIGURE 4.5 e: Film Surface Morphology (AF M image)
He:CH4:H2 =100:] :2 seem ................................................................................................ 98

FIGURE 4.5 f: Film Surface Morphology (AFM image)
He:CH4:H2 =100:] :4 seem ................................................................................................ 99

FIGURE 4.6 a: Film Surface Roughness (AF M image)
He:CH4:H2 =100:1:1 seem .............................................................................................. 100

FIGURE 4.6 b: Film Surface Roughness (AFM image)
HezCH42H2 =100:1:2 sccm .............................................................................................. 100

FIGURE 4.6 c: Film Surface Roughness (AFM image)
He:CH4:H2 =100:] :4 seem .............................................................................................. 101

FIGURE 4.6 (1: Film Surface Roughness vs Hydrogen Flow Rate
He:CH4:H2 =100:1:1-4 sccm .......................................................................................... 102

FIGURE 4.7 a: Film Growth Rate vs Hydrogen

xi

HezCH4zH2 =100:1:1-4 sccm .......................................................................................... 103

FIGURE 4.7 b: Film Growth Rate vs Substrate Temperature

He2CH4:H2 =50:3:50 sccm .............................................................................................. 104
FIGURE 4.7 0: Film Growth Rate vs Pressure

He:CH4:H2 =50:3:50 sccm .............................................................................................. 105
FIGURE 4.8 a: Thin Film Morphology (SEM image)

Ar2CH4zN2 =100:1 :1 sccm .............................................................................................. 106
FIGURE 4.8 b: Thin Film Morphology (SEM image)

Ar:CH4:N2 =100:1 :2 sccm .............................................................................................. 107
FIGURE 4.8 c: Thin Film Morphology (SEM image)

Ar:CH4:N2 =100:1:10 sccm , ........................................................................................... 107
FIGURE 4.9 a: Film Surface Roughness (AF M image)

Ar:CH4:N2 =100:1:1 sccm .............................................................................................. 108
FIGURE 4.9 b: Film Surface Roughness (AF M image)

Ar:CH4:N2 =100:1 :2 seem .............................................................................................. 109
FIGURE 4.9 c: Film Surface Roughness (AF M image)

Ar:CH4:N2 =100:1:10 sccm ............................................................................................ 110
FIGURE 4.9 (1: Film Surface Roughness (AFM image)

Ar:CH4:N2 =100:1 :20 sccm ............................................................................................ 111
FIGURE 4.9 e: Film Surface Roughness vs Nitogen F low Rate

Ar2CH4zN2 =100:1:1-20 sccm ......................................................................................... 112
FIGURE 4.10: Film Growth Rate vs Nitrogen Flow Rate

ArzCH42N2 =100:1:1-2O sccm ......................................................................................... 13
FIGURE 4.11: Schematic of four point probes ................................................ 1 14
FIGURE 4.12: Film Conductivity vs Nitrogen Flow Rate

Ar:CH4:N2 =100:1:1-20 sccm ......................................................................................... 115
FIGURE 4.13: Substrate Temperature vs Nitrogen Flow Rate

Ar:CH4:N2 =100:1:1-20 sccm ......................................................................................... 116
FIGURE 5.1: UNCD free standing ﬁlm .......................................................................... 117
FIGURE 5 .2: Structure of surface acoustic wave (SAW) devices .................................. 121

xii

FIGURE 5.3: Structure of ultra high frequency MEMS devices .................................... 122

FIGURE 5.4: Atomic force microscope ........................................................................... 124
FIGURE 5.5: Atomic force microscope cantilever .......................................................... 125
FIGURE 5.6: The silicon AF M Tip ................................................................................. 125
FIGURE 5.7: The AF M Tips coated UNCD ................................................................... 126
FIGURE 5.8: The AFM Tips coated UNCD ................................................................... 126

“Images in this thesis are presented in color,”

xiii

Chapter 01: Introduction

1.1 Introduction

Ultra-nano crystalline diamond (UNCD) thin ﬁlms have many superior properties and are
promising for many applications requiring smooth surfaces. There are various methods to
grow nano-crystalline diamond ﬁlm including hot ﬁlament chemical vapor deposition
(HFCVD), plasma torch, microwave plasma assisted chemical vapor deposition
(MPACVD), etc. The microwave plasma assisted CVD system has a number of

advantages compared with other methods.

The MPCVD system can produce smooth thin ﬁlms with large area and in a repeatable
manner. Huang [Huang 2004] reported on the growth of nano-crystalline diamond across
a wide range of pressure and power by microwave plasma assisted CVD with Ar-Hz-CH4
gas mixtures. Those experimental results established the relationship between input
variables and the resulting diamond thin ﬁlms. In order to better understand the growth of
nano-crystalline diamond thin ﬁlms, more research needs to be conducted on the

microwave plasma assisted CVD system.

1.2 Motivation

Synthesized nano-crystalline diamond ﬁlms have drawn increased attention in recent

years. The nano-crystalline diamond ﬁlms, with very small crystals size and a smooth

surface, are preferred in many applications. The thickness of the diamond ﬁlm is an
critical application dependent parameter. Some applications require very thin diamond
ﬁlms, while others need thicker ﬁlms. The development of processes to synthesis nano-
crystalline diamond ﬁlm with various thicknesses by microwave plasma assisted CVD is
reported in this thesis. Three different gas mixtures including Ar-Hz-CH4, He— Hz-CH4
and Ar- Nz-CH4 are used to grow nano-crystalline diamond in this investigation. The
process of synthesizing thin and thick nano-crystalline diamond ﬁlms using pressures

from 60 to 180 Torr is also carried out in this investigation.

The nano-crystalline diamond ﬁlm experiments are investigated with Ar-Hz-CH4 and
He-Hz-CH4 gas mixtures. Argon and helium are both noble gas. Most of research
reported in the literature used argon in the Hz-CH4 gas mixture to grow nano-crystalline
diamond. The synthetic process of nano-crystalline diamond using the noble gas helium
still is open area of investigation. A part of this thesis will research on conducting nano-
crystalline diamond ﬁlm. By adding nitrogen into gas mixture, the characteristic of

diamond ﬁlms change to become electrically conducting.

The nano-crystalline diamond synthesis investigated in this thesis is directed toward three
applications. First the potential application for diamond thin ﬁlms as coatings atomic
force microscope (AFM). The tips made by silicon or gold will be worn out very fast
when working on a hard surface. With nano-crystalline coating, the tips will have a

longer useful life.

A second potential application for thick nano-crystalline diamond ﬁlms is surface
acoustic wave (SAW) devices [B1 2000]. Since the polishing of rough polycrystalline
diamond surfaces is very difﬁcult, nano-crystalline diamond ﬁlm, with their very smooth

surface, is more favorable material for the SAW device application.

A third potential application for conducting nano-crystalline diamond is for resonator
devices. Resonators are a key component in micro electro mechanical structure (MEMS)
devices. Resonators are actuated, usually electro statically, to oscillate at their natural
resonant frequency. The nano-crystalline diamond ﬁlms provide the highest resonance

frequencies of any materials since it has a very high Young’s modulus (E >800 GPa).

1.3 Research Objectives

The research objectives of this thesis included:

(1) Develop the process and methodology to synthesis nano-crystalline diamond
thin ﬁlms.

(2) Develop the process and methodology to synthesis nano-crystalline diamond
thick ﬁlms.

(3) Develop the process and methodology to synthesis nano-crystalline diamond
conducting ﬁlms.

(4) Characterize the nano-crystalline diamond quality ﬁlms deposited over a

wide range of pressures and gas mixtures.

(5) Establish the relationship of how the input variables (pressure, power, gas
ﬂow rate...) affect to the growth of nano-crystalline diamond ﬁlms.

(6) Establish the process to grow name-crystalline diamond using the noble gas
argon and helium.

(7) Establish the reactor condition to deposit conducting nano-crystalline
diamond with nitrogen gas.

(8) Investigate the properties of grown nano—crystalline diamond ﬁlms for three
applications including (a) AF M tip coatings; (b) surface acoustic wave
(SAW) devices and (c) ultra high frequency micromechanical (UHF -MEMS)

resonators.

1.4 Thesis Outline

Chapter One is an introduction with general information and objectives of this thesis.

Chapter Two presents history, background and related literature of diamond ﬁlms
synthesis and nano-crystalline diamond ﬁlms in particularly. The nucleation techniques,
the methods to grow nano-crystalline diamond ﬁlms at substrate low temperature, and
methods to grow conducting nano-crystalline diamond ﬁlms will be also introduced in

this chapter.

Chapter Three describes the system operation and experimental methods. The microwave

plasma assisted CVD system and procedures to grow nano-crystalline diamond are

introduced. The reactor’s operating ﬁeld map for three different gas mixtures ( Ar-Hz-

CH4, He-Hz-CH4 and Ar-Nz-CH4) will also presented.

Chapter Four summarize the experiment results of this investigation. The surface
morphology, growth rate, uniformity and conductivity of nano-crystalline diamond ﬁlms

are studied in this chapter.

Chapter Five presents the applications of nano-crystalline diamond including (a) surface
acoustic wave devices, AFM tip coatings and ultra high frequency micromechanical

resonator devices.

Chapter Six summarizes the thesis and present recommendations for further development

of growth nano-crystalline diamond ﬁlms using microwave plasma assisted CVD system.

Chapter 02: Background

2.1 General information
2.1.1 Historical

The natural diamond was found by the fourth century BC in India. It is considered to be
of the highest value among precious stones. Because diamond has many outstanding
properties compared with others material, it also drew a lot of attention from scientists.

Synthetic diamond was developed in the last forties year with various methods.

In the early 19508, the process of high pressure high temperature (HPHT) synthetic
diamond was invented by General Electric. In1954, W. Eversole of Union Carbide Crop
in the United State proved that diamond showed homoepitaxial growth from carbon-
bearing gas under low pressure by the chemical vapor deposition (CVD) method

[Eversole 1962].

In 1956, B.V. Deryagin and B.V. Spitsyn synthesized diamond by the CVD method in
Russia. In these methods, the growth rate of diamond was extremely low, and graphite
also grew simultaneously with diamond so that in each case the chemical reaction of the

process had to be suspended [Spitsyn 1981].

In the early 19703, atomic hydrogen was used during the growth phase in CVD method.
In 1975, a high growth rate CVD process was announced by Deryagin’s laboratory in
Russia and also N. Setaka’s group. This is a signiﬁcant achievement as growth had only

previously been possible on diamond substrates.

In 1981, Matsumoto of NIRIM (National Institute for Research in Inorganic Materials),
Japan, made a breakthrough in diamond synthesis by developing the hot ﬁlament CVD
method, followed by development of a microwave CVD method by Kamo of NIRIM
[Matsumoto 1982]. This method drew attention, as the experiment was reproducible and
could produce crystals of good quality. Over the past two decades many researchers have

worked to signiﬁcantly advance the growth of diamond

As chips have shrunk over the years, engineers have struggled with ways of dissipating
the heat they create. Because silicon, the main component of semiconductors, suffers
electrical breaks down, some experts believe a new material will he need in the future.
Diamonds might ﬁt the bill. Diamond can withstand 500 OC; electrons and holes move

through diamond with high mobility more easily at increased temperature of 100-200 0C.

Engineers could cram a lot more circuits onto a diamond-based microchip if they could
perfect a way of making pure crystals cheaply. Chemical vapor deposition diamond
technique became available in the form of extended thin ﬁlms and free-standing plates or

windows. With CVD-diamond a huge of new applications opened up.
2.1.2 Classiﬁcations

There are four known types of natural diamond (Ia, Ib, Ila, IIb), classiﬁed according to

the presence of nitrogen in the crystal, and certain other properties.

Type I diamonds have nitrogen atoms as the main impurity. If the nitrogen is localized in
clusters it does not affect the diamond's color (Type Ia). If it is dispersed throughout the

crystal, it gives the stone a yellow tint (Type Ib). Typically a natural diamond crystal

contains both Type la and Type lb material. Synthetic diamonds that contain nitrogen are

Type lb.

Type II diamonds have no nitrogen impurities. They contain either no or other impurities.
Those containing no impurities are Type Ila and are colored clear pink, red or brown. The
color arises by structural anomalies from plastic deformation. Type IIb are the natural

blue diamonds which contain scattered boron within the crystal mat1ix.

Synthetic diamonds can also be categorized according to this scheme. However there can

still be a wide variation in some properties between diamonds of the same type.

Almost all synthetics are of type lb, having an even distribution of nitrogen atoms
substituted for carbon atoms in the lattice (up to about 500ppm). It is believed that in the

earlier stages of their history, all natural diamonds were type Ib

Most natural diamonds (~99.9%) are of type Ia, with a large amount of nitrogen
concentrated in various aggregates in the crystal. The initially type Ib diamonds are
considered to have changed to type Ia after many years in a HPHT (High pressure high

temperature) environment, in which the nitrogen diffused and coalesced into aggregates.

Types Ila and IIb are very rare in nature but can be synthesized for industrial purposes.
Natural diamonds consisting of several different types in one stone are sometimes seen.
Diamonds occur in a variety of colors - steel, white, blue, yellow, orange, red, green,
pink, brown and black. Colored diamonds contain impurities or defects that cause the

coloration, whilst pure diamonds are always transparent and colorless.

Diamond is an insulator, but due to the fact that it contains defects and impurities, it can

behave like a semi-conductor, which makes it useful for several electronic applications.

2.1.3 Crystal Structure

 

Figure 2.1: Diamond Structure

Diamonds typically crystallize in the cubic crystal system and consist of tetrahedrally
bonded carbon atoms. The diamond cubic crystal structure (Figure 2.1) consists of two
interpenetrating face-center cubic (FCC) lattices, displaced from each other by one
quarter of the body diagonal. Each carbon atom is tetrahedrally coordinated (using sp3
atomic orbital), creating strong, directed sigma bonds with its four neighboring carbon

atoms. The bond length and lattice constant are 1.54 and 3.56 angstroms, respectively.

Graphite is the most common form of carbon. In graphite, each carbon atom is covalently
bonded to three carbon atoms to give trigonal geometry. Bond angle in graphite is 120°.
Each in—plane carbon atom combines with its three neighbors using hybrid sp2 atomic

orbits, with a covalent 6 bond length of 1.42 angstroms. The repeating layers are pi

bonds, perpendicular to the plans with a 3.35 angstrom lattice constant. Three out of four
valence electrons of each carbon atom are used in bond formation with three other carbon

atoms while the fourth electron is free to move in the structure of graphite (Figure 2.2).

 

Figure 2.2: Hexagonal Graphite

Diamond is carbon in its most concentrated form. Diamond is distinctly different from
the common graphite which is also composed of carbon. Diamond's particular
arrangement of carbon atoms, or its crystal structure, the feature that deﬁnes any

fundamental properties.

2.1.4 Properties of diamond

Diamond has many superior properties compare with other materials such as extreme
hardness, high thermal conductivity, high breakdown voltage, large band gap and high
electrons and holes mobility (Table 2.1). Therefore diamond has many potential

applications in industry and MEM device.

10

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Diamond Si Ga-As CVD
Band Gap [eV] 5.45 1.12 1.43 5.5
Breakdown Field [V/cm] 107 3 x 105 4 x 105 107
Resistivity [9 cm] 10‘3— 1016 1.5 x 105 108 108
Electron Mobility [ cm2/V.s] 1900-2200 1350 8500 1350—1500
Hole Mobility [ cm2/V.s] 1600 480 400 1000
Saturation Velocity [Km/s] 220 82 80 220
Mass Density [g/cm3] 3.51 2.33 5.23 3.51
Atomic charge [C] 6 14 31 6
Dialectric Constant 5.7 1 1.9 13.1 5.6
Optical Transparency UV to Visible to
microwave mid-[R
Thermal Conductivity [W/cm.K] 20-23 1.5 0.5 10-21
Thermal Expension Coeff
[K"] 0.8 x 10'6 2.6 x 10'6 5.8 x 10'6 2.0 x 10'6
Hardness [ Kg/mm2] 10,000 1,000 600 10,000

 

 

 

Table 2.1: Characteristics of Diamond compares with Silicon and Ga—As

ll

Ultra-Nanocrystalline Diamond (UNCD), a form of industrial diamond in which the grain
size is in the range of several tens to hundreds of nanometers [Reinhard et a1. 2004],
captures many of the best properties of natural diamond in thin ﬁlm form. UNCD has
unique properties not found in any other carbon-based material. UNCD is currently being
evaluated for a wide variety of applications including MEMS (RF & Optical-MEMS,
BioMEMS), cold-cathode electron sources, chemical process pump seals,

bioelectrochemical electrodes, and others.

2.2 Chemical Vapor Deposition of Diamond

2.2.1 Introduction

The complex chemical processes occurring in chemical vapor deposition of diamond are
fascinating and have been studied by many scientists. How does one understand the
process of growing diamond in chemical vapor deposition? This section describes a
model for understanding the complex gas phase chemistry, environment, surface and bulk

chemical in CVD process to growth diamond (Figure 2.3).

2.2.2 The gas-phase chemical environment

Many studies of the gas-phase chemistry, during diamond chemical vapor deposition, in
the past two decades [Celii 1989], [Harris 1989], [Corat 1993], [McMaster 1995]. Most

studies, both experimental and computational, discuss the CVD environment with respect

12

to substrate temperature, atomic hydrogen concentration, hydrocarbon chemistry,

deposition uniformity etc...

 

Reactants

 

 

 

 

 

 

H2+CH4

 

 

 

   
    
 

Activation

CH4+ H _’ CH3+H2

Flow and Reaction

-l

 

 

 

l A Diffusion A l

 

 

 

 

 

 

 

 

 

 

 

SUBSTRATE f5

 

Figure 2.3: Schematic of processes occurring during growing CVD

diamond [Butler 1993]

13

2.2.2.1 Substrate temperature

In hot ﬁlament and microwave system, the range of substrate temperature for diamond
deposition is from 400-11000C with the more typical values being 700-10000C [Bachm
1991], [Zhu 1991], [Buttle 1998]. This range of substrate temperature allow various
surface phenomena to occur including various adsorption, de—sorption and abstraction
reactions to occur that lead to diamond growth [Grotjohn 2001]. If the substrate
temperature becomes too high, the diamond will be converting to graphite. In microwave
plasma systems, a heating or cooling device is sometime integrated within the substrate
holders to ensure the proper substrate temperature. Substrate temperature plays an
important role to improve the growth rate. Figure 2.4 shows the relation between growth
rates versus substrate temperature. Film growth rates are seen to increase as the substrate

temperature is increased.

01%)-

Growth Rate/ pm h'1

 

 

.0 .0 .0 .o .o
8 -¥ N (a) b
O O O C
l l l 1

§ .

 

500 600 700 800 900

Substrate Temperature / °C

Figure 2.4: Film growth rate versus substrate temperature

(COz/CH4, 50/50%) [Pether 2002].

14

2.2.2.2 Atomic Hydrogen

Two types of radical are needed in the growth of CVD diamond including atomic
hydrogen and carbon-containing growth species. First, atomic hydrogen is needed so that
the surface is almost entirely covered with hydrogen to permit growth in the diamond
phase of carbon and not the graphite phase [Grotjohn 2001]. Second appropriate carbon-
containing growth species must be supplied to the growth surface. Atomic hydrogen is

the most critical determinant of diamond ﬁlm quality and growth rates.

In hot-ﬁlament systems, atomic hydrogen is produced heterogeneously by thermal
decomposition of H2 on the hot ﬁlament surface. The atomic hydrogen produced diffuses
rapidly away from the ﬁlament, resulting in a concentration proﬁle near the ﬁlament. As
the hydrocarbon content of the gas is increase beyond a critical value, the H
concentration drops because of graphite covers the ﬁlament [Celii 1989]. This critical
hydrocarbon fraction corresponds closely to the solubility limit of carbon in hydrogen

[Somm 1990].

In plasma-enhanced systems such as microwave, RF and DC are jet reactors, H is
produced homogeneously in the plasma. The external energy input couples directly to the
free electrons in the plasma, which can dissociate the hydrogen via the reaction.

H2+ e' H+H+e’

————>
This reaction often proceeds through successive vibration excitation of H2 by electron

impact, leading ﬁnally to dissociation. In general the new hydrogen is dissociated by

15

either electron impact or thermal dissociation. Typically, the lower pressure discharge
(less than few tens of torr) has lower gas temperatures and the electron impact
dissociation process dominates. The higher pressure (greater than few ten of torr) has
higher gas temperatures (greater than 2000K) and thus thermal dissociation of hydrogen

is the dominant dissociation process.

2.2.2.3 Hydrocarbon Chemistry

There are two dominant carbon-containing radicals important to diamond growth: CH3
and C2H2 [Goodwin 1998]. The methyl radical CH3 is generally acknowledges being the
most important species for diamond growth. It is oﬁen obtained from the methane (CH4)
input. feed gas. The CH4 chemical reactions in the typical diamond deposition
environment primarily occur as neutral-neutral chemistry rather than via electron
collision process, which are more important for the hydrogen dissociation. In microwave
plasma CVD reactor system, the hydrocarbon chemistry reactions occur on a time scale
much shorter than the residence time of deposition gas in the discharge chamber. Hence,
the hydrocarbon chemistry typically reaches an equilibrium condition for the discharge

gas temperature and atomic hydrogen concentration found in the discharge.

The CH3’s concentration depends on substrate temperature [McMaster 1994]. Below
1000 K the substrate temperature dependence of the CH3 mole fraction can be described
by activation energy of 3-4 kcal/mole. The explanation for this effect is recombination of

CH; with H2 to form CH4 or CH3 with another CH3 to form C2H6, in the cool gas layer

16

near the substrate [Corat 1993]. In general the Cl radicals are most often postulated to be
important for polycrystalline diamond growth using H2/CH4 mixture. Altemately, the C2
radical is the key growth species for nano-crystalline diamond growth using Ar/H2/CH4
mixtures [Grue 1995]. Species with three (C3) or more carbon atoms are generally not

important for diamond growth [Frenk 1989].

2.2.3 The growth species

The question of which carbon-bearing gas phase species is the dominate species for the
growth of diamond ﬁlm has been of great interest, both from academic and process
optimisation standpoints. For the reactor designer, it is important to identify the “growth
species” to maximize its concentration. The observation that MPCVD and HFCVD both
deliver diamond ﬁlms at similar growth rates indicates that the diamond growth precursor

is likely to be a neutral species.

The modelling of gas phase reaction kinetics and consideration of measured diamond
growth rates suggests that CH3 and C2H2 are possible diamond precursor species [Harris
1988]. Table 2.2 shows the results of isotope labelling studies in which a 2:1 mixture of
'3 C methane and 12C acetylene was introduced into a HFCVD reactor in such a way as to
minimise isotopic scrambling between the two species [D’Evelyn 1992]. The '3 C mole
fractions of the resulting (homoepitaxial and polycrystalline) ﬁlms were determined by
Raman spectroscopy and were found to be very similar to those of the input CH4. It was

assumed that CH4 and CH3 were in equilibrium; therefore CH3 was concluded to be the

17

diamond precursor species. Similar results were obtained by Johnson [Johnson 1992]

using a cavity plasma reactor, as shown in Table 2.2.

 

 

 

 

 

 

 

13C Mole Fraction (%)
CVD Method Film Type Film CH4 C2H2
Hot-ﬁlament Polycrystalline 58.2 i 3.6 61.6 i 5.5 32.4 i 5.6
Hot-ﬁlament Homoepitaxial 56.8 i 1.2 58.6 i 5.4 34.9 i 5.3
Microwave Polycrystalline 77 83 29

 

 

 

 

 

Table 2.2: Summary of results for C mole fraction (%) in the diamond ﬁlm, CH4, and

C2H2 for Hot-Filament [D’Evelyn 1992] and Microwave Plasma CVD [Johnson 1992].

Lee carried out a set of experiments in which jets of: (1) CH3 and H2; (2) CH3 and H; (3)
C2H2 and H; were directed at diamond seed crystals [Lee 1994]. Epitaxial diamond
growth on these crystals was only observed for incident jets of CH3 and H, whereas
replacing CH3 with C2H2 resulted in a largely graphitic deposit. This study and a number
of others involving a wide variety of CVD diamond deposition methods [Celii 1992]
have concluded that CH3 is the major diamond precursor in low-pressure low-power
diamond CVD reactors (e. g. HF, MPCVD and ﬂame CVD). However, it should be noted

that C2, or C atoms may be the dominant growth species at higher powers (1'. e. >5 kW) in,

for example, a DC arc jet reactor [Yu 1994].

2.2.4 Diamond Surface Chemistry

One of the most important aspects of diamond surface chemistry is the reaction between
atomic hydrogen and the diamond surface. During growth of diamond, the atomic

hydrogen bombards the diamond surface continuously, so most of the diamond surface is

18

hydrogenated, and therefore non-reactive with incoming hydrocarbon species. The
fraction of surface sites which are not hydrogenated (the open site fraction f') is
determined by a dynamic equilibrium between the two reactions

CdH+H ——> c’d +112
And

C} + H ———> CdH
Where CdH represents a hydrogen-terminated surface site and Cid an equivalent site

without hydrogen.

The nature of OH bonding on hydrogenated diamond surface was studied by Thoms
[Thorns 1995], Struck [Struck 1993] and McGonigal [McGo 1995]. These studied show
that the (100), (110), and (111) surfaces are mainly covered by the monohydride (CH)

species.

During diamond growth, the diamond surface is nearly fully saturated with hydrogen.
This hydrogen coverage limits the availability of sites where hydrocarbon species may
chemisorbs, and blocks migration sites once they are adsorbed. The temperature range
between 600-1200 0C is well above the temperatures at which physisorbed species
desorbs [Zang 1988].

2.3 The ultra-nanocrystalline Diamond Synthesis

2.3.1 Carbon Dimer Growth Processes

19

Nanocrystalline diamond ﬁlms (UNCD) posses very ﬁne grains (with the crystal sizes on
the order of nanometers) and a very smooth surface as compare to microcrystalline ﬁlms.
UNCD can be grown using Ar/CH4 in a lower hydrogen concentration enviromnent Several
research groups have employed spectroscopic techniques to measure the predominant
species present during growth as a function of process parameters, especially under
conditions of high argon concentration, typically Ar/H2/CH4 gas mixtures. Correlation
between materials grown and species present in the growth environment is an important

step in identifying the primary growth species.

As H2 input is reduced and replaced by Ar, C2 emission is greatly increased. This
increase in C2 emission is interpreted as being due to the increased C2 ground state popu-
lation [Gruen 1995]. This rise in C2 population is correlated with the observed increase in
growth rate, and thus supports that C2 is a growth species for UNCD synthesis. Thus,
dicarbon (C2) is believed to be the key growth species [Gruen 1995] for nano-crystalline diamond
growth instead of methyl (CH3) and acetylene (C2H2), which are believed to be the

important species in traditional CH4/H2 polycrystalline diamond growth.

The nano-crystalline diamond growth process is described as follows:

a) One C2 adds to the reconstructed monohydride surface by inserting itself into ﬁrst one C-H
surface bond without abstraction of the terminating hydrogen bond (step 1).

b) The C2 molecule then rotates about the newly formed bond to insert its other carbon into
the C-H bond across from it, thus forming a (100)-oriented surface dimmer row (step 2),

producing an adsorbed ethylene-like structure.

20

c) A subsequent C2 molecule then inserts itself into the adjacent surface C-H bond,

parallel to the newly inserted surface C2 dimmer, to produce a surface with two adjacent
ethylene-like (steps 3 and 4).

d) The original state of the (110) surface is ﬁnally recovered by the formation of a C-C
single bond between adjacent ethylene-like groups (step 5) and thus producing a new layer
on the diamond surface. This direct insertion growth mechanism for C2 is unique in that it is
not dependent on the abstraction of hydrogen atoms from the surface. Specially, the path for
the formation of a C-C single bond between adsorbed, two-carbon moieties via step 5 does

not involve any gas-phase atomic hydrogen.

In 1999, BM. Gruen [Gruen 1999] reported: (1) the reaction of a singlet C2 with the C=C double
bond of the C9H12 cluster gives carbene structures, which lead to the formation of new
diamond critical nuclei during growth, (2) on the other hand, the reaction of singlet C2 with
the HC-CH single bond or C-H bonds of the C9H14 cluster results in a cyclobutene-like
geometry, which leads to growth on the (100) surface in a series of steps, (3) the nucleation
rates increase dramatically under conditions where small fractions of the reconstructed
(100) surface are un-hydrided and C2 concentrations in the plasma reach levels of 1012 cm'3,
and (4) low hydrogen content plasma favor these conditions [Gruen

1999].
2.3.2 Ultra-Nanocrystalline Diamond Renncleation Growth Model

Ultra-nanocrystalline diamond ﬁlms have potential advantages when compared with

polycrystalline diamond CVD because the average surface roughness is lower on the order

21

of a few 10's of nanometers. Ultra-nanocrystalline diamond ﬁlms usually are believed to grow

by continuous re-nucleation.

A mam-crystalline growth model explains the structural evolution of the ﬁlm based on a
substrate seeded with diamond nuclei that grow isotropically [Huang 2001]. Very high
heterogeneous re-nucleation rates (1 O10 cm‘zsec'l) ensure that growth occurs and results in the
formation of smooth, phase—pure nano-crystalline diamond ﬁlms. This high secondary
nucleation rates allows the transition from microcrystalline to nanocrystalline diamond

ﬁlms.

24 UMa-nano-crystallineDiamondFTlmDepositionTechniques

In general, nano-crystalline diamond ﬁlm grows in high concentration of Ar (75-99%), 0.5-2% CH4
andzeroto afewpercentageofH2. Theresultingcrystal sizes areusuallysmallerthanSOmnand
often are as small as 10 or less nm. There are some techniques which have been involved to
synthesize mane-crystalline diamond ﬁlms such as: Hot ﬁlament CVD, Microwave plasma CVD,

Radio-ﬁequency Plasma CVD and DC are plasma.

2.4.1 Hot-ﬁlament CVD

Masumoto et a1. [Masumoto 1982] gave the ﬁrst description of a process using hot-

ﬁlament CVD (Figure 2.5). The hot-ﬁlament assisted process Operates at lower gas

activation temperatures and low pressure (1-80 Torr). The substrate is between 5 and 20

22

to grow diamond has some advantages when compare with others like simplicity (Figure
2.5), low cost, scaleable and can be used to coat complex shapes and internal surfaces.
Two major drawbacks of hot- ﬁlament CVD are: (1) material from the ﬁlament can
contaminate deposited ﬁlms and (2) the range of gases available for use in HFCVD is
limited by the sensitivity of the ﬁlament to oxidising or corrosive species.

Process
Gases ‘

Substrate

Filament

Heater

1

To
Pump

Figure 2.5: Hot-ﬁlament system diagram

The ﬁlaments (tungsten, tatalurn, rheniurn materials), evaporates to a small extent and
contaminates the growing diamond ﬁlm. This metallic contamination is not too much of a
constraint for coatings used in mechanical applications such as tools or general wear
parts; however, it is a nuisance when envisaging electronic applications such as active

components, as well as optical or sensor devices.

Wang et a1. [Wang 2004]. have grown UNCD on 2" Si (100) wafers by decreasing the
depodﬁmprenneandmumsingmmgasmhrunewimaMﬁlmmmchmquvaage
grainsimsofapproximately4—8mnwereachieved

23

2.4.2 Microwave Plasma CVD

Microwave plasma assisted chemical vapor deposition (MPCVD) systems have more
advantage than other chemical vapor deposition (CVD) systems like hot ﬁlament CVD,
direct current CVD arts or combustion ﬂame in terms of have a wide operating pressure
regime and high growth rate [Grotjohn 2001]. The range of pressure operated in MPCVD
systems range from 10 mTorr to over 240 Torr. MPCVD is one of the most popular

techniques used to grow nanocrystalline diamond ﬁlms in the laboratory.

The plasma is generated in a reactive gas mixture by a high-frequency electric ﬁeld, such
as microwaves (Figure 2.6), or by electron cyclotron resonance (ECR), i.e. a combination
of electric and magnetic ﬁelds. By using these methods, the coatings are very uniform (:t
10 % of average thickness), over large area (200 mm and more), smooth, and of high
purity. By this process, the large areas of uniform, homogeneous, polycrystalline thin

diamond ﬁlms were obtained.

 

Microwave Power

 

 

Figure 2.6: Microwave plasma CVD diagram

24

Gruen at al. [Gruen 1999] grew name-crystalline and ultra-nanocrystalline diamond ﬁlms
with a argon-carbon (C60 in argon) microwave plasma and controlled the diamond crystal
microstructure by argon additions to methane-hydrogen microwave plasma discharges in a
microwave plasma CVD reactor (ASTeX PDS-17). It was found that nanometer sized diamond
could be synthesized with either C60 or CH4 carbon precursor. Cross-section and plane view
SEM images show that the morphology, grain size, and growth mechanism are affected by the
ratio of argon to hydrogen in the gas mixture. The transition from microcrystalline to
nanocrystalline which depends on ratio of argon to hydrogen was conﬁrmed by X-ray
diffraction and Raman spectroscopy. The nano-crystalline diamond was synthesized at an
Ar/H2 volume ratio of 99% and CH4 volume percentage of 1%. The nanocrystalline diamond
was synthesized at 0-2% of H2 and 1% of CH; (vol%). The C2 dimer concentration is
promoted signiﬁcantly by increasing the argon concentration [Gruen 1999]. A critical

process in this deposition is believed to be the continuous renucleation by the C2 dimer.

Nanocrystalline diamond ﬁlms were also synthesized on a 4" Si (100) wafer with a
hydrogen ﬂow rate of 100 sccm and a methane ﬂow rate of 10 sccm using a microwave
plasma CVDsystanTlmbasisofmenmoaymlhmdimnonddeposiﬁmmﬂusprocesswasavay
high nucleation density. The silicon substrate was scratched twice by dry diamond powders with the
sizes of 250 nm and 5 nm respectively [Yoshi 2001]. The high nucleation density, approximately
1x10ll cm'z, led to a smooth (RMS=8.4 nm by atomic force microscopy) and ﬁne-grain (about
10 nm observed by ﬁeld emission scanning electron microscopy) diamond ﬁlm with 3.5 pm in
thickness. The FI'IR (fourier transform inﬁared spectrometer), spectra showed C-H bands: Sp3 -

CH2 symmetric stretch at 2850 cm’1 and sp3 -CH2 asymmetric stretch at 2925 cm", in the ﬁlm.

25

Hong et a1. [Hong 2002] used the same technique and similar conditions to deposit nanocrystalline
diamond ﬁhns on a 4" Si(100) wafer but modiﬁed the two-step scratch seeding procedure with
dry diamond powders of the sizes of 1 um and 5 nm for tribological characteristics study.
A slightly thinner ﬁlm (2.2 pm thick) with approximately the same size crystals (10—15 mm)

showed a very low surface roughness value (10 mn).

Bhusari et a1. [Bhusari 1998] deposited diamond ﬁlms with grain sizes ranging ﬁ'om 4 nm to a
few hundreds of nanometers in methane, hydrogen, and oxygen gas mixture by an AsTex 5
kW microwave reactor. The growth results of the quartz substrates pretreated with two different
diarnond-powder sizes, 4 nm and 0.1 um, were compared. The ultra-smooth and highly
transparent nano-crystalline diamond ﬁlms were coated on the quartz substrates (1) using 4 11m
powder pretreatment and low (<20%) methane concentration, and (2) using 0.1 um powder
pretreatment and high (>20%) methane concentration. According to the in situ OES (optical
emission spectroscopy) study, the C2 dimer continued to increase as methane concentration
increased, while other hydrocarbon species that decreased signiﬁeantly as methane concentration
irraeasedTlnrs,itwasspearlatedtl1atQ maybethepredominant growmspeciesathighermethane

fractions.

Shardaetal. [Sharda 2003]comparedthe optieal properties ofmicrocrystallineandnanoa'ystalline
diamond ﬁlms fabricated on silicon substrates by microwave plasma chemical vapor
deposition with a mixture of 5% methane in hydrogen The substrate was pretreated with

bias enhanced nucleation. The nanocrystalline diamond ﬁlm grown at 700 0C had a very

26

high optical absorption coefﬁcient, i.e. >104 cm" (higher than that of the microcrystalline
diamond ﬁlm) even though it was smoother than microcrystalline diamond ﬁlm. Nevertheless,
the nano-crystalline diamond ﬁhn grown at 600 °C, was smoother, had 78 % transmittance in the

inﬁared region, and thus had demonstrated a potential for application as optieal windows.

Ulcznski [Ulezn 1998] reported to grow diamond on borosilicate glass substrates for protective
coatings purpose. The glass substrates were Corning code 7059 and Corning code 7050.
The ﬁlms are grown by low-temperature microwave plasma-assisted chemical vapor
deposition on seeded glass substrates. A smooth diamond ﬁlms (the thickness less than

2 pm), both patterned and unpattemed, achieved with near ideal transmission throughout

the visible

2.4.3 Radio-Frequency Plasma CVD (RFPCVD)

The power source of radio-ﬁequency plasma CVD uses with frequencies ranging from
hundreds of kHz to tens of MHz. A schematic drawing of a Radio-Frequency Thermal
Plasma CVD (RFPCVD) reactor is shown in Figure 2.7. Several types of RFPCVD
systems have been used to deposit diamond such as RF thermal plasma and RF glow
discharge plasma systems. The ﬁrst reported growth of diamond using RF thermal
plasma was in 1987 by Matsumoto [Matsumoto 1987]. High growth rates diamond
deposition (in the tens of um/h) over substrates as large as 10 cm in diameter using RF
thermal plasmas were achieved by a Toyota group [Kohza 1993]. Similar to DC thermal

plasmas, RF thermal plasmas exposes the substrates to a high heat load, and the substrate

27

temperature needs to cool down enough for diamond growth. So the challenge of this

method is to control substrate temperature and boundary layer thickness.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Torch Head
C l' t Outlet 2 :
00 mg wa er J l
g L

Chamber Flange O O
O O.._. RF C011
0 O

, Thermal Plasma
Cooling water Inlet [q
l 1
Water Cooled Flange Substrate

 

 

 

 

—’ Substrate Holder

 

Figure 2.7: Radio-Frequency Plasma CVD diagram

Using a radio ﬁequency plasma assisted CVD (RFPACVD) method, an appropriate thickness of a
nanocrystalline diamond layer was deposited as an anti-abrasive coating on cemented carbide
substrates. The nanocrystalline coating reduced the ﬁiction coefﬁcient in sliding against wood
[Niedzi 2001]. The results ofthis study are helpful in the selection ofthe optimum thickness ofthe
nano-crystallinediamorxl ﬁlrnstobecoatedonthecemented earbidetoolstoimprovecuttingofthe

millsusedinthewood industry.

Erz et a1. [Erz 1993] fabricated nano—crystalline diamond, optical transparent ﬁlms on silicon and

quartz substrates using methane-oxygen-hydrogen mixture by a remote tubular microwave CVD. In

28

this investigation, they used different diamond powder with grain sizes ranging ﬁom 0.01-3 pm to
enhance diamond nucleation on the substrates. The results showed that a nucleation density up to 3 x
1010 cm2 was achieved by scratching the substrates with 10 nm diamond powder. The high
nucleation density led to a ﬂat diamond ﬁlm with a smooth surface However, by increasing the ﬁlm
thiclcnessﬁom 1 umto 10 um,thesurfaceroughnessincreasesmorethan6times (30110 nrnto

200 nm).

2.4.4 D.C Arc Jet Plasma CVD

A typieal dc arc jet plasma CVD reactor describes in ﬁgtne 2.8. The gas rrrixture Ar/H2 is
incorporated into aprimaryAr plasma ﬂowinthetwintorch assembly. Thesegas ﬂow 5 aremixed
and expansion into the main reaction chamber. Methane is introduced into the Ar/H2 plasma through

an annular injection ring positioned 10 cm downstream ﬁom the output nozzle.

ArIH, Ar

H

   
   
  

Methane .,
Injection )
Ring \O->

IIIIIIIIIIIII*Ibar ﬁll-.II‘ II... A (CRDS)
,...... -... ...1. 0

 

Substrate

Figure 2.8: D.C arc plasma CVD reactor [Mankel 2003]

29

Nistor et a1. [Nistor 1997] grew ﬁne-grain diamond ﬁlms on silicon substrates in methane-
hydrogen-argon gas mixture with ﬁxed argon ﬂow (50 sccm) and varied methane ﬂow ﬁom 5-50
sccm, and hydrogen ﬂow at 45 sccm by a D.C. arc discharge plasma After the ultrasonic seeding
process (ultrasonic seeding with 5 nm diamond powder suspension in ethanol), a pulsed excimer laser
irradiation generated by an excimer KrF laser (pulse duration 15 nanosecond) was used to remove
the undesirable non-uniformities in the surface distribution of the seeded crystals, while leaving the
uncoalesced particles for subsequent growth tmdisturbed The two-step seeding
procedure led to highly smooth ﬁlms owing to the irradiation of pretreated substrates by
laser assisted disintegration of the coalesced seeds and removal of too large residue diamond
particles. The improvement of the growth of nanocrystalline diamond ﬁhns was obtained by the
combination of the uniformly seeded substrates and a high methane concentration (50% of

ar'gon-methane-hydrogen mixture).

2.5 Pre-nucleation Techniques

This section describes the pre-nucleation techniques used to prepare the substrate before
growing nanocrystalline diamond ﬁlms. The process plays an important role in enhancing
the initial nucleation density.

Growth of diamond begins when individual carbon atoms nucleate onto the surface in

such a way as to initiate the beginnings of a sp3 tetrahedral lattice. When using natural

diamond substrates (3 process called “homoepitaxial” growth), the template for the

30

required tetrahedral structure is already present, and the diamond lattice is just extended
atom-by-atom as deposition proceeds. But for non-diamond substrates (“heteroepitaxial”
growth), there is no such template for the C atoms to follow, and those C atoms that
deposit in non-diamond forms are immediately etched back into the gas phase by reaction
with atomic H. So pretreatment of the substrate is necessary in order to obtain a
nucleation density sufﬁcient to allow the growth of a continuous diamond ﬁlm on non-

diamond substrates [Liu 1995].

Once nucleation of carbon has occurred, the homoepitaxial diamond growth can
proceeded. The individual crystals become progressively larger and eventually grow into
each other, leading to characteristic columnar growth of polycrystalline diamond ﬁlms. A
continuous ﬁlm is formed at this point. In UNCD diamond growth, pre-nucleation is also

necessary as the ﬁrst step get the smooth of UNCD ﬁlms.

There are ﬁve techniques used for pre-nucleation on the substrate surface. The simplest
and most commonly pre-nucleation technique to seed the substrate surface is using
mechanical polishing with micro or mane-diamond powder. The second technique is
pretreatment using the Rotter method [Rotter 1999] and then ultrasonic scratching with
nano-powder liquid. The third technique is using a spin coating slurry containing nano-
diamond powder. The fourth technique is using bias enhanced nucleation (BEN) method.

And the last technique is using tungsten (W) ﬁlms.

31

Piazza [Piazza 2005] reported research of seeding on substrate surfaces by the scratching
method for UNCD growth. The study focused on the effect of diamond powder crystal
sizes for seeding on the substrate surface. The results show that the nucleation increases

as the seed particle size decreases (Figure 2.9)

 

(a) 0)) (C)

Figure 2.9: SEM pictures of ﬁlm deposited aﬁer 01 hours using different powder

(a) using micro powder; (b) using nano power; (c) using ultra-nano powder

The results also show that the diamond powder size using for seeding effected the surface

morphology of the diamond ﬁlms (Figure 2.10)

32

 

(a) (b)
Figure 2.10: SEM pictures of ﬁlm deposited after 10 hours using different powder as

seeds. (a) using micro powder; (b) using nano powder

Lee reported growing UNCD ﬁlms using the BEN pre-treatment method [Lee-Y 2005].
The nucleation process was carried out with a CH4/H2 plasma and a negative DC bias
voltage system. By using BEN method, the nucleation site density is greater than 10'1

sites/cm2 and growth rate is up to l um/hr (Figure 2.11).

 

Figure 2.11: SEM morphology of ﬁlm deposited after 3 hours using BEN seeding method

33

For comparison reason, various pretreatment methods to enhance the surface nucleation
density were studied by Chen et.al [Chen 2005]. UNCD ﬁlms were grown by MPACVD
system under the same conditions (Ar1CH4% = 99:1%, 150 Torr) after pre-treated with
four different methods: scratching, spin coating, ultrasonic, and bias DC. The results are

show in Figure 2.12.

 

Figure 2.12: SEM morphology of UNCD ﬁlms using different seeding methods (03 hrs)
(a) scratching (using 0.1 pm powder); (b) spin coating (using 3 nm powder);

(c) ultrasonic (using 3 nm powder); (d) bias DC (~100V).

Other research about enhance nucleation of UNCD used tungsten (W) ﬁlms. Naguib
[Naguib 2005] studied the pretreated method using a thin tungsten ﬁlm applied onto a

silicon surface prior to ultrasonic seeding. The thickness of the tungsten layer varied from

34

36 to 100 angstroms. The results for nucleation density are from 101' to > 1012 sites/cm2.
Figure 2.13 (a), (b) show the results of SEM images for two UNCD ﬁlms grown at the
same condition and for the same time, using two pretreated methods: a) without tungsten
ﬁlm added, (b) with tungsten ﬁlm added. The nucleation density has been increased by

using the tungsten ﬁlm.

 

Figure 2.13: SEM morphology of UNCD ﬁlms using tungsten seeding methods

(a) without tungsten added; (b) with tungsten ﬁlm added (100 angstroms thickness)

Table 2.3 summaries the results of nucleation density from difference pretreatment

methods.

35

 

 

 

 

 

 

 

 

Pretreatment methods Nucleation density (cm? Substrate References
No pretreatment 103- 105 Silicon [Bauer 1993]
Scratching 106—10IO Silicon [Ascarelli 1993]
Ultrasonic scratching 107_10r1 Silicon [Popovici 1992]
Spin Coating 106— 1010 Silicon [Smolin 1993]
Biasing 1(11’110‘I Silicon [Stoner 1992]
Tungsten ﬁlm IOU—>1012 Silicon [Naguib 2005]

 

 

 

 

 

Table 2.3: Nucleation densities of diamond after various pre-treatment

2.6 UNCD at low temperature growth

In UNCD growth by a MPACVD system, the substrate temperature is typically around
700 0C to obtain high quality diamond ﬁlm at a good growth rate. For most micro-
electronic device applications, depended on materials, the substrate temperature needs to
keep at 500 0C or lower. So growth of UNCD at low temperature while maintaining good

ﬁlm quality (with reasonable growth rates) is a new challenge for UNCD researchers.

Xiao reported the growth of UNCD diamond at low temperature (i.e. substrate
temperature range from 400 -800 0C) [Xiao 2004]. The initial enhanced nucleation
method plays a very important role for growth of UNCD at low temperature. Ultrasonic

seeding with nano-diamond powder was utilized. The UNCD ﬁlms growth was

36

performed using a Cyrannus Iplas MPECVD system. Since the thermal conductivity of
argon is much lower than hydrogen, and the power levels are also lower for plasma
formation, the gas mixtures Ar-CH4 = 99:1% is used to reduce the substrate temperature.
The results for growing UNCD diamond at 400 0C was a growth rate of 0.2 um/h as

compare with 0.25 pm/h at 800 °c.

 

(b)

Figure 2.14: SEM morphology of UNCD ﬁlms

(a) UNCD ﬁlm deposited at 800 °c ; (b) UNCD ﬁlm deposited at 400 °c
Figure 2.14 shows the surface morphology of diamond ﬁlms growth at 800 0C (a), and

400 0C (b). The surface morphology of UNCD at 400 0C is similar with UNCD at 800 0C.

37

 

Figure 2.15: Low temperature UNCD coating for bio-MEMS application.

Figure 2.15 shows the applications of low temperature UNCD. The low temperature
UNCD ﬁlm is used to fabricate hermetic protective coating for bio- MEMS devices

(because the melting point of aluminum is very low).

2.7 Conducting UNCD ﬁlms

The possibility of doping diamond and changing it from an electrical insulator to a

semiconductor opens up a wide range of potential electronic applications.

Researchers at Argonne National Laboratory (ANL) reported growing conductivity
diamond by adding nitrogen gas to Ar-CH4 gas mixtures [Bhatta 2001]. The conductivity
at room temperature increases dramatically with nitrogen concentration, from 0.016 (1%

N2) to 143 (o.cm)" (20% N2).

38

1% 20%

 

Figure 2.16: Surface morphology of conducting diamond ﬁlm 1% and 20% N2.

Figure 2.16 shows the surface morphology of conducting diamond ﬁlm with 1% and 20%
nitrogen in Ar—CH4-N2 gas mixtures.

The diamond ﬁlms changed from insulating to conducting because nitrogen atoms are
incorporated into the narrow boundaries between the grains (Figure 2.17) leading to

enhanced electron transport [Bhatta 2001].

 

Figure 2.17: Surface morphology of conducting diamond ﬁlm

39

Chapter 03: System Operation/Experimental Method

3.1 Introduction

This chapter describes the experimental system used for this thesis research including the
microwave plasma assisted chemical vapor deposition (MPACVD) reactor, microwave
power supply, wave guide system, gas ﬂow control system and computer control system.

This chapter also describes procedures to set up the experiment and system operation.

3.2 Experimental Systems

The system as Show in (Figure 3.1) is used in this thesis. It consists of (1) Microwave
power supply, (2) Directional power coupler, (3) Flexible waveguide, (4) Transition unit,
(5) Cavity side wall, (6) Base plate, (7) Water cooling pipes, (8) Quartz dome, (9)
Excitation probe, (10) Air cooling entrance, (11) Sliding short, (12) Variable input gases,
(13) Gas inlet valve, (14) MKS Mass ﬂow controller, (15) Pressure controller, (16)
Pressure read out, (17) Pressure gauge, (18) Monitoring computer, (19) Throttle valve,
(20) Nitrogen purge system, (21) Roughing pump, (22) Exhaust gas, (23) Holder base

plate, (24) Process chamber, (25) Chiller, and (26) Substrate holder.

40

 

 

 

 

 

 

 

 

 

Figure 3.1: MSU-Microwave Plasma Assisted CVD System

3.2.1 Microwave Power and Wave Guide System

The microwave power source used in this thesis experiment is a Cober model 86F 2.45-

GHz, 6 kW supply (Figure 3.2). The Cober-SF6 supplies microwave energy into the

cavity through a rectangular and coaxial waveguide (Figure 3.3).

41

 

Figure 3.2: Control Board of Microwave Generator model S6F

Coaxial Waveguide Rectangular Waveguide

Cober
Power

Supply

 

Figure 3.3: Microwave Plasma Assisted CVD System.

42

To Cavity Applicator

   
   
  

To Incident power meter

-------
uuuuuuuu
.......

To Reﬂected power meter

Water out

Figure 3.4: Microwave power and Wave Guide System

The microwave power and wave guide system (Figure 3.4) consists of:
(1) Magnetron
(2) Circulator
(3) Dummy load.
(4) Directional power coupler

(5) Rectangular Waveguide

43

The microwave generator Cober model S6F is a complete self-contained power source
with 6 kW continuous power output and operates at 2.45 GHz frequency. The air cooling
is mounted on top of the cabinet. A portion of the air is exhausted through the waveguide
to the applicator. Two of the major components, the magnetron tube (1) and the circulator
(2), are directly water cooled. All indicators and operating controls are mounted on the
door for easy monitor (Figure 3.3). The power input is approximately 12 kVA. The
microwave power Cober S6F is equipped with a waveguide arc detector. When an arc

occurs, the control circuit is immediately disabled and the amber “are” light comes on.

The microwave power supplied by the magnetron (1) is propagated into the cavity
applicator through rectangular waveguides (5). The reﬂected power, which is reﬂected
back ﬁom the cavity applicator, passes through the dual-directional coupler (4) and is
directed by the circulator (2) into a matched dummy load (3), where it is absorbed and
dissipated as thermal energy. The circulator and the matched dummy load protect the
power source from being damaged by preventing the propagation of the reﬂected power
back into the power supply. The dual-directional power coupler attenuation factors for
incident and reﬂected power is 60 dB. The incident power Pin and reﬂected power me are

measured by incident and reﬂected power meter.

3.2.2 Transmission System

The transmission system guides the microwave energy into the cavity applicator (Figure

3.5).

44

The microwave energy ﬁ'om the generator is propagated through a rectangular waveguide
(1), a ﬂexible waveguide (2) and a transition unit (3). Microwave energy is then coupled
into the cavity applicator (6) by the excitation probe (4) and coaxial waveguide (5). The
excitation probe is located at the center of the sliding short (7). The excitation probe can
move up and down along the reactor axis by manual adjustment in order to get the best

position for incident power matching.

1 2

.
. . .

. .
. . .
.0 I'.

4 . _
-‘ . .' -‘ ~m ".
'- A ‘ i. t u
. t

.’ . .' .' . 7 ‘1. l
-_ e »

n c ‘
.I ... f C
.' I: l u _4 .—

. . .

n I I

o' o o . I
. . .
,' . .‘ 2 .‘r—
.0 I.- _ ' '
. . . A,

. . .
1. ...
' u o ‘ '

4.
n' c o' be
. .
. . .
. .
. . .
. .
- n 1

Microwave source

       

 

 

Figure 3.5: Transmission System

(1) Rectangular Waveguide (2) Flexible Waveguide
(3) Transition Unit (4) Excitation Probe
(5) Coaxial Waveguide (6) Cavity Applicator
(7) Sliding Short

45

3.2.3 Vacuum pump and gas ﬂow control System

3.2.3.1 Vacuum pump and nitrogen purge system

The vacuum pump system is very important in CVD diamond system. Through the

control system, it will keep the pressure at the desirable constant pressure.

 

 

 

 

 

   

 

Cable to MKS pressure
controller

/
...—El

 

 

 

 

 

 

 

Figure 3.6: The Vacuum and Nitrogen purge System

(1) The process chamber (2) Glass window
(3) Throttle valve (4) Nitrogen purge
(5) Roughing pump (6) Exhaust gas

46

Figure 3.6 shows the vacuum and nitrogen purge system. The vacuum pump (5), used for
this system, is two stage rotary vane vacuum pump (Alcatel 2063 CP).
The throttle valve (3) used in this system is a MKS 653 A. The nitrogen purge (4) is used

to bring the pressure up to atmosphere pressure when the experiment is done.

3.2.3.2 The Gas Flow Controller System

Figure 3.7 shows the gas ﬂow controller system used in the MPACVD system. The
gases, used for nano-crystalline diamond ﬁlm deposition experiments are H2 or N2, Ar or
He and CH4. The gases from gas tanks (4) ﬂow into the chamber through four MKS
mass ﬂow controllers (5) (range ﬁom 10 to 1000 sccm). The gas ﬂow is automatically

controlled by Lab-view software on computer (6) and gas ﬂow controller (7).

The gas ﬂow control is monitored by a 4-channel MKS type 247 C ﬂow controller (7).
The impurity of source gases used for PACVD system are: Hydrogen (99.999%),
Methane (99.999%), Nitrogen (99.995%), Argon (99.999%) and Helium (99.995%). The
source gases have high purity to minimize the introduction of impurities into the process

chamber during ﬁlm deposition.

Two baratron capacitance manometers MKS type 6278 are used for monitoring the
pressure inside the chamber. The Baratron capacitance manometer MKS type 627 B
determines the pressure in the process chamber by measuring the change in capacitance

between the diaphragms and an adjacent dual electrode. The differential capacitance

47

signal is converted into a useable output by signal conditioning circuitry and directly

transferred to the MKS pressure controller (8).

48

Microwave Power

   
        

 

 

 

v .. .. -
0000000!
0°0°0°0‘1
0, 0, 0, 0,0
‘ J

0
Ii0

D

      

   

 

 

 

 

 

Figure 3.7: The Gas Flow Control System
(1) Cavity (2) Process chamber
(3) Gas inlet valve (4) Gas Tanks
(5) Mass ﬂow controller (6) Monitoring Computer
(7) 4-channel read out (8) Pressure Controller
(9) Baratron (10) Transducer
(11) Pressure read out (12) Throttle Valve

49

The MKS pressure controller MKS model 651 instrument used in this system is a self-
tuning pressure controller for throttle valves. It provides a read-out for an attached

capacitance manometer.

The 4-channel readout MKS model 247C is used to control and display from the mass
ﬂow controllers (5). The ﬂow rate set point can be adjusted either through front panel

controls or remotely through the rear panel analog interface.

The mass ﬂow controller MKS model 1159B (ﬁgure 12) is used to measure and control
the ﬂow of gases from gas tanks (4). It can also be used as a pressure controller when

connected to a suitable pressure transducer. The gas ﬂow range channel is shown in

 

 

 

 

Table 3.1.
Channel 1 Channel 2 Channel 3 Channel 4
Gas H2 Hz Ar CH4
Range (sccm) 1 0 1000 500 10

 

 

 

 

 

Table 3.1: The gas ﬂow range

50

 

3.2.4 Computer control system

The computer is used to control and automatically monitor the experiment procedure in
the MPACVD system from start to shut down. The program software used for control is

Lab-View.

 

Set experiment pressure

Set experiment running time

Set gas ﬂow

Experiment start-up sequence

Start timer as pressure reaches its set point

P‘PP’P?‘

 

 

 

 

 

1. Monitor operating pressure and power.
2. Check timer

 

 

 

Operating pressure over set point
Reﬂected power is over 25% of
incident power (disabled).

Time expires

 

 

 

 

Normal shut down procedure

 

 

Emergency shut down:

 

H

. Turn off microwave power
2. Turn of all gas ﬂow

3. Automatic throttle valve
controlled by pressure
controller

 

 

 

Figure 3.8: Computer Control System Diagram

51

Figure 3.8 shows the monitoring computer control ﬂow chart used for the MPACVD
system. The operating pressure and the run time are ﬁrst set in the CVD program. The
experiment system is then set up and the feed gas ﬂow to the process chamber is
established. After the CVD system is working with the Lab-View control program, the

throttle valve operates in a remote mode to adjust the pressure in the chamber.

During the experiment, the pressure and running time are monitored and controlled as
preset values. If for some reason, the reﬂected power is more than 25% of incident power
value or operating pressure exceeds preset value, the microwave power will be shut down
by computer. In that case, the throttle valve still maintains the pressure at the preset
value. Under normal operation, the CVD program control directs the system into a

normal shut down sequence at the end of the last state (when the running time is expires).
3.2.5 Microwave Plasma Cavity Reactor

Figure 3.9 shows the microwave plasma cavity reactor used for the PACVD system. The
cavity is made of brass. The inside diameter of the cavity is 7 inches. The thickness of the
cavity wall is 0.125 inches. The cavity, which forms the conducting shell, is electrically
shorted to a water-cooled base plate and a water cooled sliding short via ﬁnger stock. The
sliding short controls the applicator height L, and the excitation probe extends below the
sliding short a distance LP, Both Ls and I,p can move up and down along the longitudinal
axis of the applicator cavity wall. The applicator height L8 is adjusted to approximately

21.5 cm and the probe depth Lp is about 3.2 cm [Zhang 1993]. The cavity length is set to

52

get the TM013 mode. This mode reduces the near ﬁeld effect caused by the coaxial

excitation probe (Figure 3.9).

The microwave power is propagate into the cavity applicator through a mechanically
tunable coaxial excitation probe which is inside a coaxial waveguide and is located in the
center of the sliding short. The cavity, with volume bounded by the sliding short, side

wall and base plate forms the cylindrical electromagnetic excitation region (Figure 3.10).

The base plate is internally water cooled and also air cooled from out side. It also
included the input gas feed plate and gas distribution plate. A quartz dome (ﬁve inches
inside diameter) is sealed by Buna-N O-ring (Copolymer of butadiene and acrylonitrile)

in contact with base plate assembly.

The thermally ﬂoating substrate holder setup assembly (Figure 3.9 and 3.11) includes a ﬂow
pattern regulator, a metal tube (stainless steel, CD = 64 mm, ID = 57 mm and h = 47mm), a
quartz tube with ID = 95 mm, O.D =100 mm, and height=50 mm, and a holder-base plate. The
ﬂow pattern regulator (made by molybdenum, I.D =3.016" and OD =4.038") is a plate with

a series of holes arranged in a circle right inside the big circumference.

53

 

 

Microwave Power

 

 

Excitation Probe '

 

 

Cavity Side Wall ——>

Finger stock

if

 

Ls
Sliding Short "‘

 

(lllllllllllllllllllllllllllllllll lll

 

 

Air cooling inlet

 

Gas Inlet——’

 

 

Variable Height
State

 

 

 

 

Pyrometer H ___________ .1
Mirror

 

Lp
Quartz Dome

 

Pyrometer Viewing
Location

 

 

J
’I
f

_ 1'— Base Plate

‘—Water Cooling

 

I

 

Quartz tube
Holder Base Plate

 

 

 

Process gas outlet

 

“—‘Process Chamber
. ‘ Throttle Valve
Glass Wlndow
T I Exhaust
Nitrogen Purge System Roughrng Pump

Figure 3.9: Microwave Cavity Plasma Reactor

54

 

Figure 3.10: Cavity Applicator

There are ﬁve holes (D = 6 mm), one in the center and four holes symmetric and off the center
by 28 mm, used to measured the temperature ﬁom the back side. The gas ﬂow coming out the
gas inlet, into the quartz dome, ﬂows through the plasma. The conﬁguration is designed to

increase the uniformity of the ﬁlm deposition by changing the ﬂow pattern in the plasma

discharge and inﬂuencing the shape of the plasma discharge [Zhang 1993].

The premixed input gases are fed into the gas inlet of the base plate assembly. The substrate is
placed on top of the ﬂow pattern regulator, which is supported by a quartz tube. Quartz tubes of
different heights may be used to change the position of the substrate with respect to the plasma
to optimize the ﬁlm deposition (ﬁom 47 mm to 50 mm). A stainless steel tube which serves as

an electromagnetic ﬁeld resonance breaker is placed inside the quartz tube. The tube prevents the
plasma discharge ﬁom forming undemerth the substrate.

55

‘1"? m "'1

 

Temperature
measured

Pattern regulator

holes

Base plate

 

Figure 3.11: Substrate holder assembly (Top view)

The stainless steel tube and quartz tube are placed on a substrate holder base plate which has
30 mm diameter hole at its center to pass the hot gases from within the
quartz dome to the exhaust roughing pump. The base plate, the annular input gas feed
plate, and the gas distribution plate introduces a uniform ring of input gases into the quartz
dome where the electromagnetic ﬁelds produce a microwave discharge. A screened
view window (Figure 3.12 a) is cut into the cavity wall for viewing the discharge. During
an experiment, the viewing window is used to observe the plasma size inside the quartz
dome. The plasma size needs to be large enough to cover the substrate but small enough so

it does not touch the dome. Figure 3.12 (b), (c) and ((1) show the plasma with different gas

mixtures as viewed through window.

56

you... 30‘. 0 . ......
C. O. C O .....O...

cc. 0 c m
o c o cone..."-

 

Figure 3.12 a: The screenviewingwindow

 

57

Figure 3.12 b: Ar-H2-CH4 gas mixture plasma

 

Figure 3.12 c: He-Hz-CH4 gas mixture plasma

 

Figure 3.12 d: Ar-Nz-CH4 gasmixtureplasrna

58

An air blower (Dayton model 4C443A) with 100 CFM (cubic foot per minute) ﬂow rate
blows a cooling air stream into the air cooling inlet to cool the quartz dome and cavity side
wall. The air exits the cavity through the air blower outlet and through four holes (optical
access port) in the base plate. The air blower existing inside the Cober model S6F microwave
power supply adds another air cooling stream into the microwave cavity plasma reactor. Three
Teﬂon pieces in the coaxial waveguide were drilled with four of 1/8" diameter through holes.
This allows the cooling air from the air blower in the microwave power supply to ﬂow through the
coaxial waveguide, onto quartz dome and cavity side walls. This air ﬂow, then exit out of the

air blower outlet and the optical access ports.

A recirculating chiller (Nestlab model CFI‘ 300), which controls the temperature of the input
coolant liquid, is used for the cooling system (Figure 3.13). The cooling water ﬂow is 3 gpm at 60

psi. The coolant temperature is range ﬁom 5 0C to 35 0C.

 

Figure 3.13 The chiller Neslab model CFT 300

59

A water ﬂow indicator is used to monitor water cooling operation (Figure 3.14). During the
experiment, when the cooling water ﬂow is too slow or off for some reason, the operator can

emergency shut down (manually) the system to avoid overheating

 

Figure 3.14: The water ﬂow indicator

In the ﬁrture, the cooling water ﬂow needs to be improved such that monitoring is done by the

computer so that the system automatically shuts down when cooling ﬂow is low.

3.2.6 Operating Field Map

This section describes the general operating ﬁeld map for the determination of substrate
temperature. Measurements were carried out with the microwave plasma reactor under
thermally ﬂoating substrate holder set up and different gas mixtures (Figure 3.15, 3.16,
3.17). Each plot shows the substrate temperature measured as a function of pressure and

absorbed microwave power for a set gas ﬂow mixture.

60

Ts(OC)

840

790

740

690

640

590

540

490

440

390

   
  
  
   
   
 
 

240 Torr

220 Torr
’2’200 Torr
180 Torr

"160 Torr

. /,rw:;~ 140 Torr
' ,a/ x120 Torr

 

1200 Torr

80 Torr
60 Torr

 

900 1050 1200 1350 1500 1650 1800 1950 2100 2250 2400 2550 2700

1)abs (watt)

Figure 3.15: The MPACVD operating ﬁeld map

Ar-H2-CH4 = 100-4-1 (sccm)

61

900 p

220 Torr
850 ~
2% Torr
800 ~ '

180 Torr

   
 
 
 
  

 

750 r , 160 Torr
. Axe-~24 ,2’140 Torr

A 700 — //s '

03 J‘ 120 Torr

w ,'t” ,2",

{—1 _ '
650 100 Torr
600 80 Torr

550

500

 

 

450 i J l I l 1 1 l I l I L L J
1200 1350 1500 1650 1800 1950 2100 2250 2400 2550 2700 2850 3000 3150

Pabs (watt)

Figure 3.16: The MPACVD operating ﬁeld map system

He-H2-CH4 = 100-4-1 (sccm)

62

740 —

690

640

590

rs(°C)

540

490

I 1 1 41

l l 1

1950 2100 2250 2400 2550 2700

 

 

I l'

1650 1800

1

1200 1350 1500

440
Pabs (watt)

Figure 3.17: The MPACVD operating ﬁeld map system

Ar-N2-CH4 = 100-4-1 (sccm)

63

 

Figure 3.18: Substrate temperature is measured from the back side

The deposition pressure, the microwave power Pm and the substrate temperature T5 are
interrelated and interdependent. Figure 3.15, 3.16 and 3.17 show the dependence of
substrate temperature on the deposition pressure, input gas mixture and microwave power
Pubs. The substrate temperature Ts increases with either increase in the pressure, and/or the
microwave power Pubs. For a ﬁxed pressure, the plasma discharge vollnne V increases with
mcmasesmmenucmwavemwans.HE&bmmempaanueismea&uedﬁmnbacksideas

shown in ﬁgure 3.18.

For a ﬁxed microwave power P“, the plasma discharge volume V decreases with increasing
pressure. By the observation through the viewing window in the cavity wall, the lower
absorbed microwave power limit is determined by the minimum power required to maintain a
discharge volume that covers a 3" diameter substrate. The upper limit of the microwave power is

deterrninedbythe maxirnurnpowerthatcanbeusedtogenerateadischargevolmnewhichisjust

big enough to ﬁll the quartz dome without touching the quartz dome walls. The upper limit of the

microwave power is used to operate the MPACVD system without over heating the quartz dome.

3.3 Experimental Procedures

3.3.1 Prepared sample

The seeding or pro-treatment method is the ﬁrst step to prepare the sample before running
experiments. In the nanocrystalline diamond growth process, seeding enhances the initial
nucleation diamond density on the substrate surface which is critical in determining the quality
and uniformity of the ﬁhn after growth. Nucleation enhancement on the substrate surface is

especially important to grow nanocrystalline diamond at low substrate temperatures.

Mechanical scratching using micron diamond powder (0.25 um crystal size) and pre-

treatment by Rotter method [Rotter 1999] combined with ultrasonic nanopowder 3-5 nm

crystal size procedures are introduced in this section.

65

3.3.1.1 Scratch seeding procedure

Theprocedmeformechanimlscratchseedir1g(Figme 3.19)isasfollowed:

1. Placethesubstrateonthewedingstage.

2. Connect the seeding stage to a pump and turn on the power. The vacuum sucks the
substrateandkeepsthesubstratefrommoving.
3.Quicklycleanthesurfacewiﬂ1acetoneandmetlnnolbyKhnW1pem.
4.PutsomeAmplex(0.25umcrystalsized)microndiamondpowderontothesubsuatesmface.
Ifthe humidity in the room is high, bake the diamond powder at 150°C for 2 hours before

usage.

 

Figure 3.19: Prepare for scratching method

66

5. Use a wrapped in Kim WipeTM ﬁnger to polish the substrate with the combination of several

different angles of straight line motion and several different diameters of circular motion in 10

minutes. Make sure the substrate surface is scratched everywhere with a median force.

6. Pick up the substrate from the seeding stage and put it in the container, with the scratched

surface facing down.

7. Fill the container with acetone (enough to cover above the substrate).

8. Put the container in an ultrasonic bath for 30 minutes for cleaning and agitation purpose.

9. Take the substrate out and put it in another container with the scratched surface facing up.
F ill the container with enough methanols to cover the substrate (5 minutes).

10. Use Q-tip to gently wipe the substrate surface to remove any dirt or diamond powder.

11. Putthe substrateinto anopencontainer.

12. Rinse the substrate with acetone and methanol for 2 minutes each step.

13. Rinse the substrate with deionized water for 10 minutes.

14. Blow dry with a nitrogen gun (in clean room).

15. Check the substrate surface cleanness under optical microscopy. If there is dirt or diamond

powder left on the substrate surface, repeat step 6 through step 15.

Figure 3.20 shows the result of a mechanical scratch seeding method (under optical

microscope).

67

 

Figure 3.20: Silicon wafer substrate surface after using scratch seeding method.
(Optical microscopes 50X)

3.3.1.2 Ultrasonic seeding procedure

The procedure of this method is as follows:

1. Clean the substrate surface by acetone and methanol.

2. Put the substrate onto the substrate holder, then pretreatrment the surface by Rotter method:
in the MPACVD system use diarmond growth depositions condition for 30 minutes with gas
mixtures Ar-H2-CH4 = 100:4:1 and 120 Torr pressure.

3. After pretreatment, take the substrate out of the eharmber and put it into an ultrasonic bath
with ultranano diamond powder liquid (30 minutes).

4. Rinse the substrate with deionized water for 10 minutes.

5. Blow driy with a nitrogen gun (in clean room).

Figure 3.21 shows the result of a Ruttler and Ultrasonic seeding method

68

 

Figure 3.21: Substrate surface after using Ruttler and Ultrasonic method.

(Optical microscopes 50X).

3.3.2. Experimental Set-up

1.Clsanﬁreinsideﬂ1equartzdmmeandrxocelsehamberusimgacetone.
2.1oadﬂresubsuatesample(wiﬂrseeding)imotherxocelsclnmbu.
3.Sett1pﬂrenﬁnororﬂrerrnocorrplebelowﬂ1esr1bstrate(indrecelner).
4.Closetheeharnberwindow.

5.Pmmpdowmd1epressmeintheprocelschambu(lsmﬂyfor3ornmhoms)

69

3.3.3 Start up and shut down procedure

a. Start up procedure:
After the system is pumped down as low in pressure as possible then start to run experiment
as follow:
0 Turn on the water to microwave power supply.
0 Turn on the microwave power supply. The power level control knob should be zero.
0 Turn on the Neslab chiller and set the temperature at 18°C.
0 Turn on the gas tank valves.
0 Turn on the gas inlet valve.
0 Set the gas ﬂow for each channel of the 4-channel read out MKS 247C. Switch to
automatic mode.
0 Set the experimental running time, pressure, and gas ﬂows for each channel in each nrm
state of the CVD control software.
0 Adjust the cavity length to 21.5 cm by moving the sliding short position.

0 Open the roughing valve. Now the chamber pressure is controlled by the automatic
throttle valve.

0 Enable the microwave power supply when the system pressure reaches 5 Torr.

o Tum on the cooling fan.

0 Slowly increase the input microwave power as pressure increases such that the
plasma discharge covers the entire substrate surface (look through the viewing window
with safety glass).

0 Fine tune the cavity length, L, to obtain the minimum reﬂected power.

70

The experiment starts to run by itself as control by computer.

b. Shut down procedure:

When the experiment is completed, the system will perform the shut down procedure as

follows.

Turn off the rrrierowave power

Turn off the gas ﬂow channel (set the key back to manual mode)

Turn oﬂ‘ the computer program

Tum of the gas inlet valve

Turn off the gas tank valves

Turn off the chiller

Turn off the air blower motor

Open the knob of chamber window before using nitrogen to bring the charmber pressure
up to atmosphere.

Open the chamber window and unload the sample.

71

Chapter 4: Experimental Results

4.1 Introduction

This chapter describes the experimental results for nanocrystalline diarmond synthesis
from a microwave plasma assisted CVD system. The substrate material used for each
experiment was Silicon 3” wafers (100) Boron doped (P Type); the wafer thickness was
from 331 to 431 pm. The gas inputs were H2, N2, Ar, He and CH4. The cooling

temperature of Chiller was usually control at 18 0C. The applicator height is L, = 21 .5cm.

4.2 Nano-crystalline diamond films growthby H2/Ar/CH4 gas mixtures

This section presents the results of MPACVD diamond growth experiments using

H2/Ar/CH4 gas mixtures.
4.2.1 Film Morphology

Figure 4.1 (a), (b), and (c) displays the surface morphology of diamond ﬁlms with
hydrogen varied from 1 to 4 seem (AFM microscope). Figure 4.1 (d), (e) and (f) shows
the surface roughness increases with hydrogen ﬂow rate. From Figure 4.1 (d), the
average surface roughness (RMS) is 12.4 mm for a hydrogen ﬂow rate of 1 seem. When
the hydrogen ﬂow rate is increased to 4 seem, the surface becomes roughly with an RMS

of 19.6 nm (Figure 4.1 f). Figure 4.1 (g) shows the relation between surface roughness

72

and hydrogen ﬂow rate. The surface roughness also increased with hydrogen ﬂow rate at

a higher pressure of 160 Torr as shown in Figure 4.1 (g).

 

Figure 4.1 a: Film Morphology (AFM)

Pressure =120 Torr, gas mixtures Ar:CH4:H2 =100:1:13ccm , deposition time = 8 hrs

73

 

0 5.00 pm 0 5.00 pm
Data type Height Data type Deflection
2 range 200.0 nm 2 range 10.000 nu

diamcndcoating_4_12_2004.000

Figure 4.1 b: Film Morphology (AFM)

Pressure =120 Torr, gas mixtures Ar:CH4:H2 = 100:1:2sccm , deposition time = 8 hrs

74

 

0 5.00 pm 0 5.00 pm
Data type uetaht Data type Deflection
2 range 200.0 an 2 range t0.000 an

coat1n9_03_08_2004.022

Figure 4.1 c: Film Morphology (AFM)

Pressure =120 Torr, gas mixtures Ar:CH4:H2 = 100:1:4sccm , deposition time = 8 hrs

75

 

nm Section Analysis

 

 

 

 

 

S—
L 2.031 um
RMS 12.406 nm
1c DC
_ n3(1c) 10.229 mm
o Rmax 49.736 nm
92 36.482 nm
Rz Cnt valid
Radius 1.127 pm
0 _ Sigma 135.54 nm
'7 l l I
0 2 00 4.00 6 00
um .
Surface distance 2.11) um
Spectrum Horiz distan:e(t) 2.031 um
Vent distance 14.361 nm
0 405 °

    

:‘HL 1'.

Surface distance
Horiz distance
Vert distance
Angle

Spectral period

   

Spectral Freq

 

Spectral RMS amﬁ

 

 

diamondcoating_4_19_2004.001

Figure 4.1 11: Surface Roughness (AFM)

Pressure =120 Torr, gas mixtures Ar:CH4:H2 = 100:1:1sccm , deposition time = 8 hrs

76

 

m. Section Analysis

-95
_ I

O 2.00 4.00

Spect rum

12:1“; .‘. Aug). nub. .~ “v-9...” ..

Min

   

diamondcoating_4_12_2004.000

 

 

 

 

 

L 3.027 pm
RMS 15.776 nm
1c DC
RaCC) 12.279 rim
Rmax 73.953 nm
Rz 46.500 run
R2 Cnr valid
Radius 2.915 pm
Sigma 116.68 Hm
I
6.00

Surface distance 3 109 um

Horiz distanceic) 3 027 um

Wrtﬁsunm bLEHrm

1.208 °

 

Av.‘e

Surface distance
Horiz distance
Vert distance
Angie

Spectrai period

    

Spectra} Freq

 
 

Spectra] RVS amp

 

 

  

Figure 4.1 e: Surface Roughness (AFM)

Pressure =120 Torr, gas mixtures Ar:CI-I4:H2 = 100:1:2sccm , deposition time = 8 hrs

77

 

 

 

 

 

 

 

 

0 nm Section Analysis
o—
...
L 4.121 pm
RMS 19.635 nm
1c DC
Ra(‘a<) 16.818 nm
0 _ Rmax 80.241 nm
R2 52.88? mi
Rz Cnr 8
Radius 17.210 pm
Si ma 31.721 nrn
§1 | V l g
I 0 2 00 4.00 6 00
um Surface distance 4.273 um
Spectrum Horiz distance(L) 4.121 um
Vert distance 78.733 nm
Angie 1.094 °
Sur'Hce 31(33qu
mmi: zizrar-ce
vert distance
Angie
Surface distance
Horiz distance
' Vert distance
Ang1e
Spectrﬂ period DC
DC I“, Spectrai Fred 0 Hz
coaring_03_oa_2ooa.022 Spear“ “'5 ”'9 ”"003 ""‘

Figure 4.1 f: Surface Roughness (AF M)

Pressure =120 Torr, gas mixtures Ar:CH4:H2 = 100:1:4sccm , deposition time = 8 hrs

78

Surface Roughness (nm)

30

25

20

15

1O

 

Surface Roughness vs Hydrogen

 

 

0 120 Torr
I 160 Torr

 

 

I
L— . .
_ o
0
L
0 1 2 3 4
Hydrogen (sccm)

Figure 4.1 g: Surface Roughness vs Hydrogen Flow Rate

Ar:H2: CH4 = 100:1-4:1 sccm

79

 

Figure 4.1 (h) shows the surface roughness versus thickness of UNCD ﬁlm. The surface

roughness is rougher as UNCD ﬁlm get thicker.

 

 

Surface Roughness vs Thickness
70 ~
60 _ O
50 — 9
g .
(I)
40 _
52 .
.C
8’
CE 0
8 30 ~
g
3
a)
20 ”o
O
10 r
O 1 l 1 A 1 l 1 J
0 10 20 30 4O 50 6O 70 80
Thickness (urn)

Figure 4.1 h: Surface Roughness vs Film Thickness

Ar2H22CH4 = 100:4:1-2 sccm

80

4.2.2 Film Growth Rates

The growth rate varies depending on input factors like pressure, gas mixture and
temperature. Figure 4.2 (a) shows the growth rate versus pressure. The growth rate
increases with pressure and hydrogen ﬂow rate. Figure 4.2 (b) shows the growth rate
versus methane ﬂow rate. The growth rate increases up to 1.06 um/h at a methane ﬂow
rate of 2 seem. In the case of substrate temperature, there are also correlations with

growth rate. Figure 4.2 (c) shows the growth rate increase with substrate temperature.

 

 

Growth Rate vs. Pressure
0.45 c OH2=4$ccm
IH2=2 sccm
0.4 r 0
AH2=lsccm
0.35 c I
o
0.3 _
g A
30.25 — '
8
r2 02 r A
€30.15 r '
O o
0.1 — . ‘
A
0.05 P i
i
O l 1 l l J
50 80 110 140 170 200

Pressure (Torr)

Figure 4.2 a: Growth Rate vs Pressure

ArzCH4zH2 = 100:1 :1-4 seem

81

Growth Rate (um/h)

.0
A

1.2

.o
oo

.9
ox

0.2

Growth Rate vs Methane

 

1 l l 1 J 1 l

0.8 1 1.2 1.4 1.6 1.8 2 2.2
Methane (sccm)

Figure 4.2 b: Growth Rate vs Methane Flow Rate

Ar2CH4:H2 = 100: 1-2: 4 seem , 160 Torr

82

Growth Rate (um/h)

Growth Rate vs Substrate Temperature

L2 r‘

9
00
j

0.6 r

.O
A
l

(12 r

 

O Q 1 O 1 1 1 1 1 1 1

400 450 500 550 600 650 700 750 800
Temperature (°C)

Figure 4.2 c: Growth Rate vs Substrate Temperature

Ar:CH4:H2 = 100:1-2:1-4 sccm

4.2.3 Thin and Thick UNCD ﬁlm

This section describes the results of thin and thick diamond ﬁlms. Diamond ﬁlms of
various thicknesses from 58 nm to 72.3 um were deposited. When the diamond ﬁlms are
less than 50 nm thick, the ﬁlm surface is discontinuous (ﬁgure 4.3 a). Figure 4.3 (b) and

(c) show two thin diamond ﬁlms, with thickness 58 and 61.2 nm respectively. In ﬁgure

83

4.3 (d) and (e) the two thick diamond ﬁlms with thicknesses 56 and 72.3 urn are
displayed. These two thick ﬁlms retain small grain sizes on the surface. The surface

roughness (RMS) is 50.46 and 60.88 nm respectively.

 

Figure 4.3 a: Thin Film Morphology (less than 50 nm)

Ar:CH4:H2 = 100:1 :1 seem , deposition time: 75 minute.

84

 

200 nm

Figure 4.3 b: Thin Film Morphology (58 nm)

ArzCH4:H2 = 100:1 :1 seem , deposition time: 1 hour.

85

 

Figure 4.3 c: Thin Film Morphology (61.2 nm)

Ar:CH4:H2 = 1002121 sccm , deposition time: 1.25 hours.

86

9.0.3171

 

Figure 4.3 d: Thick Film Morphology (56 um)

Ar2CH4zH2 = 10011.5:4 sccm , deposition time: 52 hours.

87

200 mm

 

Figure 4.3 e: Thick Film Morphology (72.3 um)

Ar:CH4:H2 = 100:2:4 sccm , deposition time: 65 hours

Figure 4.3 (f) and 4.3 (g) show the surface thin and thick ﬁlms measured with an AFM
microscope. The roughness of the surface for the thin ﬁlm is RMS = 12.12 nm and thick
ﬁlm is RMS = 60.88 nm. In the case of the thick diamond ﬁlm, the surface roughness on
the back side (aﬁer silicon substrate is removed) is a very smooth RMS = 11.8 nm as
compared with front side is RMS = 50.46 nm for the ﬁlm thickness 56 pm (as show in

ﬁgure 4.3 (h) and 4.3 (k)).

88

 

owiul Instr-went: must-rm
\:e 10 ’(v

 

m . ml
2 In re 1 031 u:
:4 row- f {Incl-s I?
.Mr an “aunt
out 5"”: 103-3 m
Sharp: x 9e: ~19 51 l «-
Enanoe v PM 42151.1 3'!

d' IW>1.MJ6 .05 .934

Figure 4.3 f: Thin Film Morphology AFM (58 nm)

Ar:CH4:H2 = 100:1:1 sccm , RMS = 12.12 nm.

 

ovyni znnnnvnu Mi'ﬁsf‘ﬁtﬁ‘
" in size 11' {"1

 

\ 1 NH 1 141»:
mrv-er ~>r "Mi-z 255
m . IZ'Iu Meir!
Lit: sule 3:: n
>r31x d; s 14"} m
r Jle°S Uh]!

d' 1mm ‘2'? I 321320]

Figure 4.3 g: Thick Film Morphology AFM (72.3 um)

Ar2CH42H2 = 100:2:4 sccm , RMS = 60.88 nm.

89

Dioiui :rhlrmmu Vim-mica”
: 1

   

rim M131»

1 .
Q 119.1. L'w‘u

‘.~—4’

 

I 2.0?) nil-1";
I. W‘JJCI. M/d'vv

Figure 4.3 h: Thick Film Morphology in the back side AFM (56 um)

Ar:CI-I4:H2 = 100:1.5:4 sccrn , RMS = 11.8 nm.

Digiul :mtmnvn . Mm
:.m N .v 1‘
54am nu

nb-cr :F unplu

 

V’IW mg‘lo

(‘3 mm ...;ia
.

J»
v

 

 

at “we E-‘Z- ,1 43.03!

Figure 4.3 k: Thick Film Morphology AFM (56 um)

AI:CH4:H2 = 100:1.5:4 sccm , RMS = 50.46 nm.

90

88 11m

100 pm

 

Figure 4.3 m: The UNCD thick ﬁlm 72.3 um (SEM image)

Figure 4.3 m shows the thick UNCD ﬁlm (SEM image). The method to measure the
thickness of this UNCD ﬁlm based on weight gain with density of 3.51 g/cm3 gave an

average thickness of the ﬁlm as 72.3 um. In the Figure 4.3 m, the thickness is 88 um.

4.2.4 Young’s Modulus of UNCD Films

Young’s modulus is the stress of a material divided by its strain. It is a measure of
material’s strength. The Young’s modulus of nanocrystalline diamond ﬁlm is measured
by LAwave instrument (Figure 4.4). The LAwave instrument introduces a sound wave
into the sample's surface to measure the materials phase velocity dispersion curve. The

built-in matching algorithm determines Young's Modulus of the ﬁlm.

91

LAwave device includes a nitrogen-pulse laser, a digital oscilloscope, an micrometer
translation stage, an ultrasonic signal transducer and computer. The system directs a laser
beam onto a component for half a billionth of a second, causing the surface to vibrate.
The form and duration of the wave pattern from the vibration is recorded and evaluated
within seconds by an algorithm. The laser acoustic signals can be received at varying

distances between the detector and source.

 

Figure 4.4: Fraunhofer’s LAwave Instrument

Table 4.1 shows the Young’s modulus of nanocystalline diamond ﬁlms measured by

LAwave device.

92

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Substrate (Si wafer) Film (diamond)

C11 C12 C44 D (g/cm3) Y. (GPa) Poisson's D (glcm3) h (um)
177.9 63.5 79.6 2.33 617.013 0.09 3.218 2.97
177.8 63.5 79.6 2.33 617.677 0.09 3.219 2.97
178.4 63.5 79.6 2.33 602.234 0.09 3.191 2.97

177.81 63.5 79.6 2.33 611.519 0.09 3.202 2.97

176.65 63.5 79.6 2.33 647.897 0.09 3.267 2.97
180.9 63.5 79.6 2.33 576.177 0.09 3.17 2.97
179.6 63.5 79.6 2.33 588.699 0.09 3.184 2.97
178.4 608.745 3.207
156.3 63.5 79.6 2.33 864.349 0.09 3.52 1.05
156.5 63.5 79.6 2.33 861.879 0.09 3.52 1.05
156.8 63.5 79.6 2.33 868.748 0.09 3.52 1.05
156.5 864.992 3.52
169.4 63.5 79.6 2.33 692.084 0.09 3.458 1.42
169.0 63.5 79.6 2.33 708.238 0.09 3.496 1.42
167.9 63.5 79.6 2.33 717.84 0.09 3.486 1.42
167.5 63.5 79.6 2.33 723.853 0.09 3.491 1.42
168.5 710.503 3.48275

 

 

 

 

 

 

Table 4.1: Young’s modulus results of UNCD ﬁlm

The Young’s modulus of UNCD growth with Ar:CH4:H2 measured range from 608 GPa

to 864 GPa.

4.3 N ano-crystalline diamond ﬁlm deposition from H2/He/CH4 gas mixtures

This section describes the results of nano-crystalline diamond ﬁlm growth by MPACVD

system using H2/He/CH4 gas mixtures. The resulting ﬁlms were characterized using

scanning electron microscopy (SEM) and atomic force microscopy (AFM).

93

4.3.1 Film Morphology

Figures 4.5 (a), (b) and (c) display the ﬁlm surface morphology for different hydrogen
ﬂow rate, with the helium ﬂow ﬁxed at 100 sccm, methane ﬂow ﬁxed at 1 seem and
pressure at 120 Torr. The grain boundaries of crystal diamond become larger on the
surface when the hydrogen percent in the mixture increases. This makes the surface

rougher as the grain size becomes larger.

 

Figure 4.5 a: Thin Film Morphology (SEM image)

Pressure =120 Torr, gas mixtures He:CH4:H2 = 100:1 :1 seem, deposition time = 8 hrs

94

 

Figure 4.5 b: Film Morphology (SEM image)

Pressure =120 Torr, gas mixtures He:CH4:H2 % = 100:1:2% , deposition time = 8 hrs

As see in ﬁgure 4.5 (a), (b) and 0, ﬁlms grown with helium replacing argon are

consistent with the grain size expected for nano-crystalline diamond (less than 50 nm).

95

 

Figure 4.5 c: Film Morphology (SEM image)

Pressure =120 Torr, gas mixtures He1CH42H2 sccm = 100:1:4%, deposition time = 8 hrs

Figures 4.5 (d), (e) and (i) show the surface morphology of diamond ﬁhns measured
using an AFM microscope. Figure 4.3.1 d displays the surface morphology of the
diamond ﬁlm when the hydrogen ﬂow rate is 1 seem. Figure 4.5 (e) and 4.5 (1) show the

surface morphology of diamond ﬁlms with 2 seem and 4 sccm respectively.

96

 

d1 amond_08_26_0 S . 006

100.0 nm

 

Digital Instruments ManoScope
Scan si 29 10.00 pm

Scan rate 1.001 Hz
umber of samples 256
Image Data Height
Data scale 200.0 nm
Enaaoe x Pos 49783.4 um
Engage Y Dos 42151.3 um

Figure 4.5 d: Surface Morphology (AF M Image)

Pressure =120 Torr, gas mixtures He:CH4:H2 % = 100: 1 : 1%

97

200.0 rim

100.0 run

 

Digital Instruments NanoScope
ize 10.00 pm

 

Scan 5

Scan rate 2.001 142
Nuvmer of sanvles 256
Image Data Height
Data scale 200.0 nm
Engage x 905 49783.4 um
Enoaae Y 905 42151.3 um

d‘i amond_08_26_0 5 . 007

Figure 4.5 e: Surface Morphology (AFM Image)

Pressure =120 Torr, gas mixtures He2CH4:I-I2 % = 100:1:2%

98

 

d1 amnd_08_26_05 . 005

 

Digital Instruments Nanoswpe
10 00

Scan size . um
Scan rate 2.001 Hz
nunber of samples 256
Image Data Height
Data scale 800.0 nm
Engage x 905 49783.4 um
Engage Y Dos 42151.3 um

Figure 4.5 f: Surface morphology (AF M Image)

Pressure =120 Torr, gas mixtures He:CH4:H2 % = 100:1:4%

Figures 4.6 (a), (b) and (c) show the surface rouglmess versus hydrogen ﬂow rate with
helium 100 seem, methane 1 seem and pressure 120 Torr. In ﬁgure 4.6 (a), the RMS
surface roughness is 10.5 nm (1% hydrogen ﬂow rate). The ﬁlm is very smooth. When
the hydrogen ﬂow rate increases to 2%, the RMS surface roughness is 19.8 nm in ﬁgure

4.6 (b). The RMS surface roughness increases very fast to 49.8 nm with 4% hydrogen in

ﬁgure 4.6 (c).

99

W‘WWMKW‘F‘W filth WW. WWW”:

-95
_|

Section Analysis

Spectrum

 

at amnd..08..36_05 . (106

    

l
12.

 

RMS
1:
R:(lc)
Yb¢m Rmaf
R:
R: Cut
Rediux
Signm

 

 

 

5

 

 

surface distance
Mari: dist:nce(L)
Vern disrance
Angle

 

I“. '3

SurF-ce distance
Nari: distance
Vert distance
Angle

Spectral acriod
Spectral Freq
Spectral FNS :nv

4.6 a: Film Surface Roughness (AFM image)

0 H:
0.0004 nn

 

Pressure =120 Torr, gas mixtures He:CH4:H2 = 100:1 :1 seem

—
Section Analysis

"M

h-
0*

“Will 1WMiIIMJljlfll/Ul‘wflﬂi‘l all!"l 1101111

 

diatom-1.08-36.0f- .007

spectrum

 

 

 

 

 

 

SurFace distance
Hori: dizt:nce(L)
wort distance
Andie

SurFace distance
Hori: distance

Vert distance
ﬁngle
Zoeztral period

 

 

Figure 4.6 b: Film Surface Roughness (AFM image)

Pressure =120 Torr, gas mixtures He:CH4:H2 = 100:1 :2 seem

100

 

nm Secﬁon Analysis

L 3.281 pm
RNS ‘23.?97 ma
1: DC
I Ra(lc) 37.:33 nm
D -M"’W’m mar. 199.5a nm
R: 101.0? nm

R: Ent valid
Radius 3.00? pm
Sigma 123.3 nm

400
I

 

 

 

 

 

   

 

g1 I l I
I Cl 2 5 5 0 7.5 10 0
i.“ Surface distance 3.821 um
Snectrum Hori: distanceo.) 3.281 um
Wrt distance 139.53 n'n
2.435 '
M» e
Eur-Face distance
Hon-1': distance
Vert distance
*" Mb“- ~_ Spectral period DC
“1" Spectral Freq ‘3 H:
Spectral DIS am 0.013 nm

 

DC
dﬁ amand_08_26_05 . 005

Figure 4.6 0: Film Surface Roughness vs % Hydrogen (AFM Image)

Pressure =120 Torr, gas mixtures He:CH4:H2 % = 100:1:4%
Figure 4.6 (d) shows the surface roughness versus hydrogen ﬂow rate in case of

He:CH4:H2 gas mixtures. When the hydrogen ﬂow rate is increased, the surface

roughness increases.

101

Surface roughness vs Hydrogen

Surface roughness (nm
8 8 ‘5 8 8
l 7 l l ﬂ
0
o

—s
O
l

O

 

 

Hydrogen (sccm)

Figure 4.6 (1: Film Surface Roughness vs Hydrogen Flow Rate

Pressure =120 Torr, gas mixtures He2CH4zH2 = 100:1:1-4 sccm, deposition time = 8 hrs

4.3.2 Film Growth Rates

This section presents the relationship between growth rate and various input variables
including hydrogen ﬂow rate, temperature and pressure. Figure 4.7 (a) displays the
growth rate versus hydrogen ﬂow rate. The growth rate increases when the hydrogen
ﬂow rate is increased (The results are comparable with those of Ar-CH4-H2 gas
mixtures). Figure 4.7 (b) shows the relation between growth rates versus temperature.

Figure 4.7 (c) shows the growth rate versus pressure.

102

Growth Rate vs Hydrogen

.9

N

U1
1

.0
N
l

Growth Rate (um/h)
.6
O

.0

O

U:
l

 

0 l L l
0.5 1.5 2.5 3.5 4.5

Hydrogen (sccm)

Figure 4.7 a: Film Growth Rate vs Hydrogen

Pressure =120 Torr, gas mixtures He:CH4:H2 = 100:1:1-4 sccm

103

Growth Rate (pm)

(135

.o
w

.o
N
u:

.o
N

U
H
M

.0
_d

0.05

 

400

Growth Rate vs Temperature

1 i J

500 600 700 800
0
Temperature ( C)

Figure 4.7 b: Film Growth Rate vs Temperature

He:CH4:H2 = 50:3:50 sccm

104

Growth Rate vs Pressure

.0

N

Ur
l

.0
N
ﬂ

0.15 *

Growth Rate (pm)

.0
—
l

0.05 *

 

O 30 60 90 120
Pressure(Torr)

Figure 4.7 c: Film Growth Rate vs Pressure

He:CH4:H2 = 50:3:50 sccm

The Young’s modulus of UNCD growth with He:CH4:H2 measured in the range from 636

GPa to 850 GPa.
4.4 Nano-crystalline diamond ﬁlm deposition from N 2/Ar/CH4 gas mixtures

This section describes the results of growing nanocrystalline conducting diamond ﬁlms

using nitrogen in the gas mixture. The substrate temperature ranges from 400 0C to

635 0C for these experiments.

105

4.4.1 Film morphology

Figure 4.8 a, displays the surface morphology for a conducting diamond ﬁlm with a
nitrogen ﬂow rate of 1 seem. Figures 4.8 (b) and (c) display the surface morphology for

conducting ﬁlms with nitrogen ﬂow rates of 2 seem and 10 sccm.

 

Figure 4.8 a: Thin Film Morphology (SEM Image)

Pressure = 100 Torr, gas mixtures Ar2CH42N2 = 100:1:1 sccm

106

 

200 nm

Thin Film Morphology (SEM Image)

8b

Figure 4.

'2 seem

1.

N2 = 100!

= 100 Torr, gas mixtures Ar.CH4

Pressure

 

200 nm

Figure 4.8 c: Thin Film Morphology (SEM Image)

10 sccm

:1:

= 100

CH4.N2

gas mixtures Ar:

9

= 100 Torr

Pressure

107

The surface roughness of conducting diamond ﬁlms versus nitrogen ﬂow rate was
investigated. Figures 4.9 (a), (b), (c) and (d) show the surface roughness of conducting
diamond ﬁlms grown with nitrogen ﬂow rates of 1, 2, 10 and 20 sccm. With 1 seem
nitrogen ﬂow rate, the surface is very smooth (RMS = 12.6 nm). The surface roughness is
seen to increase with nitrogen ﬂow rate. When the nitrogen ﬂow rate is 20 seem, the

surface roughness is 88 nm.

Digital Instruments NanoScope

Scan size 10.00 pm
Scan rate 2.001 H2
Number of samples 256
Image Data Height
Data scale 500.0 nm
Engage X Pos 49733.4 um
Engage Y Pos -42151.3 urn

3 view angle

{1. light angle

 

X 2.000 um/div
2 500.000 nm/d‘iv

di amond_08_2 6-05 . 002

108

Figure 4.9 a: Film Surface Roughness (AFM Image)

Pressure =100 Torr, gas mixtures Ar:CH4:N2 = 100:1:1 sccm , RMS = 12.65 mm

Digital Instruments NanoScope

Scan size 10.00 pm
Scan rate 2.001 Hz
Number of samples 256
Image Data Height
Data scale 500.0 nm
Engage x Pos -19783.4 um
Engage Y Pos 42151.3 um

1‘1 view angle

ii} light angle

K}

 

X 2.000 um/div
2 500.000 nm/div

di amond_08_2 6_05 . 003

Figure 4.9 b: Film Surface Roughness (AF M Image)

Pressure =100 Torr, gas mixtures Ar:CH4:N2 = 100:1 :2 seem , RMS = 17 nm

109

Digital Instruments NanoScope
20.0

Scan size 0 pm
Scan rate 1.001 H2
Number of samples 256
Image Data Height
Data scale 1.000 um
Engage X Pos -19783.4 um
-42151.3 um

Engage Y Pos

Eij view angle
3:} light angle
l

   

X 5.000 pm/div
2 1000.000 nm/div

tran_08_ll_05.003

Figure 4.9 c: Film Surface Roughness (AFM Image)

Pressure =100 Torr, gas mixtures Ar:CH4:N2 = 10021 :10 sccm , RMS = 37.35 nm

110

 

Digital Instruments NanoScope
0.0

Scan size 0 pm
Scan rate 1.001 H2
Number of samples 256
Image Data Height
Data scale 1.000 um
Engage X Pos 49783.4 um
42151.3 urn

Engage Y Pos

7’: View angle
{’1— light angle

 

X 5.000 um/div
2 1000.000 nm/div

tran_08_11_05 . 000

Figure 4.9 (1: Film Surface Roughness (AFM Image)

Pressure =100 Torr, gas mixtures Ar:CH4:N2 = 10021120 sccm , RMS = 88 nm

111

 

Figure 4.9 (e) displays the surface roughness versus nitrogen ﬂow rate. With more

nitrogen ﬂow rate in the gas mixtures, the surface roughness increases as seen in the

diagram below.

Surface roughness (nm)

100

00
O

8

8

N
O

 

 

Surface Roughness vs Nitrogen
O
b o
t. O
O O
O 5 10 15 20 25
Nitrogen (sccm)

Figure 4.9 6: Surface Roughness vs Nitrogen Flow Rate

Pressure 100 Torr, Ar:CH4:N2 = 100:1:1-20 sccm

112

4.4.2 Film Growth Rate

This section describes the growth rate versus nitrogen ﬂow rate. The grth rate

increases when nitrogen ﬂow rate is higher (Figure 4.10).

Growth Rate (um/h)

0.25

0.2

0.15

0.1

0.05

Growth Rate vs Nitrogen

 

 

 

 

 

 

F‘ I
r + 60 Torr
I -|— 100 Torr
— I
I ' o
o
— o
. .
O 5 10 15 20 25
Nitrogen (sccm)

Figure 4.10: Film Growth Rate vs Nitrogen Flow Rate

Ar:CH4:N2 = 100:1:1-20 sccm

113

4.4.3 Film Conductivity

The electrical conductivity of diamond ﬁlm is investigated in this section. The
conductivity of nanocrystalline diamond ﬁlms were measured by four point probes
device. Figure 4.11 show the schematic of four point probe device. The resistivity (p) of
the ﬁlms determined by formula:

p = k (V/I) t
t: Thickness of the ﬁlms
V: Voltage (V)
1: Current (A)

k: Constant (depend on the shape of the ﬁlms)

 

 

 

 

 

 

Figure 4.1]: Schematic of four point probe

114

Figure 4.12 shows the conductivity versus nitrogen ﬂow rate. The conducting of the

diamond ﬁlms increase as the nitrogen ﬂow rate input increased.

Conductivity vs Nitrogen

 

 

12 f

10 — °
'7" O
.5 8 .
'9,
o
E 6 ”
.2
‘5
3
'2 4 ~
0
0

O
2 t
o
O . l l l l 1
O 5 10 15 20 25
Nitrogen(sccm)

Figure 4.12: Conductivity vs Nitrogen Flow Rate

Pressure 100 Torr, ArzCH4:N2 = 100:1:1-20 sccm

Figure 4.13 shows the substrate temperature versus nitrogen ﬂow rate. When increasing

the nitrogen ﬂow rate, the substrate temperature is increased.

115

Substrate temperature vs Nitrogen ﬂow

650 *' o

a a

8 9 §

T l l
O

Substrate temperature (00)
A
8
$

 

l l l 1

§

0 5 10 15 20 25
Nitrogen ﬂow (sccm)

Figure 4.13: Substrate temperature vs Nitrogen Flow Rate

Pressure 100 Torr, Ar:CH4:N2 = 100:1:1-20 sccm

The Young Modulus of conducting diamond ﬁlm is varies from 545 GPa to 891 GPa.

116

Chapter 5: UNCD Film Applications

5.1 Introduction

This chapter describes the applications of nano-crystalline diamond ﬁlms. Nano-
crystalline diamond captures many of the best properties of natural diamond in thin ﬁlm
form. It is currently being evaluated for a wide variety of applications based on its super
properties. Some of the applications of nano-crystalline diamond ﬁlm include a hard
coating material, a material/substrate for micromechanical systems, a surface acoustic
wave (SAW) device substrate [Bi 2002], a robust conducting coating for electrochemical
electrodes, and a freestanding ﬁlm for vacuum windows or ion beam stripping foils

(Figure 5.1).

 

Figure 5.1: UNCD freestanding ﬁlm

(Photograph provided by Dr. Reinhard)

117

Diamond can be doped from an insulator to a semiconductor, giving it the potential to be
used in many electronic devices such as piezoelectric effect devices, radiation detectors,
ﬁeld effect transistors, ﬁeld emission displays, and UV photo-detectors. Defects and
surface roughness issues still need to be addressed before diamond electronic devices can
be widely used. The surface acoustic wave (SAW) device is one type of electronic device

which can use impure, thin UNCD diamond, known as the SAW ﬁlter.

The ﬁeld emission display device is based on the electron emission properties of
polycrystalline diamond. It consumes very low power levels, and employs the idea of

using UNCD ﬁlm as an electron emitter in ﬂat-panel displays.

Generally, nano-crystalline diamond, with a smooth surface, is a suitable material for
many applications. In this chapter, three applications of nano-crystalline diamond are
brieﬂy explored: (1) Surface acoustic wave (SAW) device based on UNCD, (2) Ultra
high frequency micromechanical resonators and (3) UNCD coatings for atomic force

microscope tips.

5.2 UNCD surface acoustic wave (SAW) devices

Surface acoustic wave (SAW) devices are critical components of many modern digital
microwave and optical telecommunications systems. These devices perform complex
signal processing ﬁmctions through electro acoustic interactions in materials. Use of

these devices reduces part counts in cellular telephones and other complex

118

communications systems, making these systems increasingly portable, powerful, and

affordable.

Nam-crystalline diamond ﬁlms are a good candidate in SAW applications because SAW
devices require a smooth surface. Smooth surfaces reduce the propagation loss and
ensure the correct generation and propagation of the surface acoustic wave. With micro
crystalline diamond ﬁlms, polishing to achieve a smooth diamond surface is very
difﬁcult, especially in large wafers. Therefore, nano-crystalline is a favored material for

the SAW device application.

SAW devices, used in satellite communication or optical communication, require a high
frequency ﬁlter (greater than 2.5 GHz) [Nakahata 1992]. As digital communications
move to higher frequencies for more bandwidth, conventional SAW devices require more
difﬁcult and expensive lithography. Due to the smaller feature size required, diamond has
the highest known speed of sound and other unique desirable acoustic properties, SAW
devices, built on CVD diamond, provide operation at extremely high frequencies using
existing low-cost lithography. Since diamond is not a piezoelectric material, diamond
SAW ﬁlter requires a multilayer structure. Diamond must be incorporated with other
piezoelectric materials like ZnO. Table 5.1 gives the comparison between a diamond
SAW ﬁlter and other materials. Research on diamond SAW devices, carried out by
Nakahata, is concluded that when diamond was combined with a piezoelectric thin ﬁlm,

the SAW velocities elevated to as high as 12000 m/sec [Nakahata 1995].

119

 

 

 

 

 

 

Material Sound velocity Frequency (GHz) Feature size for
(m/s) 2.5 GHz ﬁlter (um)
LiNb 02 3500 0.9 0.35
Quartz 3200 0.8 0.32
ZnO/ Sapphire 5500 1 .4 0.55
ZnO/Diamond 10000 2.5 l .0

 

 

 

 

Table 5.1: Characteristics of SAW ﬁlter comparisons (Fujimori 1998)

SAW devices are most typically implemented on piezoelectric substrates (quartz, lithium

niobate) on which thin metal ﬁlm inter-digitated transducers (IDT) are fabricated using

photolithography. With a surface wave velocity 1><104 m 3“, diamond allows SAW

device operation near 2.5 GHz (Bi 2002). Figure 5.2 shows the structure of a SAW

 

 

 

device.
A1 electrodes ZnO
/
_-_7 I
M -_-._-_-_..
UNCD ﬁlm ——'
Si substrate

 

 

 

Figure 5.2: Structure of a surface acoustic wave (SAW) device (Bi 2002)

120

 

For application such as SAW device processes to grow thick nanocrystalline diamond
ﬁlm with thickness up to 56 um and surface smoothness of 50 nm were achieved in this

investigation as described earlier in section 4.2.3.

5.3 Ultra high frequency micro-electro-mechanical (U HF—MEMS) resonators

Micro-Electro-Mechanical Systems (MEMS) are the integration of mechanical elements,
sensors, actuators, and electronics on a common silicon substrate through micro-
fabrication technology. While the electronics are fabricated using integrated circuit (IC)
process sequences (e.g., CMOS, Bipolar, or BICMOS processes), the micromechanical
components are fabricated using compatible "micro-machining" processes that selectively
etch away parts of the silicon wafer or add new structural layers to form the mechanical

and electromechanical devices.

MEMS promises to revolutionize many product categories by bringing together micro-
electronics with micro-machining technology, making possible the realization of
complete systems-on-a-chip. MEMSis an enabling technology that allows the
development of smart products. By augmenting of the computational ability of
microelectronics with the perception and control capabilities of micro-sensors and micro-

actuators, an expansion of the space of possible designs and applications occurs.

UNCD ﬁlm is the most desirable material for many MEMS applications. The ultra high

frequency mechanical (UHF MEMS) resonator device is one of them. The resonance

121

frequency is generally proportional to acoustic velocity, which is proportional to the
square root of Young’s modulus to density ratio. UNCD ﬁlm provides the largest boost

towards even higher resonance frequencies.

 

Figure 5.3: Structure of ultra high ﬁ'equency MEMS device [Wang 2002].

Figure 5.3 shows the structure of an ultra high frequency MEMS device [Wang 2002]. A
nano-crystalline diamond micro-mechanical disk resonator with a material-mismatched
stem has been demonstrated at a record frequency of 1.51 GHz with an impressive Q of
11,555 (in resonant systems, Q is a measure of the ratio of the energy stored in it to the
energy lost during one cycle of operation). This is more than 7X higher than
demonstrated in a previous 1.14-GHz poly-silicon disk resonator. The nanocrystalline
diamond ﬁlms achieved a frequency-Q product of 1.74 x1013 that exceeds the 1 x1013 of
some of the best quartz crystals. In addition, a 1.27-GHz version with a Q exceeding
12,000 exhibits a measured motional resistance of only 100 kg with a dc-bias voltage of

20V, which is more than 34X lower than measured on a pure poly-silicon counterpart at

122

1.14GHz. At 498 MHz, Q is up to 55,300 in vacuum and 35,550 in air have been
demonstrated, both of which set frequency-Q product records at 2.75 x1013 (vacuum) and

1.77 x1013 (air) [Wang 2002].

The objective of this project was to grow conducting UNCD at low temperature with a
useful deposition rate of greater than 0.25 um/hr and with a high Young’s modulus.
Conducting UNCD ﬁlms were deposited at a substrate temperature 635 0C that yielded a
Young’s modulus of 89] GPa. The conductivity of ﬁlm was 2 (Q.crn)'l and the growth
rate was 0.4 urn/hr.

After more than six months of research, these results for the growth nanocrystalline

conducting diamond ﬁlm generally meet the UHF MEMS requirements.

5.4 UNCD Tips coated

Atomic force microscopy (AFM) is a method of measuring surface topography on a scale
from angstroms to 100 microns. This technique involves imaging a sample using a probe,
or tip, with a radius of about 20 nm. The tip is held a several nanometers above the
surface using a feedback mechanism (Figure 5.4) that measures surface—tip interactions

on the scale of nano-newtons.
Variations in tip height are recorded while the tip is scanned repeatedly across the

sample, producing a topographic image of the surface. In its repulsive "contact" mode,

the instrument lightly touches a tip at the end of a leaf spring or "cantilever" to the

123

sample (Figure 5.5). As a raster-scan drags the tip (Figure 5.6) over the sample, a
detection apparatus measures the vertical deﬂection of the cantilever, which indicates the
local sample height. Thus, in contact mode the AFM measures hard-sphere repulsion

forces between the tip and sample.

 

Figure 5.4: Atomic force microscope (Baselt 1993)

 

Figure 5.5: AFM cantilever is touching on the sample (Baselt 1993)

 

Figure 5.6: The silicon AFM Tip (Baselt 1993)

124

In combination with tip-sample interaction effects, the sharpness at the end of the tips
generally limits the resolution of AF M. The development of sharper and harder tips is
currently a major concern. UNCD ﬁlm with small grain sizes (nano-scale), high hardness
and smooth surfaces is an ideal material for AFM tips. The UNCD-coated tips are
expected to improve the accuracy of AFM-image results as well as to preserve the spatial
resolution expected from the tips remaining sharp. The diamond coating tip also is used
as a tool for fabrication. It can cut metal tracers to modify the circuit in an IC chip or
form gaps for further fabrication. Since the silicon tip is not hard enough, they are worn

out quickly when used for cutting.

Figure 5.7 and 5.8 show a silicon AFM tip coated by UNCD ﬁlm in this project. The
original silicon tip was seeded by dipping the tip into a nanopowder liquid (crystal size 3-
5 nm). Then the seeded silicon tip is coated with UNCD thin ﬁlm for 60 to 75 minutes.
The conditions to grow a UNCD thin ﬁlm on AFM tip are: a gas mixture of Ar-Hz-CH4

=100:1:1, a pressure of 120 Torr and a 2 kW microwave power.

125

 

Figure 5.7: The AFM Tips coated UNCD (SEM image)

 

Figure 5.8: The silicon AFM Tip coated UNCD (SEM image)

126

Chapter 6: Conclusions

6.1 Introduction

UNCD thin ﬁlms have a great potential for many application. The techniques to grow
ultra nano-crystalline diamond (UNCD) ﬁlm were reported in recent years. Huang
[Huang 2004] reported on the growth of ultra-nano crystalline diamond using microwave
plasma assisted chemical vapor deposition system with an Ar/Hz/CH4 gas mixture. Huang
explored a large experimental parameter spaces for the synthesis of smooth ultra-nano
crystalline diamond ﬁlms. However, in order to utilize previous developed techniques
and to explore new process techniques to grow the ultra-nano crystalline diamond ﬁlm
for speciﬁc applications with varied thicknesses of ﬁlm, more research needed to be

done.

After two years, the objectives of this thesis which were to develop the process
technologies and methodologies to grow a wide range of thicknesses and conductivities
of ultra-nano crystalline diamond ﬁlms have been achieved. Three gas mixtures including
Ar/Hz/CH4, He/HZ/CH4 and Ar/Nz/CH4 were investigated to grow UNCD ﬁlms.
Thickness studies of ultra-nano crystalline ﬁlms were carried out with demonstrated
thicknesses from 58 nm to 72 pm. Uniform, low-stress, UNCD ﬁlms were deposited over
a wide pressure range (60-180 Torr) and temperature range (400-850 C). Film surface
roughnesses as low as 12 nm (AFM microscope) was obtained. The highest growth rate

of 1.12 um/h was achieved at 180 Torr, H2 /Ar/CH4 = 4/100/2 sccm and 3 kilowatt

127

power. The routine and repeatable synthesis of smooth and uniform ultra-nano crystalline

diamond ﬁlms have been demonstrated for applications.

6.2 Summary

6.2.1 UNCD ﬁlms growth by Ar-CH4-H2 gas mixtures

This section summarizes the results of nano-crystalline diamond ﬁlms grth with Ar-

CH4-H2 gas mixtures.

6.2.1.1 Effect of variable inputs:

a) Hydrogen ﬂow rate
When hydrogen ﬂow rates is increased from 1 to 4 sccm:
o The surface roughness increased 1.5 times (120 Torr) and 2.15 times (160 Torr)
o The growth rate increased 2.11 times (120 Torr) and 2.17 times (160 Torr)

o The substrate temperatures increased from 580 0C to 625 0C (120Torr) and from

543 °c to 660 °c (160 Torr).

b) Pressure
When the pressure is increased from 60 Torr to 180 Torr:
o The growth rate increased 13.33 times (Ar-CH4-H2 =100-1-4 sccm) and 17.5

times (Ar-CH4-H2 =100-1-2 sccm)

128

o The substrate temperature increased from 430 0C to 600 0C (Ar-CH4-H2 =100-1-l
sccm) and 470 °C to 700 0c: (Ar-CH4-H2 =100-1-4 sccm)

0 Surface roughness increased 6 times (Ar-CH4-H2 =100-1-4 sccm)

6.2.1.2 Results of thin and thick ﬁlm

Continuous UNCD ﬁlms were successfully grown with thicknesses ranging from 58 nm
to 72 pm.

0 The thinnest continuous ﬁlm with a thickness of 58 nm was achieved using a
deposition pressure of 120 Torr, a gas mixture of Ar-CH4-H2 =100-1-1 seem, an
incident power of 1.5 kW, and a deposition time of 1 hr.

0 The thickest ﬁlm with a thickness of 72.3 um was achieved using a deposition
pressure of 180 Torr, a gas mixtures of Ar-CH4-H2 =100-2-4 seem, an incident

power of 2.2 kW, and a deposition time of 65 hrs.

The Young’s modulus of ultra-nano crystalline diamond, with Ar-CH4-H2 gas mixtures,

ranged from 608 to 864 GPa.

6.2.2 UNCD ﬁlm growth by He-CH4-H2 gas mixtures

Helium is an inert gas and it was used to replace argon for UNCD deposition. This

section summarizes the results of nano-crystalline diamond ﬁlms growth with He-CH4-H2

gas mixtures.

129

6.2.2.1 Effect of variable inputs:

a) Hydrogen ﬂow rate
When the hydrogen ﬂow rates increased ﬁ'om 1 to 4 seem:
o The surface roughness increased 4.7 times (120 Torr)
o The growth rate increased 2.3 times (120 Torr)
b) Pressure
When the pressure increased from 60 Torr to 120 Torr:
o The growth rate increased 36 times (He-CH4-H2 =30-3-3O sccm)

o The substrate temperature increased from 430 0C to 800 0C (Ar-CH4-H2 =30-3-30

sccrn)

The Young’s modulus of ultra-nano crystalline diamond grown with He-CH4-H2 gas

mixtures range from 771 to 850 GPa.

6.2.3 UNCD ﬁlm growth by Ar-CH4-N2 gas mixtures

This section summarizes the results of nano-crystalline conducting diamond ﬁlms growth

with Ar-CH4-N2 gas mixtures.

6.2.3.1 Effect of variable inputs:

a) Nitrogen ﬂow rate

130

When nitrogen ﬂow rate is increased from 1 to 20 sccm (100 Torr):
o The growth rate increased 2.29 times
0 The conductivity increased 35.5 times

0 The substrate temperature increased from 450 0C to 650 0C

b) Pressure
When the pressure increased ﬁom 60 Torr to 120 Torr:
o The growth rate increased 2.8 times (Ar-CH4-N2 =100-1-1 sccm)

o The substrate temperature increased ﬁ'om 450 0C to 600 0C (Ar-CH4-N2 =100-1-4

sccm)

The Young’s modulus for conducting UNCD ﬁlm was measured to be 89lGPa with the

substrate deposition temperature at 635 0C.
6.3 Discussion
This section gives some recommendations for future research. There still are some
issues for growing UNCD using the MPACVD system that need to be investigated

beyond this thesis such as:

(1) Growth Rate:
The maximum deposition rate achieved onto 3 inch silicon wafers was 1.12 um/h

(180 Torr, Ar-CH4-H2 =100-2-4). With these results, the process costs are still too

131

high for some commercial applications of UNCD ﬁlms using the Microwave Plasma
Assisted CVD system. In the future, more research needs to be done to improve the
growth rate of UNCD by modiﬁed the cavity, cooling system, substrate heater, and

nucleation treatment method.

(2) Thin ﬁlm:
The thinnest continuous UNCD ﬁlm achieved in this research was 58 nm. More
research needs to be invested for improve or enhance nucleation on the substrate

surface to get even the thinner UNCD ﬁlms.

(3) Growth diamond ﬁlm at low temperature substrate conditions (below 400 0C):

Lower the substrate temperate decreases the growth rate for UNCD deposition. There
are many applications for diamond coating where the substrate material must remain
at or below 400 0C. Improving the deposition rate at low substrate temperature is

remaining an important direction for further investigation.

132

TABLE A.1: Experiment data for Ar/H2/Cll4

Appendix

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Ar/Hz/CH4 Thickness Time Pressure Pabs Ts Ls RMS
(seem) (tun) (hr) (tort) (watt) (°C) (cm) (um)
100-1-1 0.058 0.75 120 1733 500 21.5 12.12
100-1-1 0.068 1.0 120 1793 506 21.5
100-1-1 0.088 1.5 120 1899 515 21.5
100-1-1 0.153 2 120 1701 510 21.5
100-1-1 0.292 2.5 120 1760 520 21.5 12.36
100-1-1 0.153 16 60 1034 470 21.5
100-2-1 0.178 3 120 1880 505 20.5
100—1-1 0.181 4 160 1240 575 21.5
100-4-1 0.215 8 120 1161 565 21.5
100-4-1 0.275 8 120 1045 560 21.5
100-1-1 0.318 4 160 1400 570 21.5 13.97
100-1-1 0.374 8 80 994 485 21.5
100-2-1 0.418 4 120 1226 525 21.5
100-2-1 0.470 4 120 1740 550 21
100-4-1 0.524 4 120 1161 535 21.5
100-2-1 0.563 4 120 1766 540 21
100-2-1 0.849 8 120 1119 545 21.5
100—2-1 0.854 8 120 1326 560 21.5 12.90
100-1-1 0.912 8 i 160 1730 595 21.5
100-2-1 0.943 8 160 1680 585 21.5 14.11
100-2-1 0.996 8 160 1941 613 21.5 15.08
100-1-1 1.068 8 120 1393 550 21.5 12.40
100-4-1 1.190 8 120 1361 555 21.5 19.60
1002-] 1.212 8 120 1394 550 21.5 13.2

 

 

 

 

 

 

 

 

133

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

100-2-1 1.237 8 160 1643 600 21.5 15.59
100-2-1 1.262 8 120 1295 550 21.5 13.7
100-2-1 1.284 8 160 1641 620 21.5
100-2-1 1.425 16 100 1075 490 21.5
100—2-1 1.768 8 160 1523 590 21.5 18.88
100-0-1 1.780 16 100 919 450 21
100-4-1.6 2.580 24 160 1650 660 21.5
100-2-1 2.855 8 160 1550 605 21.5
100-2-1 2.950 8 160 1700 647 21.5
100-4-1 2.980 8 160 1992 660 21
100-2-1 3.042 8 160 1520 600 21.5 16.65
100-4-1 3.133 8 180 1987 705 21.5
100-1-1 3.517 15 160 1560 580 21.5
100-1-1 3.767 22 160 p 1760 623 21
100-2-1 4.023 12 160 1550 572 21.5
100-1-1 4.479 16 160 1700 585 21.5
100-2-1 4.637 15 160 1800 615 21
100-2-1 4.967 24 160 1740 620 21.5
100-1-1.2 5.022 24 160 1650 610 21
100-4-1 7.078 30 120 1556 560 21.5 33.5
100-1-1 7.550 22 160 1900 613 20.5
100-2-1 7.600 20 160 1600 622 21
100-4-1.4 8.962 12 160 2054 662 21.5
100-2-1.4 9.200 24 160 1894 636 21.5
100-4-1.4 10.70 12 160 1990 708 21
100-4-1.4 16.8 24 160 1870 652 21.5
100-2-1.5 17.08 30 140 2114 675 21
100-4-1.5 17.57 25 160 2090 664 21.5 38.35
100-2-1 17.8 50 160 1835 626 20.5
100-2-1 19.58 50 160 1990 624 20.5

 

 

 

 

 

 

 

 

134

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

100-2-1.5 20.7 24 160 1820 685 21
100-2-1 22.62 60 160 1854 623 20.5
100-4-2 26.07 25 160 1970 690 20.5 44.52
100-4-2 26.47 25 160 2030 694 21.5
100-4-1.4 33.70 48 160 1913 684 21
100-4-1.5 45.10 48 160 1892 690 21
100-4-1.4 50.46 65 160 1990 661 21.5
100-4-1 .4 50.87 55 160 1976 679 21.5
100-4-1.5 56.0 52 160 2013 721 21 50.47
100-4-2 72.3 65 180 1979 740 21 60.88
100-4—2 77.74 70 180 1856 740 21
T5: The temperature measured at center of substrate.
TABLE A.2: Experiment data for He/Hz/CH4
He/HZ/CH4 Thickness Time Pressure Pabs Ts Ls RMS
(scorn) (um) (hr) (ton) (watt) (“0) (cm) (nm)
100-1-1 0.050 1 120 1751 613 21.5
100-1-1 0.058 3 120 1830 625 21.5
100-1-1 0.146 2 120 1621 630 21
30-30-2.25 0.148 3 30 1105 580 21.5
100-1-1 0.290 4 120 2110 678 21.5
30-30—2.25 0.397 3 40 1250 607 21.5
30-30—2.25 0.484 6 30 1480 595 20.5
30-30-2.25 0.564 65 10 825 430 21.5
100-1-1 0.615 8 120 1717 630 21.5 10.57
100-2-1 0.837 8 120 1060 634 21.5 19.8
30-30-2.25 1.01 6 40 1350 641 20.5
100-4-1 1.837 8 120 1940 685 20.5 49.8

 

 

 

 

 

 

 

 

135

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

30-30-2.25 1.96 6 120 1180 800 21.5
100-4-2 12.37 36 140 1777 850 21.5
30-30-2.25 13.22 72 60 1540 705 20.5
TABLE A.3: Experiment data for Ar/N2/CH4
Ar/Nz/CH4 Thickness Time Pressure Pabs TS Ls RMS
(seem) (11m) (hr) (ton) (watt) (“(2) (cm) (nm)
100—2-1 0.087 3 160 1880 645 21.5
100-1-1 0.49 22 50 1113 400 21.5
100-2-1 0.7 8 100 1070 465 21.5 17.0
100-1-1 0.8 22 60 1325 425 21.5 20.79
100-10-1 1.28 8 100 1698 640 21.5 37.35
100-1-1 1.58 12 100 1776 545 21.5 12.65
100-10-1.5 2.45 6 100 1435 635 21.5 13.49
100-5-1 2.49 20 100 1690 585 21.5 20.8
100-20-1 3.1 16 100 1430 630 21.5 88.0
100-2-1 3.49 84 60 970 413 21.5
100-2-1.5 4.2 30 120 1650 560 21.5

 

 

 

 

 

 

 

 

136

 

 

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