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Diamond based-materials :
h [electronic resource] /
b synthesis, characterization and applications
by Qiang Hu.
[Tampa, Fla] :
University of South Florida,
Title from PDF of title page.
Document formatted into pages; contains 159 pages.
(Ph.D.)--University of South Florida, 2011.
Includes bibliographical references.
Text (Electronic dissertation) in PDF format.
ABSTRACT: The studies covered in this dissertation concentrate on the various forms of diamond films synthesized by chemical vapor deposition (CVD) method, including microwave CVD and hot filament CVD. According to crystallinity and grain size, a variety of diamond forms primarily including microcrystalline (most commonly referred to as polycrystalline) and nanocrystalline diamond films, diamond-like carbon (DLC) films were successfully synthesized. The as-grown diamond films were optimized by changing deposition pressure, volume of reactant gas hydrogen (H2) and carrier gas argon (Ar) in order to get high-quality diamond films with a smooth surface, low roughness, preferred growth orientation and high sp3 bond contents, etc. The characterization of diamond films was carried out by metrological and analytical techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM) and Raman spectroscopy. The results of characterization served as feedback to optimize experimental parameters, so as to improve the quality of diamond films. A good understanding of the diamond film properties such as mechanical, electrical, optical and biological properties, which are determined by the qualities of diamond films, is necessary for the selection of diamond films for different applications. The nanocrystalline diamond nanowires grown by a combination of vapor-liquid-solid (VLS) method and CVD method in two stages, and the graphene grown on silicon substrate with nickel catalytic thin film by single CVD method were also investigated in a touch-on level.
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Kumar, Ashok .
x Mechanical Engineering Materials Science Electrical Engineering
t USF Electronic Theses and Dissertations.
Diamond B ased M aterials: S ynthesis, C haracterization and A pplication s by Qiang Hu A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Mechanical Engineering College of Engineering University of South Florida M ajor Professor: Ashok Kumar Ph.D Muhammad Rahman, Ph.D. Garrett Matthew s Ph.D. Frank Pyrtle Ph.D. Jing Wang, Ph.D. Rasim Guldiken, Ph.D. Date of Approval: April 4, 201 1 Keywords: c arbon, MPECVD HFCVD I V characteristics, CMUT s Copyright 20 11 Qiang Hu
DEDICATION T his dissertatio n is dedicated to those who keep on learning new knowledge, challenging and improving themselves in their li ves
ACKNOWLEDGMENTS First, I would like to express my heart felt thanks to Dr. Ashok Kumar my major advisor, for providing me with four years of financial support and time ly guidance related to cutting edge research topic s In addition to the academics, the generosity and the patience I have learned from hi m will benefit me for the rest of my life H is demonstrated passion toward research has and will encourage me in the pursuit of my career. Second I would also like to than k all the committee members for their valuable opinions that helped bring this dissertation to a satisf actory completion. Nanotechnology Research an d Education Center (NREC) staff Robert Tufts, Ri chard Everly and Jay Bieb er provided me great support and training in th e fabrication of the micro device and the SEM characterization of diamond films. Dr. Guldiken and his group members Onu rsal and Lynford offered me great help in the design and fabrication of the capacitive m icromachined ultrasonic transducers (CMUTs). In contemplation, these four year s of study research and working with my group partners including previous graduates, postdoctor al researchers and visiting scholars in s lab were pleasant and will remain fond memories for the rest of my life. Final ly, t his work wa s completed under the support of National Science Foundation ( NSF ) NIRT grant #EC C S 0404137 and a Graduate Multidisciplinary Scholars ( GMS ) fellowship from the Graduate School at the University of South Florida initiative G FMMD00.
i TABLE OF CONTENTS LIST OF TABLES ................................ ................................ ................................ ............. iv LIST OF FIGURES ................................ ................................ ................................ ............. v ABSTRACT ................................ ................................ ................................ ....................... ix C HAPTER 1: INTRODUCTION TO CVD DIAMOND ................................ .................... 1 1.1 Overview of CVD D iamond ................................ ................................ .............. 1 1.2 S tructure, P roperties and A pplications ................................ ............................. 5 1.2.1 Crystalline Structure of CVD Diamond ................................ .............. 5 1.2.2 Properties and A pplications ................................ ................................ 8 1.2.3 Research O bjectives ................................ ................................ .......... 1 8 1.3 Growth of D iamond Chemical Vapor Deposition Method ......................... 1 9 1.4 C haracterization T echniques ................................ ................................ ............ 2 4 1. 4 .1 Raman S pectroscopy ................................ ................................ ......... 2 5 1.4 .2 Atomic F orce M icroscopy (AFM) ................................ .................... 2 8 1.4 .3 X R ay D iffractions (XRD ) ................................ ............................... 30 1.4 .4 Scanning E lectron M icroscopy (SEM ) ................................ ............. 3 2 1.5 L ist of R eferences ................................ ................................ ............................ 3 5 C HAPTER 2 : VARIOUS FORMS OF CAR BON ................................ ............................ 3 9 2.1 Polycrystalline D iamond F ilm ................................ ................................ ......... 3 9 2.1.1 Introduction to Polycrystalline D iamond Film ................................ 39 2.1.2 Growth C onditions of Polycrystalline Diamond Film ..................... 40 2.1.3 Characterizations of Polycrystalline Diamond Film ........................ 41 2.2 Nanocrystalline Diamond F ilm ................................ ................................ ........ 4 6 2.2.1 Introduction to N ano crystalline Diamond Film ............................... 4 6 2.2.2 Growth C onditions of Nanocrystalline Diamond Film ..................... 4 6 2.2.3 Characterizations of Nanocrystalline Diamond Film ....................... 4 7 2.3 D iamond Like Carbon (DLC) ................................ ................................ .......... 51 2.3.1 Introduction to DLC ................................ ................................ .......... 51 2.3.2 Growth C onditions of DLC ................................ .............................. 5 2 2.3.3 Characterizations of DLC ................................ ................................ 5 3 2.4 Diamond N anowires ................................ ................................ ........................ 5 6 2.4.1 Introduction to VLS and N anowires ................................ ................. 5 6 2.4.2 Growth C onditions of D iamond N anowires ................................ ..... 5 8 2.4.3 Characterization of Diamond N anowires ................................ .......... 5 9 2.5 Graphene ................................ ................................ ................................ .......... 61
ii 2.5.1 Introduction to G r a phene ................................ ................................ .. 61 2.5.2 Growth C onditions of G raphene ................................ ....................... 62 2.5.3 Characterization of G raphene ................................ ........................... 62 2.6 Summary ................................ ................................ ................................ .......... 6 5 2.7 List of References ................................ ................................ ............................ 6 6 C HAPTER 3: THE ROLE OF ARGON IN THE GROWTH OF NAN OCRYSTALLINE DIAMOND FILM S ................................ ................................ .... 6 9 3.1 The R ole of Ar in Nanocr ystalline Diamond Film ................................ .......... 69 3.2 E xperimental D escription s ................................ ................................ ............... 70 3.3 Raman A nalyses of Nano Diamond Films on Ar V ariation ............................ 71 3.4 SEM M orphologies of Nano Diamond Films on Ar V ariation ....................... 72 3.5 XRD C omparison of Nano Diamond Films on Ar V ariation .......................... 74 3. 6 Pressure I nfluences on Nano Diamond Film s ................................ ................. 7 5 3.6 .1 Raman A nalyses of Nano Diamond Films on P ressure V ariation ................................ ................................ ................................ .... 76 3.6 .2 SEM Images of Nano Diamond Films on P ressure V ariatio n .......... 77 3.6 .3 XRD C omparison of Nano Diamond Films on P ressure V ariation ................................ ................................ ................................ .... 81 3.7 Summary ................................ ................................ ................................ .......... 82 3. 8 List of References ................................ ................................ ............................ 8 3 C HAPTER 4 : NITROGEN DOPED NANOCRYSTALLIN E DIAMOND FILMS ........ 8 6 4 .1 Introduction to Nitrogen Do ped Nano Diamond Films ................................ 8 6 4 .2 Experimental D escriptions ................................ ................................ ............... 87 4 .3 Raman Analyses on N 2 V ariation ................................ ................................ .... 88 4 .4 SEM Morphologies on N 2 V ariation ................................ ................................ 90 4 .5 XRD C omparison o n N 2 V ariation ................................ ................................ .. 93 4 6 I V Characteristics and O hmic C ontact of NNCD F ilms ................................ 94 4.7 N NCD F ilms G rown on SiO 2 L ayer ................................ ................................ 98 4.8 Summary ................................ ................................ ................................ ........ 10 7 4 .9 List of References ................................ ................................ .......................... 10 9 C HAPER 5: POLYCRYSTALLINE DIAM OND FILMS SYNTHESIZED BY MICROWAVE AND HOT FILAMENT CVD ................................ ............................... 1 1 4 5.1 I ntroduction to HFCVD T echnique ................................ ............................... 1 14 5.2 R aman S pectra and S tress E valuation of P olycrystalline D iamond F ilm ...... 1 1 6 5.3 SEM of P olycrystalline D iamond F ilm ................................ ......................... 121 5. 4 AFM of Polycrystalline Diamond F ilm ................................ ........................ 122 5.5 XRD of P olycrystalline D iamond F ilm ................................ ......................... 124 5.6 Summary ................................ ................................ ................................ ........ 1 25 5.7 List of References ................................ ................................ .......................... 1 26 C HAPTER 6 : FABRICATION OF CMU TS ................................ ................................ .. 1 28 6 .1 I ntroduction to CMUTs ................................ ................................ .................. 1 28 6.2 CVD D iamond Film Grown for CMUTs ................................ ....................... 130
iii 6 .3 S tructure D esign and Fabrication Process ................................ ..................... 131 6 3.1 First Mask and Unit Cell Pattern ................................ .................... 132 6.3.2 Diamond Deposition and Second Mask for O pening Window ...... 13 4 6.3.3 Diamond Etch ing and Membrane Release ................................ ...... 13 5 6.3.4 Electrode Metallization ................................ ................................ ... 138 6.4 Summa ry ................................ ................................ ................................ ........ 139 6 5 List of References ................................ ................................ .......................... 140 C HAPTER 7: CONCLUSION S AND FUTURE WORK S ................................ ............ 1 42 7.1 Conclusions ................................ ................................ ................................ .... 1 42 7.2 Future Works ................................ ................................ ................................ 1 4 3 ABOUT THE AUTHOR ................................ ................................ ....................... End Page
iv LIST OF TABLES Table 1 Poly crystalline diamond with gas chemistry CH 4 /H 2 at ratio of 1/100 ............... 40 Table 2 The depositing conditions of pressure, temperature and Ar variations ............... 70 T able 3 The experimental conditions of 180sccm Ar flow rate at various p ressures ....... 7 6 Table 4 Deposition parameters of nano diamond films with various N 2 concent rations ................................ ................................ ................................ ..... 88 Table 5 Silver contact deposition parameters by PLD method ................................ ......... 9 6 Table 6 Structural features of diamond for different layer structures ............................. 104 Table 7 Experimental conditions of diamond deposition for CMUTs ........................... 131
v LIST OF FIGURES Figure 1 1 Formation of diamond cubic crystal structure ................................ ................ 6 Figure 1 1a. The electron distribution in sp2 and sp3 bonds of carbon atom ...................... 7 Figure 1 2 Prototype biosensor of diamond cantilever functionalized with crosslinker to detect target analytes ................................ .............................. 1 3 Figure 1 3. The schematic diagram of diamond films integrated SAW frequency filter ................................ ................................ ................................ ............... 15 Figure 1 4. Structure of p n junction and working principle of LED .............................. 17 Figure 1 5 Component construction of a typical plasma enhanced CVD system .......... 21 F igure 1 6 A p h oto of Astex CVD system ................................ ................................ ...... 24 Figure 1 7 Working mechanism of Raman scattering ................................ .................... 26 Figure 1 8 The orbital jump of electrons with respect to energy change ....................... 2 6 Figure 1 9 A typical Raman spectrum for polycrystalline diamond film ....................... 2 7 Figure 1 10 Nanocrystalline diamond image of AFM ................................ ...................... 30 Figure 1 11 XRD pattern of intri nsic nanocrystalline diamond syn thesized at 110Torr ................................ ................................ ................................ ......... 32 Figure 1 1 2 Penetration depths of various signals beneath the surface ............................ 3 3 Figure 1 13 SEM image of intri nsic nanocrystalline diamond syn thesized at 100Torr ................................ ................................ ................................ ......... 3 4 Figure 2 1 Raman spectra for polycrystalline diamond with various H 2 flow r ate ........ 4 2 Figure 2 2 Morpholog ical images of polycrystalline diamond by SEM ........................ 4 3 Figure 2 3 SEM m orphology of polycrystalline diamond synthesized with low H 2 flow rate ................................ ................................ ................................ ... 4 4
vi Figure 2 4 XRD pattern of polycrystalline diamond film ................................ .............. 4 5 Figure 2 5 Raman spectrum for nanocrystalline diamond ................................ .............. 4 8 Figure 2 6 Morphology of nanocrystalline diamond by SEM ................................ ........ 4 9 Figure 2 7 AFM images of nanocrystalline diamond film measured by non contact mode ................................ ................................ ................................ 50 Figure 2 8 XRD pattern of nanocrystalline diamond film ................................ .............. 51 Figure 2 9 Raman spectrum for diamond like carbon ................................ ................... 5 4 Figure 2 10 Morphology of diamond like carbon by SEM ................................ .............. 55 Figure 2 11 Topography of diamond like carbon by AFM ................................ .............. 5 6 Figure 2 12 Image of Si nanowi res by SEM ................................ ................................ .... 60 Figure 2 13 Image of nanocrystalline diamond nanowires by SEM ................................ 61 Figure 2 14 Optical image of graphene flake made by CVD ................................ ........... 63 Figure 2 15 Raman spectrum for graphene ................................ ................................ ...... 6 4 Figure 2 16 Image of graphene by SEM ................................ ................................ ........... 6 4 Figure 3 1 Raman spectra of nano diamond grown at same pressure ............................ 72 Figure 3 2 SEM images of nano diamond grown with various argon ............................ 74 Figure 3 3 XRD patterns of nano diamond grown at 120 HPa ................................ ....... 75 Figure 3 4 The comparison of Raman spectra under various pressu res ......................... 77 Figure 3 5 M icrographs of nanocrystalline diamond films grown with 180sccm Ar at 90 H p a ................................ ................................ ................................ ... 7 8 Figure 3 6 Micrographs of nano diamond films grown with 180sccm Ar at 1 00 HPa ................................ ................................ ................................ .......... 78 Figure 3 7 Micrographs of nano diamond films grown with 180sccm Ar at 110 HPa ................................ ................................ ................................ .......... 79 Figure 3 8 Micrographs of nano diamond films grown with 180sccm Ar at 120 HPa ................................ ................................ ................................ .......... 80
vii Figure 3 9 XRD patterns of nano diamond grown at various pressures ........................ 82 Figure 4 1 Raman spectra of nano diamond for three different concentrations o f N 2 ................................ ................................ ................................ .............. 89 Figure 4 2 Comparison of intrinsic and 5% nitrogen doped nano diamond .................. 90 Figure 4 3 Micrographs of N doped nano diamond films grown at 120 HPa ................. 91 Figure 4 4 Micrographs of 5% N 2 doped nano diamond films grown at 130 HPa ......... 92 Figure 4 5 Micrographs of 10% N 2 doped nano diamond films grown at 110 HPa ....... 92 Figure 4 6 Micrographs of 15% N 2 doped nano diamond films grown at 100 HPa ....... 93 Figure 4 7 XRD patterns of nano diamond grown at various N 2 concentrations ........... 94 Figure 4 8 I V characteristics for various interfaces ................................ ...................... 98 Figure 4 9 SEM image s of NNCD films for (a) NNCD/Si, (b) NNCD/NCD/Si (c) NNCD/S O 2 /Si ................................ ................................ ........................ 102 Figure 4 10 AFM topograghic images of NNCD films for (a) NNCD/Si, (b) NNCD/NCD/Si, (c) NNCD/SiO 2 /Si ................................ ........................... 103 Figure 4 11. XRD p attern of NNCD films for different layer structures ........................ 105 Figure 4 12. Raman spectra of three layer structures ................................ ...................... 106 Figure 4 13. Raman spectrum of nanocrystalline diamond film o n q uartz ..................... 107 Figure 5 1 Image of HFCVD 008 system ................................ ................................ .... 115 Fi gure 5 2 Raman peaks of HFCVD poly crystalline diamond at four corners of specimen ................................ ................................ .................... 117 Figure 5 3 Raman peaks of HFCVD poly crystalline diamond at center and corners of specimen ................................ ................................ .................... 11 8 Figur e 5 4. Raman peaks of MPECVD poly crystalline diamond films ........................ 119 Figure 5 5 MPE CVD polycrystalline diamond growth evolution ................................ 121 Figure 5 6 Two and three dimensional AFM images of HFCVD p oly crystalline dia mond film ................................ ................................ ...... 12 3
viii Figure 5 7 Two and three dimensional AFM images of MPE CVD p oly crystalline diamond f ilm ................................ ................................ ...... 1 24 Figure 5 8 XRD patter n of MPE CVD diamond film ................................ .................... 125 Figure 6 1 Schematic structure of a CMUT unit cell ................................ ................... 128 Figure 6 2 A CMUT constructed by connecting units in series and parallel way ................................ ................................ ................................ .............. 1 29 Figure 6 3 One unit cell of SiO 2 pattern by first mask ................................ ................. 1 33 Figure 6 4 First mask pattern after removal of exposed photoresist ............................ 1 33 Figure 6 5 Unit cell with an opening w in dow by second mask ................................ .... 134 Figure 6 6 Unit cell with removal of photor esist on the anchors ................................ 135 Figure 6 7 The unit cell with released diamond m embrane ................................ ......... 137 Figure 6 8 The patterned alu minum electrode ................................ .............................. 139
ix ABSTRACT The studies covered in t his dissertation concentrate on the various forms of diamond film s sy n thesized by chemical vapor deposition (CVD) method, including microwave CVD and hot filament CVD According to crystallinity and grain size, a variety of diamond forms primarily includ ing micro crystalline (most commonly referred to as polycrystalline) and nanocrystalline diamond fil ms, diamond like carbon (DLC) films were successfully sy nthesized. The as grown diamond films were optimized by changing deposition pressure, volume of reactant gas hydrogen (H 2 ) and carrier gas argon (Ar) in order to get high quality diamond films with a smooth surface, low roughness, prefer r ed growth orientation and high sp 3 bond ing contents etc. The characteriz ation of diamond films w as carried out by metrological and analytical techniques such as scanning electron microscopy (SEM), X ray diffraction (X RD), atomic force microscopy (AFM) and Raman spectroscopy T he results of characterization served as feedback to optimize experimental parameters, so as to improve the quality of diamond films A good understanding of the diamond film properties such as mechanical, electrical, optical and biological properties, which are determined by the qualities of diamond film s is required for the selection of diamond films for different applications. The nanocrys talline diamond nanowires grown by a combination of vapor liquid solid (VLS) method and CVD method in two stages and the graphene grown on silicon substrate with nickel catalytic thin film by single CVD method were also investigated in a touch on level.
x Microwave plasma enhanced chemical vapor deposition (MPE CVD ) polycrystalline diamond films were deposited with mixed gas CH 4 and H 2 at flow rate ratio 1:100 by changing the H 2 volume from 100sccm to 3000sccm. SEM micrographs revealed that the samples of 10 0sccm and 500sccm H 2 had agglomerates of a cauliflower like surface, whereas samples with 1000sccm to 3000sccm H 2 had a faceted surface ; the size of the faceted crystallites ranged from 200nm to 1500nm Raman spectra indicated that the samples of 100sccm H 2 contain a certain amount of graphitic phase, whereas samples of more than 1000sccm H 2 had a concentration of crystalline diamond. The XRD pattern s highly exhibited the crystallinity of deposited diamond with largely (111) and (220) plane ; (220) plane accounted for three times more than (111) plane in the whole deposited surface. MPECVD nanocrystalline diamond films displayed a nano peak at 1140cm 1 in the Raman spectrum, and showed 100nm global particles on the surface in SEM image s, pre sented (111) and (220) planes with a low ratio of 1:2 in the XRD pattern. Three dimensional AFM images provided consistent grain size with that of the SEM image s, and 9 4 nm average roughness i Diamond like carbon (DLC) did n o t exh ibit any sharp peaks in the Raman spectrum, and only showed broad bumps in the position of D band and G band, illustrating that there was no crystalline structure formed on the surface. SEM disclosed a rough surface with scattered particles embedded in the tiny boundary like ditches. The morphologies of nanocrystalline diamond films changed from scattered globa l particles to texture with the Ar flow rate varying from 170sccm to 200sccm under 120 HPa constant pressure. XRD pattern displayed similar height of (111) and (220)
xi plane. In another condition, keeping the Ar flow rate at 180 sccm constantly a sample of 110 HPa pressure presented a smoother surface, and the (111) plane was the primary structure of surface with the (220) plane dematerialized and a (311 ) plane generated. Nitrogen doped nanocrystalline diamond films, with 5%, 10%, 15% N 2 flow rate variation, were investigated, and it turned out with the increase of N 2 the nano peak in the Raman spectra dematerialized the XRD pattern revealed that the (111) peak became the major component rather than the (220) peak ; a sample of only 10% N 2 displayed texture structure. 15% N 2 doped nanocrystalline diamond films grown on SiO 2 demonstrated a smooth surface, high growth rate, and high (111) peak, indicat ing the SiO 2 layer changed the surface electron density and, therefore, changed the quality of the diamond film. The surface residual stress evaluated by the Raman shift showed that HFCVD polycrystalline diamond film had tensile stress at the corner and compressive stress in the center, whereas MPECVD polycrystalline diamond film had compressive stress on the whole surface. The roughness of MPECVD was higher than that of HFCVD. A capacitive micromachined ultrasonic transducer (CMUT) with a diamond membrane was successfully fabricated by overcoming the challe n ges such as diamond window etching, two stage diamond depositing, and vacuumed cavity formation.
1 C HAPTER 1 : INTRODUCTION TO CVD DIAMOND 1.1 Overview of CVD D iamond Electroni c semiconductors are believed to have been experienced in three generations since the 20 th century. Silicon is considered the first generation semiconductor, which changes the world by the role it plays in modern life such as computer chips, data storage hard drives, cell phones, music players, communication device s and so on. The second genera tion semiconductor s are believed to be gallium arsenide (GaAs) and indium phosphide (InP), which contribute to the revolution of wireless and information communication, and are especially important in military applications The third generation semiconduct ors such as silicon carbide (SiC) and gallium nitride (GaN) ar e characterized by wide band gap and are widely employed in electronic and optoelectronic industries. Blossom ed three decade s ago, diamond films were expected to be a future generation semicondu ctor by its many superior properties not only limited to electronic and electrical applications but also noticeable in their mechanical, thermal, chemical, physical and optical properties, as well as a wide range of applic ations especially in high temper ature or harsh environment s T hough several research works on diamond remain in the early laboratory stage, diamond has already attract ed much attention from the electronic, mechanical engineering, chemical and biological engineering, health care and medical device industr ies 
2 Artificial bulk diamond first appeared in the 1950s and developed under high pressure and high temperature conditions (referred to as HTHP technique) It has been primarily used in cutting, grinding and polishing tools . Since the early 1980s, the chemical vapor deposition method of growing diamond films from gas mixture has been subject ed to intensive worldwide research. The differe nce between the CVD method and the HTHP technique is that the CVD method yields diamond films on the substrates of various materials rather than bulk diamond, and it does not require high pressure, thus the CVD process is simple, rela xe d and can be easily carried out for laboratory research  With various reactant gas recipes and ratios, different forms of carbon mate rials can be made using the CVD technique such as diamond like carbon (DLC), polycrystalline diamond, intrinsic nanocrystalline diamond and extrinsic nanocrystalline diamond including p type or n type diamond Diamond like carbon is an amorphous carbon mixture of cubic lattice and hexagonal lattice without long range crystalline order. The content of various nano scale structures can be obtained by th e variation of the gas reactants (CH 4 :Ar at the ratio of 5%: 95% for the CVD method) from cubic dominated to hexagonal dominated, thus variable desired properties of hardness and tribology can be achieved. The primary desirable applications of DLC carbon are based upon the properties of hardness, wear resistance and slickness. There are two main factors that will determine the quality of the DLC f ilms. One factor is the amount of cubic lattice content, which the valence carbon atoms hybridize sp 3 bonding and form a four C C covalent b ond known as tetrahedral amorphous carbon. Another structure is graphite of sp 2 bonding which valence electron s dis tribut e in the normal state of the carbon atom. 100% sp 3 indicates the hardest and
3 100% sp 2 indicates the slickest due to the fact that the length of the sp 3 bond is significantly less than the length of the sp 2 bond. Fur ther details will be given in chapt er 1.2.1 The second factor that affects the quality of DLC films is the fractional content of hydrogen atoms and carbon hydrogen bond ing s ( C H ) that result from the carbon source gas, methane (CH 4 ). The large amount of concentration of hydro gen atom s and C H bond ing s in DLC will degrade the quality almost as much as the residual sp 2 bonded graphit ic carbon does  Since sp 3 dia mond is the hardest known material in the world it is used to make stylus for hardness measurement equipment such as a nanoindentation tester, or the stylus of a profilometer for the measurement of thin film thickness. DLC coatings are also applied to the edge of the cutting tools, the bits of the mining drills etc. The superior anti wear property of sp 2 graphite is utili zed to make pencil s solid lubricants and so on. DLC films of sp 3 tetrahedral d i a mond serve as a good insulat or in electrical and electronic applications. On the contrary, sp 2 graphite is a good conductor by a special mechanism of hopping conductivity that elect r ons move in q u antum mechanical tunneling between pockets of conductive material isolated in an insulator. Such a process characterizes graphite as a semiconductor; the electron emission s under high voltage and vacuum conditions make diamond electrodes a field emission device  Polycrystalline diamond (PCD) consists of grain sizes from several hundred nanometers to several micrometers according to the gas recipe CH 4 /H 2 at a ratio from 1/100 to 5/100 with different growth rate and sp bonding composition. One of the remarkable features of polycrystalline diamond is its high growth rate (1 and a freestanding polycrystalline thick serv ing as output windows for
4 high power microwave and laser radiation sources was reported thick polydiamond was reported as having identical optical emission propert ies to that of ideal single cry stalline diamond . The rou gh surface of the PCD films has grain sizes at the order of micron; this is the main limiting factor of diamond films in most electronic, optical, biomedical and tribological applications [7 9]. Nanocrystalline diamond (NCD) film s which possess smooth surface and high sp 3 content, have therefore been recognized as the promising candidates for such applications . As evident from its name, n anocrystalline diamo nd film consists of diamond grains in a range of size from several nanometers to several te ns of nanometers. There is no strictly defined grain size between polycrystalline and nanocrystalline diamond. Due to the extremely high density of nano grain size, nanocrystalline diamond has superior outstanding properties compared with polycrystalline d iamond including its extreme hardness, low friction coefficient, chemical inertness and high thermal conductivity [11, 12]. The protocol of nanocrystalline diamond film synthesis by plasma enhanced chemical vapor deposition is as follows: the reactant gas chemistry recipe in flow rate is Ar/CH 4 /H 2 at a ratio of 98/1/1; the deposition temperature is above 700C ; the pressure is in the range of 90 HPa to 140 HPa Intrinsic nanocrystalline diamond has a high concentration of sp 3 bonding, thus high resistivity In order to accomplish high conductiv ity of extrinsic diamond films for electronic application s there are two types of doped diamond films that are fabricated, i.e. p type and n type diamond films. P type diamond films are usually made in a hot filament CVD system and the dopant element is mainly boron. Conve n tional boron sources commonly used are triethylborane [B(C 2 H 5 ) 3 ] and diborane (B 2 H 6 ) [ 13 14 ]. In
5 the past decade, a new approach of boron sup ply for p type diamond films was reported Herei n, the boron source was obtained by forcing an H 2 stream through a bubbler containing B 2 O 3 dissolved in methanol (CH 3 OH). The system control s boron concentration by using a flow controller for the gas inlet and the H 2 and B 2 O 3 /CH 3 OH/H 2 flows are controlle d in order to achieve the desired B/C ratios . N type diamond films are generally made both in hot filament and microwave CVD system s, and dopant element sources are largely molecular nitrogen gas or phosphorus contained gas such as phosphine (PH 3 ) [1 3,16,17]. 1.2 Structure s Properties and A pplications 1.2.1 Crystalline S tructure of CVD D iamond A well known fact is that structure determines the properties of materials, including mechanical, electrical, physical and chemical properties, and eventually determines the engineering performances of materials. For instance, both diamond and graphite are composed of carbon, but differ in their structure s by various bonding type s and arrangement s of the atoms. Diamond is sp 3 bonding and has a face centered cubic structure, whereas graphite is sp 2 bonding and has a hexagonal structure. As a result, diamond serves as an abrasive because of it s great hardness, whereas graphite is used as a lubricant. Diamond is the high pressure polymorph, and hence has a denser structure than graphite. The crystalline structure of diamond can be briefly described as a spe cially arranged face centered cubic ( F CC ) structure. To better understand diamond structure and its origin, a basic structural unit, tetrahedral, has to be introduced. A t etrahedral structure
6 is re presented by one atom covalent bonded with the other four atoms with 109.5 in three dimensional space (Fig.1 1 ) If four atoms are added interior to the FCC unit cell at (1/2, 1/2, 1/2) positions, each of the extra atom s is tetrahedrally coordinated with the other four atoms in FCC unit, and zinc blende structure (also called sphalerite) is formed. Zinc blende structure is named after the exact mineralogical term for zinc sulfide (ZnS) ; the four extra added atoms re present zinc ions and one corner ed atom and three centered atoms in the FCC unit cell represent four sulfuric ions (Fig.1 1 ) S imilar compounds of equivalent structures also include silicon carbide (SiC), gallium arseni de (GaAs) and zinc telluride (ZnTe) Diamond cubic crystal structure is a variant of the zinc blende structure, in which carbon atoms replace both Zn and S atoms and occupy all positions as indicated in unit cell in Fig.1. Each carbon atom bo nds to four other carbons, one is at corner and three are at the centers of faces in FCC unit. All C C bonds are covalent. Similar elements with equivalent structure a lso include group elements in periodic table such as germanium (Ge) and silicon (Si)  The different type s of bonding fo u nd in diamond and graphite are ascribed to the vari ous type s of e lectron distribution around carbon. The atomic number of carbon is six (C 6 ) ; it has six electrons outside it s nucleus. These six electrons configure in s hell subshell designation. According to the Pauli E xclusion P rinciple, each subshell can hold no more than two electrons, which must have opposite spins. Therefore, s, p,d and f Tetrahedral structure Zinc blende structure diamond cubic crystal Figure 1 1. Formation of diamond cubic crystal structure
7 subshell may each accommodate respectively, a maximum of 2, 6, 10 and 14 elec trons. In most cases, the electrons preferentially fill up the lowest energy state (referred to as ground state) in the shell and subshell, two electrons having opposite spins per state. The normal electron configuration of the carbon atom is 1s 2 2s 2 2p 2 wit h four valence electrons (the number of electrons that occupy the outermost shell) It should be noted that p subshell has three electron states ; only two of them are half occupied, thus it has one unfilled 2p electron state which implies an unstable electron configuration. However, under special circumstances, electron transition s to higher energy states are possible to form a stable bonding structure. For instance, the s and p orbitals combine to form hybrid sp n orbitals, where n indicates the number of electrons contained in p orbital, which may have a value of 1, 2, or 3. The driving force for the formation of hybrid orbital is a lower energy state for the valence electron bonding [ 18, 19] In diamond the sp 3 hybrid bonding has an electron configur ation of 1s 2 2s 1 2p 3 In the current research, graphite and diamond are mostly concerned and the corresponding s p 2 and sp 3 bonding can be describ ed in terms of electron occupation in orbital Carbon sp 2 bonding (graphite) : 1s 2 2s 2 2p 2 1s 2 2s 2 2p 2 Carbon sp 3 bonding (diamond): 1s 2 2s 1 2p 3 1s 2 2s 1 2p 3 Figure 1 1a. The electron distribution in sp2 and sp3 bonds of carbon atom
8 Further explanation of orbi t al hybridization in perspective of chemistry, especially for paraffin compound family (C n H 2n+2 ), can be found a little different from this due to the C C bonding, C=C ( bond) and C H ( bond) bonding s involved  1.2.2 Properties and A pplications Diamond is well known as the hardest material in the world. In addition to that, diamond has numerous outstanding properties such as mechanical, thermal, optical, electronic, electrochemical and biocompatible properties presented by modulus, l ow coefficient of friction, anti wearing, high thermal conductivity, high refractive index, high dielectric constant, chemical inertness and biological compatibility that make it a promising material in many applications of industr ies. Each aspect of diamond propert ies corresponds to a certain series of applications that can be applied to industr y medical health, civil life, etc. Remarkable mechanical properties are characteristic of diamond as a conventional material including specific values of the hardest hardness (10,000 kg/mm 2 ), high tensile and a low coefficient of friction (0.05 dry)  Diamond is extreme ly hard, so it is an ideal material used for cutting tools such as scalpe ls, knives and tip s of mineral drills. On the other hand, the thermal cond uctivity of diamond is about 20 W/cm K at room temperature ; this value is b elieved to be the highest in any known materials and is significant to heat ing dissipation in cutting tool application s [21 22 ] Conventional cutting tool materials are high speed steel and tungsten carbide (WC) which are not suitable for new materials such as Al Si alloys, metal matrix composites and fibre
9 reinforced plastics [2 3 ] During th e high speed cutting process, the friction between the cutting edge and workpiece generates a large amount of heat ; the high temperature that result s from this friction softens the workpiece material that will influence the precision of the parts, and form s a strong adhesion to the surface of the cutting edge. Diamond is expected to alleviate this problem, increas ing productivity and improv ing the quality of the machined surface due to it extr a o r dinary mechanical properties and high thermal conductivity as mentioned above [2 4 ] Diamond, with its special tetrahedral crystal structure, is not readily coated on classical substrate materials. The most favor ed material for diamond coating is Co cemented tungsten carbide (WC Co) which can form strong adhesive bon ding with diamond The element Co has a catalytic effect that induces carbon diffusion into substrate and forms a graphite layer that weak en s the diamond coating. Chemical solutions can be applied to reduce the Co effect . Alternative solutions also include the insertion of interlayer materials such as amorphous carbon, metallic materials and ceramics [2 6 ]. The tribological property of low friction coefficient (0.12 0.2) of diamond make s it a good choice of solid lubricant in between surfaces. These films could find application s as ultrathin anti friction and anti wear protective coatings, hydrophobic coatings, gas diffusion barriers and dielectr ic layers in electronic devices [2 7 ] The t hermal conductivity of diamond is ranged from 10 20 W/cm K dep ending on the impurity content and crystalline defects on the surface. Copper, the most commonly used metal for heat dissipation, has a conductivity of 4 W/cm K. By comparison, diamond is believed to be the highest thermal co nducting material in the indust ry. This characteristic finds its application in semiconductor devices and circuit s as
10 a heat sink to avoid the efficiency loss of devices due to heat generation. Heat is believed to be transferred in solid by charge particles, electrons, and vibration of phonon. For diamond, the decrease of impurit ies increase s the thermal conductivity and causes less grain boundary and thicker film result ing in higher thermal conductivity [21, 22, 28]. Other applications of diamond utilizing its excellent thermal properties include heat exchanger, heat spreader, even in a dosim etric medical device that count s the number of the released photons on the sample during the heat delivering process . Each material has its own characte ristic wavelength that determines its optical property For instance, people standing under sunlight can see their shadows, indicat ing that sunlight can not transmit through the human body. B ut when people get a physical check in the hospital, an X ray can easily go through body. Diamond s with different structures and grain sizes (nanocrystalline, polycrystalline and amorphous) ha ve their own characteristic wavelength. N anocrystalline has a wavelength of 546nm, polycrystalline 552nm and amorphou s diamond 557nm according to the Raman shift calculation using the equation, D (cm 1 ) = (1/l o 1/l R ), Eq. 1 where D is the Raman Shift in units of wavenumber (cm 1 ) l o is the laser wavelength which is 514nm in the Renishaw Raman Spectro photometer and l R is the Raman radiation wavelength. DLC films are typically transparent in the infrared region (wavelength 700nm ~ 106nm), weakly absorbed in the visible spectrum (wavelength 380nm ~ 750nm), and increasingly absorbed with decreasing wavelength in the U V (wavelength range 10nm ~ 400nm). Wavelength spectra are not exactly defined, so an overlap can be seen in
11 different sections. The refractive index of diamond is 2.4, which has been found to be dependent on the preparation, hydrogen content and CH bonding of the films, and can be adjusted from 1.7 to 2.4 due to growth conditions. A higher index of refraction usually indicates DLC with stronger crosslinking, greater hardness, and better wear resistance [30 32]. Due to its IR transparency, DLC can be used f or optical applications, such as antireflective and scratch resistant wear protective coatings for IR optics Besides their application as protective optical coatings, diamond like carbon films can be used for the fabrication of optical components. Using anisotropic O 2 RIE etching in combination with hard masks such as SiO 2 or Al, patterns with well defined rectangular profiles can be obtained. In combination with the IR transparency of the films, this enables the recording of IR diffractive optical compon ents with good control of surface and pattern quality [ 30, 33 ] Base d on a similar principle, other optical applications of diamond include infrared windows, lenses, A TR units, X ray windows, etc. CVD diamond films, particularly heavily boron doped diamon d films, are being used increasingly in electrochemi cal research because of the excellent electrochemical properties exhibited such as high conductivity, sensitivity and reproducibility, low background currents, chemical inertness and biocompatibility when immobilized and functionalized with biological substances. Applications utilizing electrochemical properties of diamond include electrodes, electro chemical detectors, bio chemical sensors, and waste water treatments. Diamond electrodes have an advantage over the conve ntional electrode materials that are easily poisoned or corroded, and in addition, the
12 wide range of operating potentials increase the versatility of application especially in harsh environments. One challenge in diamond biosensor application (electroanalysis) is its selectivity to ward the target analyte. A general approach to solv ing this problem is to chemically functionalize the electrode surface in order to incorporate a selective chemical that respon ds to the analyte concerned. The strategies can be divided into the physical adsorption of the electrode modifier onto the carbon surface, or the covalent binding of the species concerned . A d iamond group in Argonne National Lab claimed that a new innovative method has been developed to construct hybrid organic inorganic interfaces on conducting diamond thin films. The UNCD films are immersed into a special chemical solution and voltages are appl ied then radicals of the solution react with the diamond surface to form strong carbon carbon bonds. Th ese C C bonds attract biomolecules such as proteins that are covalently bound in a process called functionalization  Small biologica l molecules such as amino acids, DNA are considered building blocks of molecular engineering on surfaces due to their inherent nature for molecular recognition and self assembly. The interaction between molecules and solid surfaces is significant to u nderstand the protein surface bonding and the development of bioan alytical devices as well as biocompatible materials. Figure 1 2 is a n example of a diamond cantilever biosensor that measures the frequency response
13 Figure 1 2. Prototype biosensor of diamond cantilever functionalized with crosslinker to detect target analytes Receptor biomolecules, such as antibodies, are attached to a micro cantilever made of ultrananocrystalline diamond thin film (UNCD). The cantilever is vibrated by an electrical field. W hen the sensor is exposed to a gas or liquid containing biological toxins (targeted analytes) t he toxins are selectively captured by the receptor biomolecules, which makes the cantilever heavier and changes its vibration frequency. Different biological toxins and other biomolecules can be detected by attaching different receptor molecules to the cantilever. CVD diamond film is expected to be an ideal material for radiation detectors or dosimeters because of its excellent features such as high radiation resistance, low leakage current, hi gh operati ng temperature, and high radiation stability The high resistivity and high band gap of diamond allows for a simple structure and extremely low
14 number of free carriers that will result in very low noise and power dissipation Different thicknes s of intrinsic diamond or extrinsic diamond with various degree of doping can be utilized to ray and non ionizing UV radiation. The thickness of polycrystalline diamond free standing window can be as much as several h undred micrometers [36, 37] Among all materials, diamond possesses the highest surface acoustic wave (SAW) velocity which is about 18,000 meters/second in longitudinal elastic wave [ 38 ] Longitudinal wave is an elastic wave such that the direction of displacement at each point is normal to the direction of wave propagation. This feature can be u tiliz ed to fabricate surface acoustic wave (SAW) devices such as GHz band signal filters optical and telecommunications even biosensors The brief principle of the SAW filter is to convert an input electric radio frequency (RF) signal to a SAW at the input interdigital transducer (IDT) which propagates along the surface to the output IDT and then reconvert s it to an output electric RF signal there. In this pr ocess, the signal frequency that can be effectively converted from the input RF signal to the SAW and effectively reconverted from the SAW to the output RF signal is determined by f = v / r (where v is the phase velocity of the SAW and r is the wavelength: w avelength = electrode width 4), so that only this frequency signal can pass through this device. The function flow chart is as follows : Input RF signal IDT SAW IDT output RF signal Because the freq uency is determined by the velocity of SAW and the wavelength that can be obtained by adjusting the electrode width, in order to get the expected high frequency, to find the high velocity material such as diamond film for the SAW substrate
15 can offer the be st solution for this approach. Figure 1 3 shows the schematic diagram of diamond films integrated SAW frequency filter. Figure 1 3 The schematic diagram of diamond films integrated SAW frequency filter As evident in figure 1 3 a typical layer structure of commercial product of a diamond SAW device is the IDT/ZnO/diamond/Si structure. IDTs can be aluminum, gold, sil ver, titanium, copper etc. The commonly used materials for piezoelectric layers can be zinc oxide ( ZnO ) silicon dioxide ( SiO 2 ) aluminum nitride ( AlN ) or gallium nitride (GaN) and lithium niobate (LiNbO 3 ) as well as lithium tantal a te (Li Ta O 3 ) for surface acoustic wave A typical high temperature pie zoelectric material is langasite (lanthanum gallium silicate, also referred to as LGS in literatures), which can work at 1400 C and match the excellent high temperature performance of diamond [39 41]. S ome other novel properties of diamond that have been reported in prestigious academic journal s such as Science or Nature, but have not been fully investigated are superconductivity at low temperature and light emitting diode (LED) of diamond Piezoelectric Material Film (ZnO) Substrate CVD Diamond Film IDT SAW Wavelength Output RF Signal Input RF Signal
16 Diamond is well known as an insulator. Boron has one less electron and a smaller atomic radius than carbon, and is easily incorporated into diamond. As boron acts as a charge acceptor, boron doped diamond present s a p type semiconductor i.e. it is featured by hole carriers. When boron doped diamond was synthesized at high pressure (nearly 100,000 atmospheres) and temperature (2,500 2,800 K) superconductivity was discovered below the transition temperature at 4k. The discovery of superconductivity in diamond structu red carbon suggests that Si and Ge, which are locate d in the same group in the periodic table and have a similar diamond structure, may similarly exhibit superconductivity under the appropriate conditions  LED is an acronym for light emitting diode. When a semiconductor diode which is a p n junction in phy sical nature, is forward biased (switched on), a current is generated when electrons recombine with holes In the meantime energy release s in terms of photons and light with different color (wavelength) is emitted This effect is called electroluminescence and the color of light (commercially red, green and blue) is determined by the energy gap of the semiconductor. In a diode, the current flows easily from the p side to the n side, but not in the reverse direction. LEDs present many advantages over incandescent light sources including lower energy consumption longer lifetime improved robustness, smaller size, faster switching and greater durability and reliability  Figure 1 4 shows the structure of the p n junction and the working principle of LED.
17 Figure 1 4 Structure of p n junction and working principle of LED M ost light emitting elements are locate d in the main groups II to V in the Periodic Table. The larger the band gaps of these insulating materials, the shorter the wavelength of the emitted light is. Shorter wavelengths are desirable because they potentially allow greater data storage, but suitab le materials at these wavelengths are harder to find. Blue lasers are thus a much greater challenge than red or green ones GaN can emit blue laser s at wavelengths below 450 nm at room temperature . For light emission at even shorter wavelengths, diam ond is a potentially promising material because of its large band gap (about 5. 47 eV). Koizumi et al report a diamond on diamond (homoepitaxial) pn junction made by CVD that emits at 235 nm which is locate d in the ultra violet spectrum The pn junction was formed from a boron doped p type diamond layer and phosphorus doped n type diamond layer grown epitaxially on the ( 111 ) surface of a single crystalline diamond. The pn junction exhibited good diode characteristics, and at a forward bias of about 20 vol ts strong ultraviolet light
18 emission at 235 nanometers was observed and was attributed to free exciton recombination  There are two types of band gap in semiconductor materials : direct band gap and indirect band gap. If the minimal energy state of conduction band has the same value of k vector as the maximal energy state of covalence band, this band gap is defined as direct band gap. K vector is crystal momentum that describes the electrons (waves) in crystal lattice. Theoretically only semiconductors having direct band gap can be ma de to emit light. Diamond is not direct band material. The ultraviolet light emi ssion of a diamond pn junction was ascribed to the recombination of free exciton  which is a n electrically neutral electron hole pair that exists in semiconductors, insulators or liquids. Ex c iton can transport energy without transporting net electric charge . As stated above, structure deter mines properties, and properties determine applications; diamond film by chemical v apor deposition method is an amazing material that has many excellent properties that cause it to have a few potential, novel application s in almost all walks of life. Of course, there are still several challenges that need to be overcome before diamond films achieve their market value. Further research should be conducted in order to make practical application of diamond films to better serve the li v e s of human being s 1.2.3 Research O bjective s A good understanding of the diamond film properties such as mechanical, electrical, optical and biological properties, which are determined by the qualities of diamond films, is significant to the choice of diamond films for different applications.
19 One of the most important approaches to grow ing diamond films is microwave plasma enhanced chemical vapor deposition method (MPECVD). In the current dissertation, u sing MPECVD method, diamond films with different crystallite sizes and structures such as amorphous diamond, polycrystalline diamond an d nanocrystalline diamond were synthesized in order to get different properties for various purposes of application. Intrinsic diamond is a good insulated material, by adding the dopant elements, typically boron and nitr ogen or phosphorus, p type and n type semiconductor diamond films with certain conductivities can be fabricated for electronic devices. T he characterization of as grown diamond films have been carried out by Raman spectroscopy, scanning electron spectrosco py (SEM), X ray diffraction (XRD) and atomic force microscopy (AFM). The optimized parameters have been obtained to grow the expected high quality films according to the results of characterization techniques. For nitrogen doped diamond films, the electric al properties, ohmic contact and surface fea tures on SiO 2 layer were investigated. Residual stress analyses of polycrystalline diamond films by HFCVD and MPECVD were evaluated by Raman spectra. T he application part is mainly about the design and fabrication of capacitive micromachined ultrasonic transducer with integrated diamond membrane 1.3 Growth of D iamond Chemical Vapor Deposition Method Chemical Vapor Deposition ( CVD) is a method of growing semiconductor thin film s, such as silicon carbide ( SiC ) silicon dioxide ( SiO 2 ) silicon nitride ( Si 3 N 4 ) gallium nitride (GaN), and diamond (C) etc on a substrate by the reaction of vapor phase chemicals that contain the required constituents. The structural forms of the films
20 deposited by the CVD method can be amorphous, polycrystalline and monocrystalline. The reactant gases are activated by various energy forms such as thermal, plasma and reacted on and/or above the temperature controlled su rface to form the thin film. The reactive gas species of intrinsic diamond film include methane (CH 4 which is a carbon source ) hydrogen (H 2 ) and argon (Ar); s ometimes, donor gas such as nitrogen (N 2 ) or pho phine (PH 3 ) is introduced for n type diamond fil ms whereas diborane ( B 2 H 6 ) or boron trioxide ( B 2 O 3 powder, mixed with methanol or aceton e to be volatile) is introduced for p type diamond films Changing the gas recipe results in different forms of diamond films such as amorphous diamond (CH 4 :Ar at 5%:95%), polycrystalline diamond (CH 4 :H 2 at 10%:100%) and nanocrystalline diamond films (CH 4 :H 2 :Ar at 1%:1%:98%). By regulating the process conditions, such as the gas recipe introduced, pressure of the reaction chamber, temperature of the substrate and th e way the plasma generated different qualit ies and forms of diamond films can be grown on the silicon substrate A typical c hemical v apor deposition system mainly include s four sect i ons: reactor gas supply, power energy supply ( microwave plasma or therm al hot filament ), vacu um and exhaust ion and heating and cooling part s Figure 1 5 illustrates the major components of a typical CVD system. The g as supply section is composed of cylinders of various reactant gases (such as argon, hydrogen, methane and nitrogen), stainless pip e lines, mass flow controllers and several valves. Its function is to offer gases required for chemical reaction at a certain flow rate in order to achieve high quality diamond film formation. Among them, methane is the carbon source gas. Hydrogen is the plasma precursor and argon can adjust the
21 pressure in the chamber. The stainless steel chamber is the place where deposition happens. Concerned with the chemical reaction process, the chamber is also referred to as the reactor. Figure 1 5 Component construction of a typical plasma enhanced CVD system The m ass flow controller (MFC) can offer a desired gas flow rate in a unit of sccm which means standard cubic centimeters per minute. Sometimes a unit of slpm is also used, which means standard liters p er m inute ; o ne s lpm equals to 1000 sccm. Gas and liquid can be measured in volumetric or mass flow rates such as liters per second or kilograms per second. Two measurement s can be converted by the density of material. There are two types of mass flow controller s: digital and analog; the digital mass flow controller is able to measure more than one type of gases without calibration whereas the analog mass flow controller can only be valid for one specific gas. The mass flow controllers in our lab are analog so they are for specific gas es Each mass flow controller H2 N2 CH4 Ar MFC MFC MFC MFC Plasm a RF generator Exhaust P um p Hot water Cold water Sample Stage Stage Heate r
22 has its maximum value (for instance 100sccm) and its percentage scale range from 0 to 100%. The real flow rate is calculated by the readout value (percentage value) times the maximum value. For example, when t he readout value is 50 (50%), and the maximum value of this mass flow controller is 30sccm, the real mass flow rate is 30 sccm times 50%, i.e., 15sccm. The flow rate of the analog mass flow controller is specified as a percentage of its calibrated full sca le flow rate. The m ass flow controller is calibrated with nitrogen N 2 gas, because the k factor of other gases is nearly linear to N 2 In order to use the specific mass flow controller for another gas, a calculation has to be made according to the k factor charter in the manual of the mass flow controller. The s econd part of the CVD system is the energy supply that is main ly a n RF gene rator to produce plasma. Plasma is highly ionized gas (or a mixture of several gases) composed of ions, electrons and neutr al particles, that is generated by micr owave radiation from normal gas; it is believed to be the fourth state of matter, in sequence of solid, liquid and gas. The maximum power of the Astex CVD system is 1. 5 k ilo w atts with a frequency of 2.45 G Hz. M icrowave radiation s from the RF generator go through a waveguide, which is normally a rectangular pipe made of metals or dielectric materials and functions as a transportation tunnel of radiation runs into reactant gas mixture in the CVD chamber, and fina lly yields the plasma. Carbon ions bombard onto the surface of the substrate and experience complex chemical rea c tions, such as surface reactions, nucleations, diffusions, step growth, and eventually form the diamond films. The unreacted particles are pump ed out of the chamber by vacuum system. Each CVD reactor needs a mechanical pump to get a desired vacuum level. Astex CVD diamond deposition requires 1 m T orr of pressure before input t ing the reactant gas.
23 The pressure is measure d by a mechanical pressure gauge that is connected to the chamber. The pressure can also be minor ly adjusted by a needle valve in order to get an expected pressure during the deposition process. The unreacted particles, impurities, and some other by products of plasma reactions are exhausted by a mechanical pump through the pipelines. Microwave p lasma enhanced CVD diamond deposition requires a high temperature, approximately above 700 C. When the power of the RF generator and the frequency of microwave radiation are not powerful eno ugh (for instance, Astex CVD has only 1. 5 kw and 2.45G Hz), a heater is needed to increase the temperature of the sample in order to meet the temperature requirement of diamond deposition. The heating section is made of DC bias power convert, sample stage, conduction metal wires sheathed by ceramic beads that hook up the converter and sample stage. The sample stage is made of graphite, which is a material of high electrical resistance, so as to absorb energy from the converter and release the heat to the sample. A chromium alumina thermal couple is fixed on the bo ttom of the stage and the sensible tip of the thermal couple stands up above the surface of the stage to monitor a real time temper ature inside the chamber. Tap water or chilly water is supplied to run through pip e lines wound inside the stainless steel wall of the CVD chamber to cool down the system. The Astex CVD system utilizes tap water because of the low power of the RF generator and relatively low working temperature of the system.
24 Fig ure 1 6 A p hoto of Astex CVD system 1.4 Characterization T echniques After diamond films were deposited on the silicon substrates, characterization techniques have to be carried out to prove that what were deposited by chemical vapor deposition system under certain conditions were in fact diamond, rather than some other material. Characterization techniques for diamond films include Raman spectroscopy, atomic force microscopy (AF), X ray diffracti on (XRD) and scanning electron microscopy (SEM). Raman spectroscopy can offer information on the bond type of the film ; sp 3 represents diamond whereas sp 2 indicates graphite. Atomic force microscopy can tell the topography of the deposited films such as roughness, grain size etc. X ray diffraction provides the crystalline structure of the thin films which determines what material it is. Scanning electron microscopy tells the morphology of the film surface. Each technique has its own special feature s and ad vantage s based on totally different principle s A combined imple menta tion of all these techniques will provide a comprehensive description, determination and understanding of observed material.
25 1.4 .1 Raman S pectroscopy When a light with a certain wavel ength or frequency, whether it is visible light or X ray or electron beam, hits on the atoms, the elastic or inelastic scattering of radiation occurs Elastic scattering that has no energy loss lead s to the application of optical image, X ray diffraction a nd transmission electron microscopy. Inelastic scattering that loses part of the incident beam energy is utilized to characterize the electron level energy change and lead s to the techniques of scannin g electron microscopy and Raman spectroscopy Raman scattering phenomenon was found in 1928 by Indian p hysicist and Nobel laureate C .V. Raman When a monochromatic (sing le wavelength ) laser hits on an electron, a photon is excited and the electron energy state jumps from ground state to a new state. The energy difference between the two states causes a wavelength shift in the emitted photon from the excitation wavelength. If the final state is in a higher energy status than the original sta te, which means a bsorption of energy, the excited photon will shift to a lower frequency in order to maintain an energy balance. Because of the energy change, it is believed to be an inelastic sca ttering. This phenomenon offers information about the intern al microstructure of the crystals, molecules such as chemical bonding types vibrational or rotational modes Inelastic Raman scattering is very weak compared with elastic Rayleigh scattering about 1 inelastic photon found in 10 million elastic photon s Th e most common light source in Raman spectroscopy is an Ar ion laser. Raman analysis is an ideal analytical technique because it is non contact and non destructive, requires a small sample area and provides quick outcome. Figure 1 7 illustrates the working mechanism of Raman scattering phenomenon.
26 Fig ure 1 7 Working mechanis m of Raman scattering Figure 1 8 shows the orbital jump of electrons in terms of energy change during the process of photon emission. Raman scattering can be classified as Stokes Raman scattering for energy loss photon emission, and Anti Stokes scattering for ener g y gain emission. Elastic Rayleigh scattering has no energy change with electron transportation. Fig ure 1 8 The orbital jump of electrons with respect to energy change
27 The commonly used wavelength s of lasers that excite the Raman scattering can be 514nm, 633nm, 780nm. The shorter the wavelength, the more powerful energy the laser has; thus laser with short er wavelength s c an excite more efficient Raman scattering. The intensity of the Raman peak reflects the number of photons generated. The characteristic Raman frequency can tell the composition of material. Frequency changes in Raman peaks can be utilized to calculate the stress/strain status of films; for instance, a 10cm 1 peak shift for silicon represents 1% strain [ 47 ]. Polarization of Raman peaks indicates the symmetry and orientation of the crystal lattice. The width of Raman peaks provide information about the qualit y and crystal degree of films [48 50 ] Raman analysis for CVD diamond films will ascertain the type of C C bonding (either sp 3 or sp 2 ) Figure 1 9 is a typical Raman spectrum for polycrystalline diamond film. Fig ure 1 9 A typical Raman spectrum for polycrystalline diamond film 1000 1200 1400 1600 1800 Intensity (Counts/sec) Raman Wavenumber (/cm) 1336
28 As evident in figure 1 9 the Raman spe ctrum is expressed in wavenumbers which can be explained as the number of wavelengths per unit distance and which have units of inverse centimeters (cm 1 ) In order to convert between spectral wavelength and wavenumbers of shift in the Raman spectrum, the following formula can be used: where w 0 is the excitation wavelength, 1 is the Raman spectrum wavelength . A n ex ample calculation is given to diamond film by the Re n i shaw Raman spectroscopy which has an incident laser with a wavelength of 514nm. If the Raman shift (wavenumber) of nanocrystalline diamond is 1140cm 1 then the wavelength of the excited photon is 546n m; if the Raman shift of polycrystalline diamond is 1350cm 1 then the wavelength of the excited photon is 552 nm; if the wavenumber of graphite is 1500cm 1 then the wavelength of the excited photon is 557nm. A simple conclusion can be made from above calculation that nanocrystalline diamond ha s more solid structure, and rel e ase s more energy to break it up. 1. 4 .2 Atomic F orce M icroscopy (AFM) Atomic Force Microscopy abbreviated as AFM, was invented in 1986 by G. Binnig and H. Rohrer at IBM. It was o ne of the most important tools for imaging and measuring the surface of material on the order of nanoscale. The theory and operation of an AFM are similar to those of a stylus profilometer An AFM generates a very small electrostatic force (nano Newton) be tween the probe tip and the scanned surface hence a much higher re solution can be achieved. The most important component of AFM is a
29 cantilever with a sharp tip (probe) at its end. Generally the cantilever is made of silicon or silicon nitride (Si 3 N 4 ) wit h a tip 1 10 nm in diameter which is used to scan on the surface of a specimen. When the tip approach es a sample surface, there exist s a force between the tip and the sample ; this force leads to a deflection of the cantilever A smart laser beam deflection system is introduced to measure the deflection of the cantilever. An incident laser hit s on the polished back (like a mirror) of cantilever, and then is reflected to a posit ion detector. The position detector reads out the deflection and calculate s the force according to kz, where F is the force, k is the stiffness of the lever, and z is the distance the lever is bent. The calculated forces are mapped to form the topography of the surface. The operation mode of c antilever can be classified into contact mode and non contact mode. Contact mode is for hard materials, and non contact mode is generally for moist surface s Diamond film, due to its extreme hard prope rty, is recommended to use in non contact mode in order to reduce the wear out of the tip. In contact mode, the force between the tip and the surface is kept constant by maintaining a constant deflection, and therefore constant height above the surface. Th e topography is mapped by measuring direct deflection of the cantilever. In non contact mode, the cantilever oscillates at a frequency slightly above its resonance fr equency with an amplitude of a few nanometers (<10 nm). When the cantilever comes close to the sample surface, the amplitude and phase of the vibrating cantilever change; this change is related to the force on the surface and can be easily measured to cons truct a topographic image of the sample surface D ue to the principle of scanning, image s of non contact mode are referred to as phase image
30 amplitude image and frequency image [ 51, 52 ] One of the advantages of AFM is the formation of three dimensional images on material surfaces. Fig ure 1 1 0 Nanocrystalline diamond image of AFM 1.4 .3 X R ay D iffractions (XRD) X rays are electromagnetic radiation with wavelengths in the range of 0.1 angst rom to a few angstroms, which are comparable to the size of atoms and are ideally suited for probing the structural arrangement of atoms and molecules in a wide range of materials. The energetic X rays can penetrate deep into the materials and provide information about the crystallographic structure, crystallite size (grain size) and preferred orientation of film or bulk materials X rays are produced generally by X ray tube where electrons from heated cathode filament s are focused and accelerated through a high vo l tage potential of tens to hundreds of kilovolts W hen incident electrons beam strike s on a stationary or rotating solid target (usually powdered Cu and Mo [ molybdenum ] with corresponding wavelengths of 1.54
31 and 0 .8 and served as anode ) the incident electrons can eject electrons from the inner shell of the target atoms ; then electrons from the high energy level of target atoms fill the vacancies left by incident electron beam s and in the meantime X ray photons with the same wavelength of target atom s are emitted in all dir e ctions and go through a small round window where X ray s exit S ince powders contain large amount s of fine and randomly oriented particles and each par ticle can be seen as a crystal, it ensures some particles are properly oriented such that a possibl e set of crystallographic plane s will be available for diffraction. D iffraction conditions of X rays f o r a periodic arrangement of atoms are based on the famous Br a : n 2 d sin where n is an integer is the wavelength d is the interplanar spacing and is the diffraction angle It show s that diffraction occurs only when traveled distances of rays reflected from successive planes are integer number ( n ) of wavelength XRD pattern is a plot of scattering intensity v ersus the scattering angle 2 By varying the incident X ray angle planes with different d spacing in polycrystalline material can be identified. Identification of materials is achieved by comparing X ray diffr action pattern with an internationally recognized database containing reference patterns for more than 70,000 phases. The widths of peaks in particular phase pattern s provide the crystallite sizes. Large crystallites give rise to sharp peaks, whereas wide r peaks correspond to reduced grain sizes. Peak broadening also occurs as a result of variations in d spacing caused by micro strain. In general, the XRD pattern indicate s phase presence by the position phase concentrations by peak heights, amorphous co ntent by background hump, and crystallite size /strain by peak widths [51 55]
32 Fig ure 1 1 1 XRD pattern of intrinsic nanocrystalline diamond syn thesized at 110Torr 1.4 .4 Scanning E lectron M icroscopy (SEM) The scanning electron microscope ( SEM ) is a technique to get images of a sample surface by scanning it with a high energy beam of electrons in a raster scan p attern. A n electron beam in a typical SEM is thermionically emitted from an electron gun made of a tungsten filament cathode Thermionic emission is the heat induced flow of charge carriers such as ele c trons and ions, from a hot metal cathode under vacuum. Tungsten is normally selected in thermionic electron guns because of its high melting point and low vapour pressure of all metals, thereby allowing it to be heated for electron emission. Other materials for electron emitters such as cathodes include lanthanum hexaboride ( LaB 6 ) and Zirconium dioxide (ZrO 2 ). Figure 1 12 displays different penetration depth s of various particles. 35 45 55 65 75 85 Angles (2 ) D (311) D (111) Si (400) D (220)
33 When a focused electron probe with a condensed diameter of about 0.4nm to 5nm is incident on the surface of a specimen, various signals such as Auger electrons, secondary ele ctrons, backscattered electrons and X rays are generated. The depths of the signals depend on the acceleration voltage or energy which usually rang es from 0.5keV to 40keV. When a primary beam electron collides inelastically, the energy imparted to the spe cimen atom will cause it to emit electrons that electrons Since secondary electrons do no t diffuse deep inside the specimen, they are most suitable for observing the fine structure of the specimen surface. The generation of s econdary electron s increases with a higher specimen atomic number. The secondary electron reaction can produce seco ndary electrons, X rays, Cathodoelectro lumin es cence and Auger electrons. Figure 1 1 2 Penetration depths of various signals beneath the surface
34 Auger electrons from top shallow layer of the surface provide compositional information; backscattered electrons give atomic number s and topographical information of materials Cathodo electro lumin e scence offers electrical information X rays from the thickest place under the surface tell elemental com position of materials, which re fer to an important analysis function as E nergy Dispers ive X ray S pectroscopy (EDS or EDX) [56 60] Figure 1 1 3 is the SEM micrograph of intri nsic nanocrystalline diamond syn thesized at 1 0 0Torr. Fig ure 1 1 3 SEM image of intri nsic nanocrystalline diamond syn thesized at 1 0 0Torr SEM provides structural analysis of CVD diamond films such as surface morphology, grain shape, boundaries crystallite sizes and features and a high magnification image can even display fine crystalline defect features the preferred growth orientation or texture structure s ; EDS (energy dispersive spectroscopy) analysis is able to determine the elemental and compositional information of the diamond surface.
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36  N. Ferreira, L. Silva and E. Corat, Diamond Relat. Mater. 11 (2002) 657.  S. Koizumi, M. Kamo, Y. Sato, S. Mita, A. Sawabe, A. Reznik, C. Uzan Saguy, R. Kalish, Diamond Relat. Mater. 7 (1998) 540  N. Casanova, A. Tajani, E. Gheeraert, E. Bustarret, J. A. Garrido, C. E. Nebel, M. Stutzmann, Diamond Relat. Mater. 11 (2002) 328.  W. D. Callister, Jr., materials science and engineering an introduction, 7th edition (2007), chapter 12 .2 & 12.4  W. D. Callister, Jr., materials science and engineering an introduction, 7th edition (2007), chapter 2.3.  Wikipedia, orbital hybridisation, online posting: http://en.wikipedia.org/wi ki/Orbital_hybridisation  http://www.sekicvdsolutions.com/diamond.html  C. E. Nebel and J. Ristein, Thin  I. Reineck, M. E. Sjstrand, J. Karner and M. Pedrazzini Diamond Relat. Mater. 5 (1996) 819. [2 4 ] Z. Xu, Synthesis, Characterization, and A pplications of CVD Micro and Nanocrystalline D iamond T hin F ilms, Dissertation (Ph.D.) -University of South Florida, 2007 [2 5 ] J. Oakes, X. X. Pan, R. Haubner, B. Lux, Surf. Coat. Technol. 47 (1991) 600 [2 6 ] F. Deuerler, H. Berg, R. Tabersky, A. Freundlieb, M. Pies, V. Buck, Diam ond Relat. Mater. 5 (1996) 1478 [2 7 ] C. Corbella, M. Rubio Roy, E. Bertran, M.C. Polo, E. Pascual and J.L. Andjar Diam o nd Relat. Mater. 18 ( 2009 ) 1 035 [28 ] C. Gu Z. Jin, X Lu, G Zou, J Zhang and R Fang Thin Solid Films, 119 (1997) 124. [29 ] A. Petitfils, F. Wrobel, M. Benabdesselam, P. Iacconi and J.E. Butler Diam ond Relat. Mater. 16 ( 2007 ) 1 062 [30 ] A. Grill T hin S olid F ilms, 355 356 (1999) 189  A. Grill and V. Patel, Diam ond Rel at Mater 4 (1994) 62.
37 [32 ] A. Grill and V. Patel, Diam ond Films Technol 1 (1992) 219. [33 ] L. L. Soares, C. R. A. Lima, L. Cescato, M. A. R. Alves and E. S. Braga, J. Mod. Opt. 45 (1998) 1479. [34 ] J. S. Foord, W Hao and S Hurst Diam ond Rel at Mater 16 (2007) 877. [35 ] http://www.anl.gov/Media_Center/Argonne_News/2004/an040920.html  M Zhang, Y Xia, L Wang and B Gu Journal of Crystal Growth 277 (2005) 382  S. Gastlum, E. Cruz Zaragoza, R. Melndrez, V. Chernov, M. Barboza Flores, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materia ls and Atoms 248 ( 2006 ) 103  Robert F. Davis, Diamond Films and Coatings : Development, Properties, and Applications, Noyes Publications, ISBN 0 8155 1323 2 ( 1992 ) 7  H Nakahata, S Fujii, K Hi gaki, A Hachigo, H Kitabayashi, S Shikata and N Fujimori Semicond. Sci. Technol., 18 (2003) s96  D.S.Ballantine, R. M. White, S. J. Martin, A. J. Ricco, E. T. Zellers, G. C. Frye and H.Wohltjen, Acoutic Wave Sensors: Theory, Design, and Physico Chemical Applications, 1 st edition, Academic Press I nc, ISBN 0 12 077460 7, ( 1996 )  Venkat. R. Bhethanaotla, Ashok Kumar, Nanocyrstalline Diamond Surface Acoustic Wave Biosensor (pro posal ) 2006  E. A. Ekimov, V. A. Sidorov, E. D. Bauer, N. N. Melnik, N. J. Curro, J. D. Thompson and S. M. Stishov Nature, 428 (2004) 542  http://en.wikipedia.org/wiki/Led  P. John, Science, 292 (2001) 1847  S.Koizumi, Science, 292 (2001) 1899  W. Y. Liang, Phys. Educ., 5 (1970) 226  P. Dobrosz, S.J. Bull, S.H. Olsen and A.G. O'Neill, Surface and Coating Tech., 200 (2005) 1755  http://en.wikipedia.org/wiki/Raman_spectroscopy  G. Turrell and J. C orset, Raman Microscopy: development and applications, 1 st edition, Academic Press Inc, ISBN 0 12 189690 0, ( 1996 )
38  J. J. Laserna, Modern Techniques in Raman Spectroscopy, 1 st edition, John Wiley & Sons Ltd, ISBN 0 471 95774 7, ( 1996 )  http://en.wikipedia.org/wiki/Atomic_force_microscope  Nano R2 U ser M anual, Pacific Nanotechnology Inc.  William D. Callister Jr., Materials Science and Engineering an Introduction, 7 th edition, John Wiley & Sons Inc., ISBN 0 471 73696 1,( 2007 )  B. D. Cullity, Elements of X ray Diffraction, 2 th edition, A ddison Wesley Publishing Inc., ISBN 0 201 01174 3,( 1978 ).  Introductory training manual, Introduction to X ray Powder Diffraction, PANalytical XRD System documents, NREC USF.  Hitach S800 Manual, NREC USF  U. Zeile Coatings and Shadow Casting Techniques for Eclectron Microscopy and Improvements in Coating Quality, 7 th Asia Pacific Electron Microscopy Conference, June 2000  http://en.wikipedia.org/wiki/Scanning_electron_microscopy  G. L awes, Scanning Electron Microscopy and X Ray Microanalysis, John Wiley & Sons Inc., ISBN 0 471 91391, ( 1987 )  J. Goldstein, D. Newbury, D. Joy, C. Lyman, P. Echlin, E. Lifshin, L. Sawyer & J. Michael, Scanning Electron Microscopy and X Ray Microanaly sis, 3 th edition, Kluwer Academic/Plenum Publishers, ISBN 0 30647292 9, ( 2003 )
39 CHAPTER 2 : VARIOUS FORMS OF C ARBON 2.1 Polycrystalline D iamond F ilm 2.1.1 Introduction to P oly crystalline D iamond Film The polycrystalline diamond was the first one to be studied in all forms of diamond materials Polycrystalline diamond films deposited by the plasma enhanced microwave chemical vapor depositio n (PEMCVD) method have characteristic s such as a rough surface, large and coarse grain size The polycrystall ine material differs from single crystal material in that it i s polycrystalline in nature with many grain boundaries and defects These properties determine that polycrystalline diamond films must be smooth treated before applications in electrical micro d evices Most of the applications of polycrystalline diamond films reported in literatures concentrate d on the cutting tools The main challenge to diamond coated cutting tools is the adhesion of diamond films onto the metal substrates because of the great difference in thermal coefficient of diamond and substrate materials (particularly metallic substrate); another reason is the high deposition temperature of diamond (above 750 C ) causing thermal residual stress during the cool ing process that lead s to the delaminati on of diamond films from the substrates [1 4 ].
40 2.1.2 Growth C onditions of P oly crystalline D iamond Film A characteristic of the polycrystalline diamond film growth is that its chemistry gases only contain methane (CH 4 ) and hydrogen without any addition of argon The ratio of CH 4 to H 2 is approximately 1%CH 4 to 99%H 2 not exactly, so sometimes a CH 4 :H 2 recipe at 1sccm:100sccm or 2sccm:100sccm can be found in literatures. Since the low volume of reactant gases and both of these two gas es join the chemical reaction, the pressure during deposition is ve ry low, in the range of 30Torr to 50Torr. The deposition temperature is above 730 C up to 800 C. Poly crystalline d iamond films can be successfully grown by Astex CVD system using various chemistry gases according to the protocol stated above. Table 1 list s the gas recipe and deposition conditions. Table 1. Poly crystalline diamond with gas chemistry CH 4 /H 2 at ratio of 1/100 H 2 (sccm) CH 4 (sccm) Pressure (HPa) Temperature ( C ) 100 1 20 650 500 5 30 650 1000 10 50 650 1500 15 2000 20 3000 30 An u ltrasonic slurry seeding method for diamond nucleation on Si substrate was carried out to prepare the sample. Commercial available (Sigma Aldrich) nano diamond powder (5 nm) was mixed in methanol to make the slurry. The Si wafer was cut into 2cm 2cm square pieces to serve as samples. A beaker with Si piece s submerged in methanol diamond slurry was put in Bransonic 1510, a tabletop ultrasonic cleaner. The Si
41 samp les were ultrasonic agitated for 15 minutes in order to get uniformed seeding. After oscillation seeding, the s amples were rinse cleaned by methanol to remove the residues such as contaminants or impurities on the surface, and then they were dried by compr essed nitrogen just before setting into the CVD chamber. The CVD process was carried out under the pressure of 50 Torr at 6 50 C for one hour. Torr and Pascal are different units for measuring the pressure. In engineering, one Torr is considered equal to one HPa ; in the present dissertation, Torr and HPa are used as equivalent units unless special notification. 2.1.3 Characterizations of P oly crystalline D iamond Film Characterization of polycrystalline diamond films by Raman spectros copy, AFM, XRD and SEM indicated that the chemical combination of H 2 ( 30 00sccm) and CH 4 ( 30 sccm) pr oduced the best quality films. The high Raman peak indicates that more sp3 bonding existed in the film which means there is more diamond structure presented. Compared with the H 2 flow rate recipe, the background hump of 100sccm and 500sccm H 2 indicates a certain amount of amorphous carbon existed in the film. Wavenumber is the reciprocal of wavelength, which is proportional to the frequency. The interaction between laser and atoms produces photons; if the emitted photons contain higher e nergy, it means they are hard to be generated; if the released photons have less energy, it means they are readily to be ac tivated. So the righ t shift of Rama n peak indicates that it release s less e nergy to activate the Raman scattering and the bonding between atoms is at a higher energy level The peak corresponding to the 3000sccm located at the left most side
42 of the figure indicates that it has the most solid and stable structure. Figure 2 1 is a Raman spectroscopy for polycrystalline diamond film. Fig ure 2 1 Raman spectra for polycrystalline diamond with various H 2 flow rate An SEM image of polycrystalline diamond film reveals the morphology of the surface as well as the shape and size of crystallites As evident from SEM images polycrystalline diamond has faceted crystallite s Figure 2 2 is a n SEM image of polycrystalline diamond. 1000 1200 1400 1600 1800 Raman wavenumber (cm 1 ) Intensity (counts/sec) 100sccm 500sccm 1500sccm 2000sccm 3000sccm
43 a 1000sccm H 2 with 30k magnification b 2000sccm H 2 with 30k magnification c 3000sccm H 2 with 30k magnification d 3000sccm H 2 with 50k magnification Fig ure 2 2 Morpholog ical images of polycrystalline diamond by SEM Polycrystalline diamond crystallites with 1000sccm H 2 shown in f ig ure 2 2a are multiple faceted with diameters more than 0.5 m ; the crevice can be clearly seen and the surface is rather rough. Crystallites displayed in f ig ure 2 2b with 2000sccm H 2 are a mix ture of middle size (about 0.2 m diameter ) and small size grains eac h of them accounts for approximately half of the surface. T he p olycrystalline diamond with 3000sccm H 2 in f ig ure 2 2c has a similar grain size to that in f ig ure 2 2b, but it is more closely packed than that of f ig ure 2 2b. It is also apparently observed in f ig ure 2 2c that
44 many dimples and rectangl ular flat s exist on top of the crystallites. The dimples might result from H 2 etching effects du r ing deposition. It can be concluded that CVD is a self conflicted process; during this process, plasma etching and chemical deposition occur simultaneously ; diamond formation is an outcome of a dynamic balance of etching and depositi on With reference to t he Raman spectra in f ig ure 2 1, it is conclu sive that the polycrystalline diamond with 3000sccm H 2 obtains the best quality of films. The chemical deposition of CH 4 /H 2 at 1/100sccm and 5/500sccm at low pressure and low H 2 volume in the reactor during the deposition process yielded a polycrystalline diamond whose surface top ograp hy is absolutely different from that of the faceted morphology. Particularly for the sample of CH 4 /H 2 with 1/100sccm, the diamond peak in Raman spectra ( f ig ure 2 1) was almost inhibited by the broad G band. Figure 2 3 illustr at es the surface view of these two samples that were subjected to 100sccm and 500sccm H 2 flow rate s a. 100sccm H 2 flow rate b. 500 sccm H 2 flow rate Fig ure 2 3 SEM m orphology of polycrystallin e diamond synthesized with low H 2 flow rate.
45 The temperature and pressure of CVD grown diamond locate d in the zone in the carbon phase diagram that is good for growth of stable graphite, however, is metastable for diamond. The atomic hydrogen functions as the graphite etching phase and form s the diamond structure in the CVD process Because of the low flow rate (in fact low volume) of H 2 the graphite etching action does not proceed thoroughly, leaving most part s of the surface in graphitic form and amorphous carbon. For the same reason, the sample of 500sccm H2 yields identical surface morphology to that of the 100sccm sample, but produces more diamond phase as illustrated in the Raman spectra in f ig ure 2 1. X RD shows the crystalline structure of the CVD polycrystalline diamond film Figure 2 3 is the XRD pattern of polycrystalline diamond film. Figure 2 4. XRD pattern of polycrystalline diamond film The sample was prepared by ultrasonic slurry seeding for 10 minutes. The reactant gas chemistry is CH 4 /H 2 with 1 5sccm/ 1 500sccm ratio of flow rate. The deposition was carried out at 650 C temperature and 50torr operating pressure for 10 42 52 62 72 82 Intensity Angles (2 ) D (111) Si (400) D (220) POLY 10HRS
46 hours. As can be observed, the polycrystalline diamond film primarily consisted of (111) and (220) oriented plane, and (220) plane account ed for almost three times more than (111) plane. Both peaks were rather sharp indicat ed that perfect symmetric crystal line were being formed, therefore the as deposited polycrystalline diamond film has a high quality. 2.2 Nanocrystalline D iamond F ilm 2.2.1 Introduction to N ano crystalline D iamond Film Polycrystalline diamond has a coarse grain size and it is difficult to avoid the cavities in the as deposited diamond layer during the growth process. The introduction of Ar into the gas chemistry recipe of polycrystalline diamond results in the formation of ultra fine nanocrystalline diamond. Compared with th e micro scale grain size ( > 1 m ) and rough surface(0.5 1 m ) of polycrystalline diamond films, nanocrystalline diamond films have a much smaller grain size which can be less than 5nm in diameter and smooth surface (15 30nm) Since n anocrystalline diamond films are expected to exhibit superior performance i n physical, chemical, electrical and mechanical properties than polycrystalline diamond films do, n anocrystalline diamond films have been most extensively studied and employed in al most all industr ies [5 8 ] 2.2.2 Growth C onditions of N ano crystalline D iamond Film Intrinsic nanocrystalline diamond films are grown by a chemistry gas mixture of CH 4 H 2 and Ar. Among them, CH 4 always accounts for 1% in flow rate (sccm), H 2 can take 1% up to 5%, and the remaining part is Ar. In the current experiment
47 nanocrystalline diamond film was prepared by CH 4 /H 2 /Ar at 2 sccm/2sccm/ 200 sccm for 3 h ou rs. T he deposition pressure was 120HPa and the temperature was 750 C The pressure of nanocrystalline diamond deposition is much higher than polycrystalline diamond, because of the introduction of argon, which has b ig molecules and accounts for 98 % of the total gas chemistry. Many researche r s concluded that hydrogen play s a n important role in decreas ing diamond grain size to the nano scale. The seeding and cleaning procedure of nanocrystalline diamond deposition is the same as the polycrystalline process described before. 2.2.3 Characterizations of N ano crystalline D iamond F ilm Unlike the Raman spectroscopy of polycrystalline diamond film that only displays one peak at 1330cm 1 presenting polycrystalline diamond, the nanocrystalline diamond Raman spectroscopy has four peaks that indicate nanocrystalline diamon d, polycrystalline diamond, G band (graphite) and D band (diamond structure) respectively. In the crystalline solids the Raman bands are relatively sharp ; the widths broaden with the increasing temperature and decreasing grain size However, the presence of impurities, vacancies or some other imperfections in the crystal lattice results in additional broadening of the Raman bands. In particular, amorphous materials are characterized by very broad features that are difficult to give a quantitative interpretati on [ 9 ] The laser wavelength of Raman is 514nm, which is locate d in the range of visible electromagnet spectrum from 380nm to 740nm. This wavelength is believed to readily present graphitic phase rather than diamond structure especially the nanocrystallin e
48 diamond bond which is shorter and stronger than that of the polycrystalline diamond bond. Fig ure 2 5 Raman spectrum for nanocrystalline diamond The morphology of nanocrystalline diamond observed using SEM is absolutely different from that of polycrystalline diamond. Figure 2 6 is a sample synthesized by CH 4 /H 2 /Ar at 2sccm/2sccm/180sccm under 110HPa. The surface displays uniform distribution of ca u liflo we r like diamond crystallites with 100nm to 200nm diameter of cluster, which are composed of hundred s of grains. The grain boundaries can be observed in dark color with about 0.5nm wide, which is rich in sp 2 bondings. 1000 1200 1400 1600 1800 Intensity(counts/sec) Raman Wavenumber(/cm) Ar196sccm/120hpa Nano peak G band D band
49 Fig ure 2 6 Morphology of nano crystall ine diamond by SEM Figure 2 7 is the AFM images of nanocrystalline diamond film measured by non contact mode, which can precisely record the tip sample distance by controlling atomic forces between the cantilever tip and sample surfaces. Fi g ure 2 7a is a two dimensional AFM image with 10m10m scanning area displaying a ca u liflo we r like surface morphology of nanocrystalline diamond films; fig ure 2 7b is a three dimensional image showing the maximum height of 893.81nm in the scanned area ; the height gradient can also be observed on the surface. The average roughness of this specimen is about 94nm and the surface is covered by mixed different sizes of diamond grains.
50 a Two dimensional AFM image b Three dimensional AFM image Figure 2 7 AFM images of nanocrystalline diamond film measured by non contact mode Figure 2 8 represents the XRD pattern of nanocrystalline diamond film synthesized by feeding a gas eous mixture of CH 4 /H 2 /Ar with a 2sccm/2sccm/200sccm flow rate ratio. The deposition was carried out at 700C temperature and 130torr operating pressure for 26 hours. As can be observed, identical to the XRD pattern of polycrystalline diamond film in f ig ure 2 4 nanocrystalline diamond fi lm primarily consisted of (111) and (220) oriented plane, and (220 ) plane accounted for almost three times more than (111) plane. Both peaks displayed broad width, rather than sharp peaks illustrated by polycrystalline diamond film, indicating much smaller diamond crystalli tes were being formed ; therefore the as deposited nano crystalline diamond film has a high er surface density and quality.
51 Fig ure 2 8 XRD pattern of nano crystalline diamond film 2.3 D iamond L ike C arbon ( DLC) 2.3.1 Introduction to DLC Amorphous carbon film is also referred as diamond like carbon (DLC) film which is a metastable form of amorphous carbon with a significant fraction of sp 3 bonding. Crystalline materials have atoms with long range order in a rep eated or periodic way over l ong atomic distances. In amorphous materials there is no long range atomic order These materials are characterized by lack of a systematic and regular arrangement of atoms over long atomic distances. Amorphous carbon film can be seen as short range ordere d diamond film, because it has sp 3 bonding as well as similar properties of diamond such as a high mechanical hardness, elastic modulus, chemical inertness, and optical transparency etc. even though properties of amorphous carbon film are not as good as th ose of nanocrystalline diamond film or polycrystalline diamond film. B onding s in amorphous carbon consist of a fraction of sp 3 bonded carbon sites and the hydrogen 42 52 62 72 82 Angles (2 ) Intensity D (111) Si (400) D (220) Nano 26HRS
52 content, C H bond; sp 2 sites are a third significant factor in amorphous carbon, particularly for the electronic properties [10 12 ]. 2.3.2 Growth C onditions of DLC The deposition of amorphous carbon film can be achieved by a CVD technique by feeding a mixture of methane (CH 4 ) and argon (Ar) with a typical flow rate of 5% to 95% respectively. For example, the following characterized amorphous carbon film was synthesized by CH 4 /Ar at 10sccm/190sccm for 2 h ou rs. The deposition pressure was 120HPa and the temperature was 900C. Ev en though amorphous carbon film deposition is similar to that of polycrystalline diamond film, both have two reactant gases : amorphous diamond has argon and methane; whereas polycrystalline diamond ha s hydrogen and methane but the p ressure difference is v ery high (polydiamond 50HPa, amorphous carbon 120 HPa) due to a great atom size difference between argon and hydrogen. The Astex CVD system is equipped with a 1KW, 2.45GHz microwave generator. Usually the H 2 plasma is ignited at 400 watts for diamond growth. However, the output energy at this level is not powerful enough to generate argon plasma, because argon is a kind of noble gas that is chemically inert and difficult to be ionized by low power and low frequency microwave. In order to s uccessfully deposit amorphous diamond with stable plasma of mixed chemistry consisting only or Ar and CH 4 an alternative approach of H 2 plasma ignition has to be adopted. Similar to the H 2 plasma generation as that of polycrystalline diamond and nanocryst alline diamond described above, once H 2 plasma is ignited, the Ar and CH 4 is fed into the CVD reactor subsequently according to the flow
53 rate of the recipe; then the H 2 molecular supply is reduced to a zero flow rate leav ing behind CH 4 and Ar in the chamb er. Mak ing a sensitive balance of pressure, gas flow rates, power supply and depositing temperature so that the value of reflected watts on microwave generator panel is not over 3 ( indicating a stable plasma ) ; the amorphous diamond film is finally being gr own by Astex system. 2.3.3 Characterizations of DLC Figure 2 9 is the Raman spectrum of diamond like carbon grown by 10sccm CH 4 and 190sccm Ar under 120HPa at 900C. From the figure, it is evident that there are no app arent sharp peaks. Three broad bumps corresponding to nano diamond polydiamond and graphite can be observed illustrating that the Raman spectrum analysis is good at revealing the bonding types of materials such as sp 3 or sp 2 bonds, rather than crystallinity of material. From the obvi ous height and width difference of the peaks, Raman spectra can still provide information about long range ordered atom arrangement (crystalline with sharp and high peak) and short range disordered atom arrangeme nt (amorphous phase with broad bump), even t hough both carbon phase forms are sp 3 and sp 2 bondings.
54 Figure 2 9 Raman spectrum for diamond like carbon Figure 2 10 illustra tes the SEM image of DLC. The top view of DLC demonstrates neither faceted particles like polycrystalline diamond, nor regularity crystallites like nanocrystalline diamond. The surface of DLC displays coarse and irregular cluster s Big island like agglomerates are the main feature of the surface separated by boundary like ditches that consist of tiny particles randomly oriented. The dark groves predominantly consist of sp 2 bondings ( graphitic phase ) whereas the tiny particle is rich in sp 3 bondings ( diamond phase ) The ent ire surface exhibits a rough and homogeneous distribution of small particles. 1000 1200 1400 1600 1800 Intensity(counts/sec ) Wavenumber (/cm) 10CH 4 190Ar
55 Fig ure 2 10 Morphology of diamond like carbon by SEM Figure 2 11 reveals two dimensional ( f ig ure 2 11a) and three dimensional ( f ig ure 2 11b) images of DLC by AFM technique respectively. Figure 2 11a shows large agglomerates embedded in the background of uniform small particles; figure 2 11b demonstrates the roughness of surface and particle sizes of DLC film but the big island is not accurately reflected because it is out o f the measurement range leaving an artifact in corresponding position The total scanning area is 3m3m with an average roughness of about 217nm.
56 a. Two dimensional image of DLC b Three dimensional image of DLC Fig ure 2 11 Topography of diamond like carbon by AFM 2.4 Diamond N anowires 2.4.1 Introduction to VLS and N anowires The growth of diamond wires can be divided into two stages: first, the growth of Si nanowires by the vapor liquid solid (VLS) method in a therma l furnace; second, the coating of nanocrystalline diamond on the shel l of Si nanowires by the CVD method, which has exactly the same gas recipe as the deposition of diamond. The vapor liquid solid method ( VLS ) is a description of a mechanism for the growth of one dimensional nanowires such as S i nanowires or ZnO nanowires so called because it involves a vapor phase precursor, a liquid catalytic metal droplet and a s olid one dimensional nanowire VLS can occur in a thermal furnace or a CVD chamber, but both of them follow the same growth process in terms of variation of material forms, vapor liquid solid. The VLS growth process of Si nanowires can be briefly described as follows: first is the preparation of the thin metallic catalyst thin film (for instance Au) on the Si substrate
57 surface. When the sample is annealed at a temperature ab ove the eutectic point, a liquid Au Si droplet of nano size is formed functioning to lower activation energy for growth of nan owires. After the Si contained material such as SiO powder is heated and evaporated into gas phase, with the help of carrier gas Ar, Si atoms are adsorbed at the liquid alloy/solid Si interface, and the droplet ris es from the surface to grow up as Au tipped Si nanowires [13 ] Si nanowires have been expected for many applications, but the original function was to serve as interconnectors between nanoscale components. In addition, the high surface to volume ratio of nanowire makes it an interesting nanostructure for solid surface reaction and sensing. Up to now, even though production scale nanowire applications with market value have not achieved, but many simple device structures have been demonstrated, illustrating the possibilities might be available in the near future [14 ] Electronic applications of Si nanowires have been reported to make p n junctions within single wire s [15 17] and between wire s [18 ] field effect transistors (FETs) [15, 19 21]. Si nanowires can also be applied in the development of optical and optoelectronic devices such as l ight emitting diodes (LEDs) [18, 22 24], heterostructure nanowire based solar cells [25, 26]. The mechani cal properties of nanowires have also been employed to demonstrate device concepts such as mass sensors for particles of very small mass , nanowire power generators . Finally, the electrochemical applications of nanowires have been investigated La w et al. demonstrated molecular sensors based on nanowires . As well, pH sensors based on Si nanowires have been fabricated . In addition to the above mentioned applications, diamond nanowires compared with Si nanowires are
58 also an ideal material f or chemical/bio chemical, biomedical applications due to its special chemical inertness / stability bio compatible and strong bonding ability to DNA properties . 2.4.2 Growth C onditions of D i a mond N anowires In the present study, Si nanowires were grown by the VLS method in a Lindburg thermal furnace. The VLS growth of Si nanowires in the thermal furnace can be descr ibed briefly as follows. First of all a catalytic metal thin layer around 1 10 nanometers thick on th e Si substrate was prepared by an electron beam evaporation method (E beam evaporator) T he generally used catalytic metal can also be Au, Pd, Al, Ga etc. A gold catalytic thin layer with 5nm thick on Si substrate was selected to serve as the sample. The Si nanowire growth process was carried out in a vacuumed horizontal tube furnace about 10 3 Torr by a mechanical pump. The source material was the high purity SiO powders (120mg) which was contained in an alumina boat (also called a combustion boat). The boat with SiO powders was inserted into the horizontal tube furnace and kept in the middle of the furnace in order to absorb the highest amount of energy. Another boat with the sample was set next to the first one downstream of the carrier gas so that Si atoms decomposed by heat could drop on the surface of the sample. Prior to heating, the reaction chamber was vacuumed, and then the carrier gas Ar was introduced through the tube at a flow rate of 170 sccm. The source material was rapidly heated to 1030 C from room temperature within 2 h ou rs and was kept at 1030 C for 1 h ou r. finally, the furnace was cooled down to room temperature in 2 h ou rs. The chamber pressure was kept at about 30 m Torr during the entire process.
59 Once the Si nanowires ha d been succes sfully grown, the sample with Si nanowires was immersed into the uniformed diamond slurry and was then dried at room temperature in a natur al way to avoid nanowires broken by blowing compressive nitrogen After the naturally dried Si nanowire sample was placed into the Astex CVD chamber the procedure of intrinsic nanocrystalline diamond deposition was started with H 2 (2sccm)/CH 4 (2sccm)/Ar (180sccm) under the operating temperature 7 7 0C and pressure 120HPa. The nanocrystalline diamond growth time lasted 15 minutes. 2.4.3 Characterization of Diamond N anowires Figure 2 1 2 is the SEM image of Si nanowires grown in the above described conditions. The maximum diameter is 323nm and the minimum diameter is 158nm. It is difficult to measure the practical length of Si nanowires because most of them entangled with and overlapped each other and it is also impossible to find the roots of nanowires ; therefore, only the length of the section of nanowires on the surface could be measured. The maximum length obse rved in this image is about 4 m when actually the real length of Si nanowires can reach from several tens o micrometers to more than one hundred micrometers.
60 Fig ure 2 1 2 Image of Si nanowires by SEM Figure 2 1 3 is SEM image of the diamond nanowires grown according to the above mention ed conditions. Nano sized diamond particles coated around the shell of Si nanowires in a uniform and continuous way because the diamond and Si have identical microcrystalline cubic structures, a nd because the growth conditions were optimized. If the microstructures of two elements are not matched, such as diamond coating on ZnO nanowires which have a hexagonal crystalline structure, diamond will present scattered particles around the shell of th e ZnO nanowires. The maximum length of di a mond wire in this photo is 14m; the maximum diameter of diamond knot is more than 2 m.
61 Fig ure 2 1 3 Image of nanocrystalline diamond nanowires by SEM 2.5 Graphene 2.5.1 Introduction to G raphene Graphene is believed to be a new form of carbon allotropy subsequent to graphite, diamond, carbon nanotubes and fullerenes. It is a freestanding single atomic carbon layer with a hexagonal structure, which can be visualized as an individual layer extract ed from gr aphite. There are several ways to produce graphene, including mechanical exfoliatio n of graphene from graphite, high temperature (>1100C) extraction from SiC, chemical vapor deposition on a metal surface such as nickel and copper. In the present research, graphene was grown on Si substrate with a nickel layer by the CVD method. Graphe ne has been found to have prominent mechanical and electrical and optical properties, which make it a promising material for many applications such as biosensor, gas sensor, transistor, conducting electrode, capacitors etc [ 3 2 33 ]. Due to the limitation of application facilities, the current study only focus es on the characterization of graphene, and thus is not concern ed with the practical applications.
62 2.5.2 Growth C onditions of G raphene First the Si substrate with an Ni thin layer was heated in the Astex CVD chamber for 30 min utes in H 2 plasma of 200sccm The pressure of the chamber was 40HPa. The temperature of the sample surface was 700 C measured by pyrometer. Immediately after the heating treatment a CH 4 mix was introduced into the chamber with 22.2sccm. In the meantime in order to keep a CH 4 / H 2 ratio at 1 / 8 and a total flow rate of 200sccm ( the sum of CH 4 and H 2 ) the H 2 was adjusted to 177.8sccm from the previous heating flow rate of 200sccm. This is the reactant chemistry gas recipe of graphene growth [3 4 ] and the growth time was 5 min ute s. Finally, the CH 4 and H 2 supply, was shut off, then Ar was put in at 10,000sccm to cool down the temperature and purge the chamber and pipelines 2.5.3 Characterization of G raphen e Graphene created by the CVD method can produce a flake with several layers, as can be observed in an optical image with 50 magnification in figure 2 1 4 Sev er al factors influence the graphene synthesis greatly ; for instance, the growth rate and adhesion of graphene depend on the different catalytic metal s Temperature and carbon atom nucleation also play an important role in the growth process. Different orientations of graphene exhibit various growth rates; this point is similar to the growth of CVD diamond films and can b e clearly viewed in figure 2 15 by the wave like surface [3 5 3 6 ].
63 Fig ure 2 14 Optical image of graphene flake made by CVD Raman spectroscopy is a powerful tool for characteriz ing graphen e and helping to understand the behavior of electrons and phonons Because graphene fabricated by the CVD method usually contain s multiple layers, it is important to clarify the number of layers and the features of different order s of layer when analyzing the graphene. As evident in figure 2 1 5 there are three peaks positioned at 1351, 1571, and 2417cm 1 which are defined as D G and 2D (two dimensional) correspondingly. The remarkable sharp and symmetric 2D peak indicates the presence of single crystal graphene which might be multiple layers. D peak is attrib uted to the defects and structural disorders in graphene The intensity ratio of D to G peak increases with the rise of disorder degree in graphene, which may include distortions, corrugations (puckers) and non uniformity developed during the growth proces s [3 7 3 9 ]. The G and 2D Raman peaks change in shape, position and relative intensity with the number of graphene layers. This reflects the electronic structure and electron phonon interactions. Disorder can be monitored via the D peak [ 40 ]
64 Fig ure 2 1 5 Raman spectr um for graphene The SEM image in figure 2 16 reveal s the fact that the highly crystallin e graphene accounts for most of the sample, and the corrupted region that is proportional to the intensity ratio of D to G peak in the Raman spectrum on ly takes up a small portion in the sample. The SEM image also reveal s the multiple layer structure of graphene made by the CVD method. Fig ure 2 1 6 Image of graphene by SEM 2417 1571 1351 1200 1700 2200 2700 Intensity Raman shift /cm
65 2.6 Summary Polycrystalline diamond films produced by the MPECVD method utilizing gas chemistry CH 4 /H 2 at a ratio of 1/100 were deposited by changing the H 2 volume from 100sccm to 3000sccm. It turned out that the sample of 100sccm H 2 was composed of a large amount of graphitic phase and the surface presented cauliflower like m orphology. The samples fabricated by a more than 1000sccm H 2 flow rate showed classic faceted surfaces with the size of diamond crystallites ranging from 200nm to 1500nm. The XRD patterns exhibited high crystallinity of deposited diamond with mostly (111) and (220) plane, and the (220) plane accounted for three times more than (111) plane in the whole deposited surface. Nanocrystalline diamond film produced by the MPECVD method using gas chemistry CH 4 /H 2 /Ar at 2sccm/2sccm/180sccm was deposited under 110HPa pressure. The Raman spectrum revealed a nano diamond peak at the position of 1140cm 1 The SEM displayed cauliflower like surface morphology with 100nm to 200nm diamond crystallites. Two and three dimensional AFM images showed that the avera ge roughness was 94nm with a I dentical to the XRD pattern of polycrystalline diamond film nanocrystalline diamond film also primarily consisted of (111) and (220) oriented plane s and (220) plane s accounted for almost three times more than (111) plane. Different from polycrystalline diamond film, b oth peaks displayed broaden ed width indicating that much smaller diamond crystallites were being formed; therefore the as deposited nanocrystalline diamond film has a higher surface density and quality than that of polycrystalline diamond films
66 Diamond like carbon (DLC) exhibited in the Raman spectrum only broad bumps in the position of D band and G band, illustrating that no crystalline structure s were formed on the surface. The SEM discl osed a rough surface with scattered par ticles embedded in the tiny boundary like ditches. AFM demonstrated an average roughness of about 217nm in a 3m3m scanning area, which was rather high compared with nanocrystalline diamond film. Nanocrystalline diamond nanowires were successfully coated in a MPECVD reactor around the shell of Si nanowires produced by the VLS method in the thermal furnace. The maximum length of diamond wire that we have visualiz ed in SEM wa s 14 m; the ma ximum diame ter of diamond knot wa s more than 2m. The uniform ity of diamond coating on the shell of the Si nanowires was ascribed to the structure match between diamond and Si. 2.7 List of References  A Gicquel, K Hassouni, F Silva, J Achard, Current Applied Physics 1 (2001) 479  M. N. Gardos Surface and Coatings Tech., 113 (1999) 183  Diamond Science and Technology, MRS Conference, 2009  J. Asmussen and D. K. Reinhard, Diamond Films Handbook, 2002  A. Kumar, S. Balachandran, T. Weller, Micro and Nanoscale Applications of Nanocrystalline Diamond Films, Nanotech Conference & Expo 2009, Houston, TX  S. Mitura, K. Mitura, P. Niedzielski, P. Louda, V. Danilenko, J. of Achievements in Materials and Manufacturing Engineering, 16 (2006) 9  K. Okada Science and Technology of Advanced Materials, 8 (2007) 624  D. M. Gruen Annu. Rev. Mater. Sci., 29 (1999) 21
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68 [27 ] Y. Huang, X. Bai and Y. Zhang, J. Phys. C: Cond. M atter ., 18 (2006) 179. [28 ] P.X. Gao, J. Song, J. Liu and Z.L. Wang, Adv. Mater ., 19 (2007) 67. [29 ] M. Law, H. Kind, B. Messer, F. Kim and P. Yang, Angew. Chem. Int. Ed. 41 (2006) 2405. [30 ] Y. Chen, X. Wang, S. Erramilli, P. Mohanty and A. Kalinowski, Appl. Phys. Lett. 89 (2006) 223512. [31 ] C. E. Nebel, N. Yang, H. Uetsuka, E. Osawa, N. Tokuda, O. Williams, Diamond and Related Materials 18 ( 2009 ) 910 [32 ] http://en.wikipedia.org/wiki/Graphene [33 ] A. K. Geim and K. S. Novoselov, Nature Materials 6 (2007) 183 [34 ] A. Malesevic, R Vitchev, K Schouteden, A Volodin, L Zhang, G Van Tendeloo, A Vanhulsel and C V Haesendonck Nanotechnology 19 (2008) 305604 [35 ] R Vitchev, A Malesevic, R H Petrov, R Kemps, M Mertens, A Vanhulsel and C V Haesendonck, Nanotechnology 21 (2010) 095602 [36 ] E. Loginova, N. C. Bartelt, P. J. Feibelman and K. F. McCarty, New J. Phys. 11 (2009) 063046 [37 ] G. D. Yuan, W. J. Zhang, Y. Yang Y. B. Tang, Y. Q. Li, J. X. Wang, X. M. Meng, Z. B. He, C. M. L. Wub, I. Bello, C. S. Lee, S. T. Lee Chemical Physics Letters 467 (2009) 361 [38 ] A. Dato, V. Radmilovic, Z. Lee, J. Phillips, M. Frenklach, Nano Lett. 8 (2008) 2012. [39 ] A. C. Ferrari J. C. Meyer V. Scardaci C. Casiraghi M. Lazzeri F. Mauri S. Piscanec, D. Jiang K. S. Novoselov S. Roth and A. K. Geim ., Phys. Rev. Lett. 97 (2006) 187401. [40 ] A. C. Ferrari Solid State Communications 143 (2007) 47
69 CHAPTER 3 : THE ROLE OF ARGON IN THE GROWTH OF NANO CRYSTALLINE DIAMOND FILMS 3.1 The R ole of Ar in N ano crystalline D iamond Film Argon is a noble gas with atomic number 18 and it plays an important role in the chemical vapor deposition process of ultra fine nanocrystalline diamond films Un like h ydrogen and methane which are active species in the formation of the diamond film a rgon does not directly participate the chemical reaction in the reactor; it is a medium to contain energy so that the chemical reaction can occur at a desired temperature. Argon is also used to strike the initial plasma and purge the system after the deposition Plasma may be ignited by argon at 100 200 watts with a flow rate in the range of 1500 2500sccm. Another import ant role argon plays in the chemical vapor deposition process is that, by changing the volume of argon, the pressure in the reactor can be adjusted to an expected value. Temperature and pressure are two critical physical parameters to determine the quality of diamond films during the chemical vapor deposition process. The size of plasma can be tuned by the pressure ; higher pressure results in smaller plasma, which has high er density, yields high er growth rate and a small er deposition area. In the present se ction, the influences of argon on the intrinsic nanocrystalline diamond films are studied with variation of temperature and pressure
70 3.2 Experimental D escriptions A series of experiments with various volume s of argon were carried out in the Astex chemical vapor deposition system ; the Astex CVD system is ideal for small size samples and exploring the properties of diamond films. All samples follow the same preparation procedure as state d in section 2.1.2. Each sample was cut into 2cm 2cm square piec es and was put into slurry made up with the mixing of methanol and diamond powder (5nm) to ultrasonic agitate for 15 minutes in order to get uniformed seeding on the surface of substrate. After oscillation seeding in an ultrasonicator the samples were rin se cleaned by methanol to remove the residues such as contaminants or impurities on the surface, and then they were dried by compressed nitrogen just before being placed into the CVD chamber. The above descr ip tion of sample preparation can be seen as stand ard protocol of pretreatment before loading samples into the CVD chamber. Table 2 illustrates the gas chemistry, pressure and temperature of the deposition process. The deposition time for each sample was three hours. Table 2. The depositing conditions of pressure, temperature and Ar variations Ar CH 4 H 2 T emperature P ressure 200sccm 2sccm 2sccm 75 0 C 12 0 HPa 190sccm 2sccm 2sccm 7 67 C 1 20 HPa 180sccm 2sccm 2sccm 7 70 C 120 HPa 170sccm 2sccm 2sccm 7 66 C 120 HPa
71 3 .3 Raman A nalyses of Nano Diamond F ilms on Ar V ariation Th ere are two important peaks nam ed D band and G band in the Raman spectra of nanocrystalline diamond in which G band is not presented in the Raman spectrum of polycrystalline diamond films correspond ing to a Raman shift of 1350cm 1 and 1500cm 1 respectively. D and G band were first reported by F. Tuinstra and J. L. Koenig in 1970 on a paper of graphite study by the Raman spectrum . When the CVD diamond study became prosperous, especially nanocrystalline diamond films, D band and G band have garnered greater interest. According to Tuinstra and Koenig , D band at around 1350cm 1 is attributed to a particle size effect ; the intensity of D band is inversely proportional to the crystallite size in the direction of the graphite plan e. In the Nano Diamond films, D band in the range of 1200 1400cm 1 is believed to be induced by disorder such as nano size crystallites, impurities, imperfections, edges, and related to the laser excitation energy ; the intensity ratio I D /I G is proportional to the graphitic in plane crystallite size . In carbon allotropes, 0D defines fullerenes, 1D indicates nanotubes, 2D represents two dimensional graphene, and 3D is stacked graphene into 3D graphite. G band is considered 2D graphite of first order Raman peak, which means one phonon is involve d in the Raman scattering . The full width at the half maximum (FWHM ) intensity of the G band is also dependent on excitation wavelength . In Raman spectra of intrinsic nanocrystalline diamond as demonstrated in figure 3 1 diamond has a characteristic emission at 1140 and 1332 cm 1 ; g raphite has a characteristic emission at about 1580 cm 1 By compari ng the height and sharpness of diamond peaks at 1140 cm 1 the sample s with 170sccm and 180scc m Ar flow rate produced the best quality of diamond film and contained the lowest percent age of
72 graphite. The Raman lasers with 514nm and 488nm wavelength are only sensitive to sp 2 bonding because the cross sectional area of sp 2 is much bigger than sp 3 [ 5 ]. The Raman laser of 244nm wavelength (sometimes called UV Raman) provides higher energy and thus allows sp 3 bonding to be effectively observed [ 6 7 ]. Fig ure 3 1 Raman spectra of n ano d iamond grown at same pressure 3 .4 SEM M orphologies of Nano Diamond Films on Ar V ariation Magnification with a n SEM of abou t 5 00 times is good enough to observe whether the coating is continuous or not SEM magnification of about 1,000 times or more is sufficient to determine the crystal type. In order to compare the grain shape s size s and boundaries, images at 50k magnification were selected to make a thorough comparison. In figure 3 2, SEM images of 170s ccm and 180sccm Ar did n o t show much difference on morphology due t o the variation in Ar flow rate. Both of them have a 1000 1200 1400 1600 1800 Intensity(counts/sec) Raman Wavenumber(/cm) 120 HPa 180sccm 170 sccm 190 sccm 200 sccm Nano Micro
73 cauliflower like surface with round or global grains and the surfaces are rather rough grain boundaries can be clearly observed. The i mage of 190sccm Ar is flatte r than those of 170 and 180sccm. The grain shape is similar to 170 and 180 sccm, but the sizes are much sm aller, and therefore grain boundaries are difficult to observe. It also showed that many long grain s exist and mix with ro und grains The i mage of 200sccm Ar has a totally different morphology from all of the above specimen. It has a surface like the ear of wheat which some literatures also prefer to call feather like topography Grain boundaries are rather short and small er than those of cauliflower like g r ains. From the perspective of a smooth surface, 190sccm Ar provides the best quality of Nano Diamond films. However, like the composition of surface s such as different sp bondings or graphite, or DLC, generally the prese nce of amorphous carbon or graphite can be visually observed since it blackens the surface of the film. Diamond with significant indication of multiple bonds with sp and sp 2 configuration forms a spherulitic or amorphous crystal structure considered as a d efects matrix in which sp 3 diamond crystallites are embedded [8 9 ] On the other hand, diamond film with a low sp 2 (graphitic) signal by Raman exhibits a faceted crystal structure like the polycrystalline diamond in chapter two
74 a SEM image with 170sccm Ar b SEM image with 180sccm Ar c SEM image with 190sccm Ar d SEM image with 200sccm Ar Fig ure 3 2 SEM images of n ano d iamond grown with various argon 3 .5 XRD C omparison of Nano Diamond Films on Ar V ariation A measurement of the diffraction pattern of X rays exposed to the diamond film can be used to determine the components of diamond films. Diamond diffracts X rays at approximately 44, 75 (and 92) two theta when irradiated by copper radiation. The presence of graphite is indicated by a peak at approximate ly 27. The presence of amorphous carbon is difficult to determine by X ray. Comparison of XRD patterns on the height of diamond (111) peaks in f ig ure 3 3 illustrate that a 190sccm argon flow rate c reated the best quality of diamond. However, i t
75 also contains a certain amount of diamond (311) peak. Considering the single orientation diamond crystal 180sccm and 200sccm recipes exhibit better performance tha n other recipe s The peaks of 170sccm also display that a tiny amount of (311) orientated diamond existed. Because diamond can be seen as an FCC structure, taking into account planar density of (111) and (220) in FCC, (111) equals to 0.29 / ( R 2 ), and (220) is 0.177 / ( R 2 ), so (111) is supposed to grow much slower than (220). With the increase of growth time, (220) will become a major component of nanocrystalline diamond film. In the current case for three hours deposition time, both planes were in same level of occupation. Fig ure 3 3 XRD pattern s of nano diamond grown at 120 HPa 3. 6 Pre ssure Influences on Nano Diamond F ilms Pressure is a most important factor in determin ing the formation of diamond films. Higher pressure causes the increase of the plasma density which promote s the carbon dimers in the gas phase. Carbon dimers are believed to be the precursors for 35 45 55 65 75 85 Angles (2 ) D (311) D (111) Si (400) D (220) 200sccm 190sccm 180sccm 170sccm
76 diamond growth and will increase the grow th rate of diamond films [ 10 1 2 ]. The high gas pressures may cause an etching effect on the graphite susceptor by atomic hydrogen which can be treated as an independent process parallel to diamond deposition [ 1 2 ] Research also indicated that diamond crystallite size decreased with increasing pressure [ 1 3 ]. A p ressure change from 5torr to 20torr also presented a texture orientation change from (111) to (100) for HFCVD polycrystalline diamond films [ 1 5 ]. More extensive studies on process parameters such as input power, substrate temperatures, gas pressu res and gas flow rates can be referred to literatures [1 5 1 9 ]. In the current work, v arious pressure effects on nanocrystalline diamond films were examined while keeping the same Ar, CH 4 and H 2 flow rate. Table 3. The experimental conditions of 180sccm Ar flow rate at various pressures Ar CH 4 H 2 T emperature P ressure 180sccm 2sccm 2sccm 770 C 120 HPa 180sccm 2sccm 2sccm 770 C 110 HPa 180sccm 2sccm 2sccm 700 C 100 HPa 180sccm 2sccm 2sccm 745 C 90 HPa 3 6 .1 Raman A nalyse s of Nano Diamond Films on P ressure V ariation The comparison of Raman spectra under various pressures is presented in figure 3 4. The nanocrystalline peaks of 90 HPa and 110 HPa are almost overlapped, illustrating that the contents of nano size diamond crystallites grown under th ese two press ures are almost the same amount. The peak of microcrystalline diamond of 90 HPa is higher than that of 110 HPa indicating that microcrystalline diamond in 90 HPa contains more than
77 that in 120 HPa The peak of G band in 90 HPa is higher than that in 110 HPa me anwhile, the width of G peak in both pressures are almost the same, disclosing the fact that the graphite structure in 90 HPa exists more than 110 HPa The as grown diamond films under 100 HPa and 120 HPa own the highest nano diamond peak in the Raman figure, but both the microcrystalline diamond peak and G peak of 120 HPa are higher than those of 100 HPa revealing that the diamond films of 120 HPa consist of crystalline structures more than that of 100 HPa Fig ure 3 4 The comparison of Raman spectra under various pressures 3 6 2 SEM Imag es of Nano Diamond Films on P ressure V ariation Figure 3 5 shows the micrographs of nanocrystalline diamond films grown with Ar 180sccm under 90 HPa with magnification of 10k, 20k, 30k and 50k respectively. In the image of 10k, the entire film consists of many big grains about 1m to 2m in diameter and the grain boundaries can be clearly observed. The high magnification image s show that the surface s are full of ear of wheat like diamond texture 1000 1200 1400 1600 1800 Intensity(counts/sec) Raman Wavenumber(/cm) 180 sccm Ar 120hpa 110hpa 100hpa 90hpa Nano Micro
78 Fig ure 3 5 Micrographs of nanocrystalline diamond films grown with 180sccm Ar at 90Hpa Figure 3 6 Micrographs of nano diamond films grown with 180sccm Ar at 100 HPa
79 Figure 3 6 displays images of diamond at 100 HPa with magnification of 10k, 20k, 30k and 50k respectively. Compared with images of 90 HPa in the image of 10k, it contains much smaller grains with a maximum diameter of 1m, and grain boundaries are not as clear as that of 90 HPa In the high magnification image s, surfaces also exhibit ear of wheat like clusters, but many tiny round nano sized round diamond crystallite s can be observed in the image of 50k which are not observed in the image s of 90 HPa It should be noted that, in the image of 30k at the bottom of right corner, and also in the image of 50k at the bottom of left corner, the growth of extra large diamond grains was observed due to the increase of pressure, from 90 HPa to 100 HPa Figure 3 7 Micrographs of nano diamond f ilms grown with 180sccm Ar at 11 0 HPa As can be observed, big difference s of the surface appearance of nanocryst a lline diamond films in f ig ure 3 7 exist compar ed with those in f ig ure 3 5 and f ig ure 3 6. In f ig ure 3 7 the ear of wheat like surface feature disappeared in place of a mixture of tiny
80 sized ( several tens of nanometers ) and middle sized (rang ing from one to two hundred nanometers) diamond agglomerates uniformly distributed in a random orientation. Fig ure 3 8 Micrographs of nano diamond films grown with 180sccm Ar at 120 HPa Figure 3 8 is the nano diamond morphology of 180sccm Ar at 120 HPa As a result of increased pressure to 120 HPa large diamond agglomerates can be obviously noticed and account for more than half a percent of the total area. On the other hand, the tiny sized diamond particles decreased to 30% from almost 50% in the sample of 110 HPa In conclusion, nanocrystalline diamond depositions with 180sccm Ar flow rate display various surface morphologies and structures under different pressure s At the low pressure of 90 HPa and 100 HPa diamond films show an ear of wheat like feature, indicating that nano diamond grew in a preferred orientation and texture structures existed in the films. However, at the pressure of 110 HPa and 120 HPa the ear of wheat
81 like feature disappear ed in place of a mixture of different sized diamond agglomerates. With the increase of pressure, the evenly distributed surface became rough due to the growth of extra large sized diamond grains. 3 6 3 XRD C omparison of Nano Diamond Films on P ressure V ariation XRD is an extensively utilized nondestructive technique that provides crystalline information such as phase presence, grain size, strain and preferential growth orientation in the research of CVD diamond films. Special attention should be paid to the fact that an X ray can penetrate diamond more than 100m in depth; the peaks of substrate sometimes exhibit stronger peaks than those of diamond [20 ]. For boron doped microcrystalline diamond films, low gas pressure was proved to be advantageous to the growth of (220) plane . For HFCVD microcrystalline diamond, the g rain size and surface roughness decrease d in accordance with the decrease of gas pressure, meanwhile enhancing the secondary nucleation rate . Pressure influence on the growth rate presented discrepancy in the previous reports [23 2 7 ] ; the reason s stil l remain inconclusive due to so many affected factors such as growth approaches, different gas reactants, various substrates et c It seems that the growth dependence on pressure is nonlinear ; however, it has an optimized pressure value for specific condition s Figure 3 9 shows XRD patterns of nano diamond g rown with 180sccm Ar at various pressures. For samples of 90 HPa and 120 HPa (311) plane was not presented; whereas, for samples of 100 HPa and 110 HPa the (111) plane seemed the preferential growth orientation. In addition to the above observed phenomena, t he intensit y ratios of the corresponding two diamond peaks i.e. (111) and (220) for any sample s are
82 proportional to each other, thereby confirm ing the fact t hat the variation of pressure s w ill not affect the orientation of diamond growth and their concentrations. Judged from the full width at half maximum (FWHM) of diamond peaks in fig ure 3 9, the specimen of 110 HPa demonstrated the smallest grain size and best crystallinity Fig ure 3 9 XRD patterns of nano diamond grown at various pressures 3.7 Summary Under constant depositing pressure of 120HPa, a rgon changed in the range of 170sccm to 200sccm with an increment of 10sccm for each sample. Raman spectra showed that samples of 170sccm and 180sccm Ar contained a low level of graphite and high volume of diamond. These two samples also displayed a cauliflower like surface with round or global grains The ample of 190sccm Ar presented a flatten ed surface in the SEM image, whereas the sample of 200sccm Ar displayed a texture structure of ear of wheat like surface. XRD patterns illustrated that all samples consisted of mainly (111) 42 52 62 72 82 Angles (2 ) D (111) Si (400) D (220) Intensity D (311) 100hpa 110hpa 120hpa 90hpa
83 and (220) diamond planes ; only sample of 190sccm Ar presented a high sharp (311) plane. As evident from the inten sity and ratio of different peaks, the 190sccm s ample is found to b e the best quality This conclusion was also consistent with the SEM image. With a constant Ar flow rate of 180sccm, the depositing pressure s varied from 90HPa to 120HPa with an increment of 10HPa for each sample. Raman spectra revealed that the sample of 120HPa produced the highest quality of nano scale di a mond films. The SEM images of 90HPa and 100HPa samples exhibited texture structure, the 110HPa sample exhibited a flat cauliflower like outlook and the 120HPa sample exhibited a rough cauliflower like surface. XRD patter n s disclosed that of the sample s composed of (111) and (220) plane, the sample of 110HPa presented the highest peak intensity and unique (311) peak, whereas 90HPa showed t he lowest peak intensity. The peak ratios between (111) and (220) for all samples were identical and thereby demonstrat ed that the change in pressure has no effect on the texture orientations. 3.8 List of References  F. Tuinstra & J. L. Koenig, J. Chem. Phys. 53 (1970) 1126  K. Sato R. Saito, Y. Oyama, J. Jiang, L. G. Canc, M. A. Pimenta, A. Jorio, Ge. G. Samsonidze, G. Dresselhaus, M. S. Dresselhaus, Chemical Physics Letters 427 (2006) 117  M. S. Dresselhaus G. Dresselhaus, R. Saito and A. Jorio Physics Reports, 409 (2005) 47  A. Yoshida, Y Kaburagi and Y Hishiyama Carbon, 44 (2006) 2333  N. Wada, P. J. Gaczi, A. Solin, J. Non Cryst. Sids., 35/36 (1980) 534 [ 6 ] A. C. Ferrari, J. Roberson, Phys. Rev. B, 64 (2001) 75414
84  K. W. R. Gilkes, H. S. Sands, D. N. Batchelder, J. Robertson, W. I. Milne, Appl. Physc. Lett., 70 (1997) 1980  J. E. Butler and A. V. Sumant J. Chem. Vapor Depos. 14 (2008) 145  D. M. Gruen Annu. Rev. Mater. Sci. 29 (1999) 21 1. [10 ] X. Li, J. Perkins, R. Collazo, R. J. Nemanich and Z. Sitar, Diamond Relat. Mater. 16 (2006) 1784 [11 ] A. B. Muchnikov, A. L. Vikharev, A. M. Gorbachev, D. B. Radishev, V. D. Blank, S. A. Terentiev, Diamond Relat Mater ., 19 (2010) 432 [12 ] H. Sternschulte, T. Bauer, M. Schreck and B. Stritzker, Diamond Relat. Mater. 15 (2006) 542. [13 ] M. S. Kang, W. S. Lee, Y. J. Baik, Thin Solid Films 398 (2001) 175 [14 ] Z. Yu, A. Flodstrom, Diamond Relat. Mater. 6 (1977)81. [15 ] Y. Muranaka, H. Yamashita and H. Miyadera Diamond Relat. Mater 3 (1994) 313 [16 ] Z. P. Lu, J. Herberlein and E. Pfender Plasma Chem. Plasma Process. 12 (1992) 35 [17 ] R. Samlenski, G. Flemig, R. Brenn, C. Wild, W. M¨uller Sebert and P. Koidl Diamond Relat. Mater. 3 (1994) 1091 [18 ] J. W. Steeds, A. Gilmore, K. M. Bussmann, J. E. Butler and P. Koidl Diamond Relat. Mater. 8 (1999) 996 [19 ] P. K. Bachmann, H. D. Bausen, H. Lade, D. Leers, D. U. Wiechert, N. Herres, R. Kohl and P. Koidl Diamond Relat. Mater. 3 (1994) 1308  J J Gracio, Q H Fan and J C Madaleno, J. Phys. D: Appl. Phys. 43 (2010) 374017  F. J ia Y B ai F. Qu J S un J J. Z hao X J iang New Carbon Materials 25 ( 2010 ) 357  X B. Liang, L Wang, H L. Zhu, D R. Yang, Surface and Coatings Technology 202 ( 2007 ) 261 [ 23 ] T. Wang, H.W. Xin, Z.M. Zhang, Y.B. Dai and H.S. Shen, Diamond Relat. Mater. 13 (2004) 6
85 [ 24 ] S A Rakha, X. T. Zhou, D Z. Zhu, G J. Yu Current Applied Physics 10 ( 2010 ) 171  S. Schwarz, S. M. Rosiwal, M. Frank, D. Breidt and R. F. Singer, Diamond Relat. Mater. 11 (2002) 589.  R. Brunsteiner, R. Haubner and B. Lux, Diamond Relat. Mater. 2 (1993) 1263.  S.J. Harris and A.M. Weiner, J. Appl. Phys. 75 (1994) 5026.
86 CHAPTER 4 : NITROGEN DOPED NANOCRYSTALLIN E DIAMOND FILMS 4.1 Introduction to Nitrogen Doped Nano Diamond Films Theoretically intrinsic CVD diamond is an insulator, because of the in troduction of graphitic phase in the deposition process, it exhibits very low conductivity ; for example, an intrinsic CVD diamond sample can be observed directly by SEM without any special metallic coating treatme nt. Sometimes, the conductivity of diamond is desired for applications such as MEMS, biosensors, biochemical and electrochemical electrodes without spoiling the excellent properties of diamond. The addition of an impurit y called dopant, will dramatically increase the conductivity of pure semiconductor materials Diamond, which is a specific cubic structure of carbon, is a kind of pure semiconductor. The doped diamond can be classified into two different types : the n type and p type. The typical dopant of p type diamond is boron, which has been extensively studied and well accepted with a high conductive property. N itrogen and phosphorus are the widely preferred dopants Until now n itrogen doped diamond has bee n left much to be desired to improve conductivi ty. Nitrogen is an interesting element in the process of CVD diamond in two extreme aspects. First, it is hard to completely avoid, sometimes even the impurity of the ppm level in gas resource can result in 0.1% to 0.3% existence of nitrogen in diamond films. Nitrogen can also be seen ubiquitously in the natural diamond in the presence of a blue
87 color. Nitrogen is believed to modify the m orphology and texture of poly cry stalline (micro and nano size) diamond films, increase the growth rate and weaken the boundaries [1 7 ] On the other hand, nitrogen sometimes is purposely added into the gas chemistry as a dopant to grow n type extrinsic diamond films; during this process the content of nitrogen become s saturated at 0.2% in diamond films when the nitrogen flow rate reaches no more than 20% in gas flow mixtures Incorporation of nitrogen in nanocry s talline diamond films is inhomogeneous and rich in boundaries in the form of interstitial rather than subst itutional distribution The energy level of nitrogen is 1.57 eV below the minimum conduction band, making it virtually poor electrical conductivity of n type semiconductor material [ 8 15 ]. The nitrogen doped nanocrystallin e diamond films were fabricated b y the Astex CVD system. The nitrog en composition during the CVD process is conventionally defined in terms of gas fl ow rate with the unit of sccm, because it is technically difficult to quantify the content of nitrogen on the diamond surface. In the presen t study, the e ffects of various N 2 concentrations such as 5%, 10% and 15% on the quality of nanocrystalline diamond films were characterized by Raman spectroscopy, SEM, XRD as well as AFM t echniques. 4.2 Experimental D escriptions The preparation of specimen prior to being loaded inside the reactor can be re fer r ed to in the descri p tion in chapter 2.1 The reactant gases include H 2 CH 4 Ar and N 2 to e nsure that the diamond grown consists of nanosized grains ; the concentrations of N 2 that fed into the CVD chamber are at the 5%, 10% and 15% in sccm flow rate. The
88 details such as gas recipe flow rates, temperatures, pressures are listed in the following table 4. The deposition time for each of samples is three hours. Table 4 D epositi on parameters of nano diamond films with various N 2 concentrations N 2 % CH 4 H 2 N 2 Ar T emperature P ressure 0% 2sccm 2sccm 0 180 sccm 750 C 120 HPa 5% 2sccm 2sccm 10sccm 186sccm 800C 1 2 0 HPa 10% 2sccm 2sccm 20sccm 176sccm 85 0C 12 0 HPa 15% 2sccm 2sccm 30sccm 166sccm 885 C 120 HPa 4.3 Raman A nalyses on N 2 V ariation Figure 4 1 shows the Raman spectra for three different concentrations of N 2 As can be observed, w ith the increase of N 2 concentration nano diamond peaks around 1140 cm 1 were strongly inhibited. The nano peak s of the samples with 10% N 2 and 15% N 2 are hardly discernible D band in 5% N 2 has a remarkable shift c ompared with 10% and 15%N 2 indicating that higher surface compressive stress lower ed due to nitrogen highly incorpora ted into the diamond films ; additional nitrogen creates more boundaries and thus set up more space and this space relaxed the thermal stress between the diamond layer and Si substrate during the cooling process  G band in 5%N 2 is more highly oriented than those in the o the r two N 2 concentrations illustrating that crystalline graphitic phase reducing with the increase of nitrogen concentration
89 Fig ure 4 1 Raman spectra of nano diamond for three different concentrations of N 2 Because the sample of 5% N 2 has the highest nano diamond peak in f ig ure 4 1, a comparison is made in f igure 4 2 between 5% N 2 and intrinsic nano diamond with 180sccm Ar so that the function of N 2 can be further clarified in the process of CVD. It is obvious that the intrinsic diamond has an apparent nano diamond peak whereas the nano peak of extrinsic diamond is shown as severely prohibited ; this might be caused by the large diamond grain grown due to atomic N The sharpness of the peak can explain the crystallinity of crystal phase in the Raman spectrum The heights of both D band and G band in intrinsic diamond are much higher than that of doped diamond indicating that with the addition of nitrogen, the crystallinity of nano diamond film bec o me s degraded There still exists a microdiamond peak shift in D band ; the same explanation in f ig ure 4 1 can be applied to this figure. 1000 1200 1400 1600 1800 Intensity(counts/sec) Raman Wavelength(/cm) 5%N2
90 Fig ure 4 2 Comparison of intrinsic and 5% nitrogen doped nano diamond 4.4 SEM M orphologies on N 2 V ariation Figure 4 3 presents SEM image s of various N 2 concentrations at the same pressure of 120 HPa The i mage of 5% N 2 has a relatively smooth surface and almost invisible boundaries, whereas the image of 10% N 2 has a rough surface and big grain size; with the N 2 concentration increasing to 15%, the image displays a cragged surface and darkened boundaries which indicate sp 2 bondings It can be concluded that t he increased additions of N 2 result in a rough surface, big grain size and weak boundaries. 1000 1200 1400 1600 1800 Intensity(counts/sec) Raman Wavenumber(/cm) 5%N 2 Intrinsic 120 HPa 180 sccm
91 Fig ure 4 3 Micrographs of N doped nano diamond films grown at 120 HPa In order to further clarify the function of N 2 in CVD diamond deposition, nitrogen doped diamond samples with various pressures are presented below. The gas recipe is the same as that listed in t able 4 for each sample. Figure 4 4 shows SEM image s of the 5% N 2 incorporated sample with magnification of 20k and 50k respectively. As evident from the SEM images the diamond grains are elong ated compared with the image in f ig ure 4 3 with the same N 2 concentration due to the increase of pressure. The preferentially diamond growth results in roughness of surface.
92 Figure 4 4 Micrographs of 5% N 2 doped nano diamond films grown at 130 HPa Figure 4 5 reveals the surface feature of the 10 % N 2 incorporated sample with magnification of 20k and 50k respectively. Compared with image s of the same N 2 concentration in f ig ure 4 3, the diamond grain shape and size are almost the same regardless of a 10 HPa pressure difference between the two samples. Up on comparing the images of the sample without the N 2 addition ( f ig ure 3 7 ) we found that the grains are elongated in the case of the sample with the N 2 addition Fig ure 4 5 Micrographs of 10% N 2 dop ed nano diamond films grown at 11 0 HPa Figure 4 6 displays the surface morphology of the 15% N 2 incorporated sample at 100 HPa As illustrated above, the increase of pressure will produce large diamond
93 agglomerates. In this case, the pressure is 20 HPa less than the sample in f i ure 4 3, but it l arge diamond clusters and obvious boundaries can still be observed. Briefly SEM image s indicated the role of N 2 during the deposition of diamond as follows: N 2 addition causes elong ated diamond grains as a surface feature, produces a large cluste r with increased boundaries; consequently, the roughness also seem s to increase due to the rise of N 2 composition. Fig ure 4 6 Micrographs of 15% N 2 doped nano diamond films grown at 100 HPa Although there still remains controversies about morphol ogical dependence on the N 2 concentrations, excluding the influence of surface conta mination and oxidation, atomic nitrogen did change the surface feature by incorporating into boundaries, maybe sometimes, it is not apparent because of the low differences of gas composition pressures [ 17 23 ]. 4.5 XRD C omparison on N 2 V ariation Figure 4 7 presents the XRD patterns of nano diamond grown at 5%, 10%, and 15 % N 2 concentration respectively. It can be observed that for the XRD pattern of 5% N 2 (111) plane s account for the films less than (311) plane s, approximately half in half
94 However, for samples of 10% and 15% N 2 the (111) planes occupy the majority of the films, while the (311) planes only account for 1/3 of the total area. Fig ure 4 7 XRD patterns of nano diamond grown at various N 2 concentrations 4. 6 I V Characteristics and Ohmic C ontact of NNCD F ilms In CVD diamond, sp 3 bond ing s are believed to be responsible for the mechanical properties, whereas sp 2 bond ing s, which lie near the Fermi level, determine the optical and electronic properties of the films [ 24 ] The addition of N 2 into intrinsic nanocrystalline diamond films is expected to incorporate atomic N in the boundary areas in the form of sp 2 bonding, thus increas ing the conductivity of nano diamond films. In order to make a satisfying integration of diamond into semiconductor devices, high quality contacts are required. Most semiconductor materials are non oh mic contact due to the complexes of carrier transportation mechanism. Ohmic con tacts to p type diamond have been achieved by using carbide forming metals (such as Mo, Ta, Ti) in combination with annealing [25 27] Ohmic contacts to n type diamond are beli e ved to 35 45 55 65 75 85 Angles (2 ) D (111) Si D (220) D(220) 5%N2 10%N2 15%N2 Angles (2 ) D (111) Si D (220) D(220) 5%N2 10%N2 15%N2 Angles (2 ) D (111) Si D (220) D(220) 5%N2 10%N2 15%N2 Angles (2 ) D (111) Si D (220) D(220) 10%N2 15%N2 Angles (2 ) D (111) Si D (220) D(220) 10%N2 15%N2 Angles (2 ) D (111) Si D (220) D(220) 10%N2 15%N2 D(311)
95 be much more difficult to realize due to the lower conductivity than tha t of p type (boron doped) diamond Nitrogen and phosphorus are the two most promising elements to be used as donors for n type diamond in the Periodic Table. Titanium ohmic contacts to heavily phosphorus doped diamond film s have been reported as exhibiting low contact resistances [28 29 ]. Silver (Ag) wh ich is believed to not form carbide compounds, is an attractive metal to be employed for ohimc contact in the semiconductor industry. Ohmic contacts of A g monolayer by RF sputtering to B ion implant ed p typ e diamond have b een successfully fabricated [ 30 31 ]. However, A g thin layer by PLD (pulsed laser deposition) ohmic contact to nitrogen doped n type diamond films have not been reported. In the present study, ohmic contacts of A g thin layer by PLD combined annealing to nitrogen doped n type diamond films are successfully achi e ved. The advantage to us ing PLD to make a metallic contact is that the deposition takes place in a highly vacuumed environment (can reach 10 9 mbar by turbo pump), thus is avoid s any impurity and contamination. The high quality contact thin fi lm is prepared by ablation of the Ag target focused by pulsed laser beam in a controllable process of tuning the inert gas pressure and kinetic power of the laser. Table 5 is the silver conta ct deposition parameters by the PLD method. The samples were grown using the gas mixture H 2 /CH 4 /Ar/N 2 with the flow rate ratio at 2.5sccm/2.5sccm/165sccm/30sccm, hence, it contained 15% N 2 After PLD deposition, the samples were annealed at temperatures o f 100C, 150C, 250C and 300C in order to find the optimized temperature for silver deposition to obtain an ohmic behavior for Ag/ NCD interface, and 250C was found the best temperature for ohmic contact.
96 Table 5 Silver contact deposition parameters by PLD method Laser energy (mJ) 200 Deposition time (min) 10 Repetition rate (Hz) 10 Pressure (Torr) 1 10 6 Target Ag Substrate N NCD film/p Si substrate Substrate temperature ( C) 250 Distance from target to substrate (mm) 40 Current voltage (I V) characteristics were studied for the various interfaces as shown in figure 4 8. Figure 4 8a shows the Ohmic behavior of silver for the silicon substrat e and f ig ure 4 8b shows the I V curve for the interface Ag/NNCD, which is also a straight line suggesting the Ohmic behavior for the junction. Since s ilver was annealed at 250 C as the optimized temperature to obtain o hmic contacts t he annealing effect of the high temperature function ed on the interface of Ag electrodes and N NCD film which could possibly rule out the residual stress and microstructure imperfect ion s Here it is worth while to mention that the electrical resistance on Ag contact reduced 10 times measured at a heat ed temperature of 250 C [ 3 2] It can be seen from the observed I V behavior that it is linear for both forward and reverse bias. This suggests that N NCD film s are compatible to the Ag thin fil m contact, which has a high s ignificance for the practical application of N doped NCD film. It was observed that the current value for Ag/Si and Ag/ N NCD presents a big difference as shown in f ig ure 4 8a and b; t he
97 Ag / N NCD film can have nearly 100 times less electrical conduct ivity than that of the Ag / Si substrate This high resistivity feature of N NCD film combined with excellent thermo mechanical property makes it a promising material for specialized dielectric applications [ 33 ]. Under the as grown conditions, the room temperature resistivity of NNCD film was calculated as A Schottky type I V curve shown in f ig ure 4 8c was obtained for p n hetero junction between the N NCD film and p type Si substrate. The rectifying I V curve induced by the heterojunction structure proves that the majority of carrier s for the N NCD film are electron s It presents a perfect rectifying property at the voltage range from 0.5 to 0 V and indicates that the NNCD film can be used as a potential rectifier. T he turn on forward voltage is abou t 0.2 V which is close to the value of 0.3 V of turn on voltage of Ge/SiC heterojunction diode in reference [34 ]. This result indicated that nitrogen doped nanocrystalline diamond films might serve as an n type semiconductor material.
98 (a) interface of Ag / p Si (b) interface of Ag/ N NCD (c) heterojunction of NNCD /p Si Fig ure 4 8 I V c haracteristics for various interfaces 4.7 N NCD F ilms G rown on SiO 2 L ayer Interests in nanocrystalline diamond integrated MEMS devices have been increas ing over the past decade due to the excellent properties of diamond such as super hard, wide bandgap, high mechanical strength, high thermal conductivity, chemical stability and biocompatibility [1 4]. To fulfill the requirement of MEMS applications, o ne of the basic practical goals of nanocrystalline diamond film deposition (NCD) is to
99 obtain a large area of film with uniform thickness for diamond based micro device T his goal h as not been completely achieved partly due to the limi tation of CVD technique itself like the size of plasma , mostly for the complexity of the grow th mechanism of CVD deposition. N anocrystalline diamond films grown by the microwave plasma enhanced chem ical vapor deposition ( MPE CVD) technique have been extensively studied to understand the growth mechanism, surface characteristics sp 3 /sp 2 ratio, electrical properties, optical properties and mechanical properties such as morphology, roughness, hardness tri bology [6 17 ] Many researche r s demonstrated that the growth morphology of NCD films have a strong dependence on the substrate temperature, reactant gas mixture, pressure and the hydrogen treatment [18 20] O ther investigations suggest that the diamon d powder seed size and the seeding preparation method such as the mechanical seeding or the ultrasonic vibration seeding can have a great influence on the nucleation of diamond film [21, 22]. On the other hand, interests are focused on the surface charge c arrier diffusions or hydrog enation treatments to explore the electrical properties of diamond films [23 28]. However, the distribution and concentration of electrons or ions on surface and their correlation with the morphology and structural feature of the diamond films have not been explored. In reality the chemical vapor deposition is not only a simple mechanical or chemical process but it also involves a surface electric reaction that is critical for the growth process Since SiO 2 is the most commonly applied layer in MEMS, variations of diamond film on SiO 2 were significant to MEMS fabrication. In this section, n itrogen doped nanocry stalline diamond films (NNCD) with interface structures like 1) NNCD/Si 2) NNCD/SiO 2 /Si, and 3) NN CD/NCD/Si were grown using the microwave plasma
100 enhanced chemical vapor deposition method. B y particularly design ing the diverse layer structures, the important role of electron diffusion during the CVD process of diamond films was investigated The gas ch emistry for NNCD wa s H 2 /CH 4 /Ar/N 2 with the flow rate ratio at 2.5sccm/2.5sccm/165sccm/30sccm, which contained 15% N 2 ; for NCD ( intrinsic nanocrystalline diamond ) film it was H 2 /CH 4 /Ar at 1 8 0sccm/2sccm/2sccm ratio The deposition temperature was in the range of 750C~800C the pressure was around 110 HPa and the power supply was 800 W. In contrast with the intrinsic nanocrystalline diamond films, the addition of N 2 is believed to increase the grain size of diamond [29 33]. Simulations of the kinetic M onte Carlo and molecular dynamics method under various loading conditions also proved that N 2 addition increased grain size and weakened the mechanical properties of nitrogen doped diamond films . In addition, many reports demonstrated that diffusion d id happen during the CVD process on the sub surface of diamond films [23 26, 35, 36]. Fig ure 4 9 a shows the SEM image of the NNCD/Si structure D iamond agglomerate s can be obser ved with a diameter of ach grain might consist of several hundreds of crystallites. Crevice like dark boundaries indicate that the compositions and structure in boundary are not similar to the surface of the film Amorphous diamond boundaries that are rich in N atoms and sp 2 graphite bon ding s establish the conductive tunnel for charge carriers and electrons. Once diffusion occurs, charge carriers accumulate and cause diverse morphologies due to the existence of a block ed layer underneath nitrogen doped diamond films such as intrinsic diam ond or SiO 2 thin films. Figure 4 9 b show s the surface morp hology of the NNCD/NCD/Si structure. It can be seen that the dia mond agglomerates are much smaller, and the width
101 of boundaries are narrow and not as clear as that of the NNCD/Si structure. The NNC D/NCD/Si structure was formed by first depositing intrinsic nanocrystalline diamond film for two hours according to the gas chemistry Ar/CH 4 /H 2 at a ratio of 180sccm : 2sccm : 2sccm then depositing nitrogen doped nanocrystalline diamond film for another two hours in the gas chemistry of N 2 /Ar/CH 4 /H 2 at a flow rate of 30sccm : 165sccm : 2.5sccm : 2.5sccm I ntrinsic diamond films by the CVD method consist of many sp 2 bonding s i.e. a graphite structure that is high ly conductive. Similar to the case of NNCD films there exist more sp 2 bonding s of graphite than sp 3 bonding s of diamond in the boundary region of intrinsi c nanocrystalline diamond films The boundary region of intrinsic nan ocrystalline diamond serves as a channel for the migration of charge carriers and electrons and thus reduce s the density of charge carriers and electrons on the surface of diamond films This cause s the non uniformity in density in various part s of the surface and results in different morphology and roughness of the diamond surface. Figure 4 9 c shows the SEM image of the NNCD/SiO 2 /Si layer structure There is no crevice like grain boundary observed and the roughness of the surface is small er as compared to the previ ous two layer structures i.e. NNCD/Si and NNCD/NCD/Si. In perspective of charge carrier diffusion, it is proposed that the 100nm thick SiO 2 layer has blocked the diffusion and formed a uniform density of charge carrier on the surface of the diamond film, thus result ing in a better quality o f nanocrystalline diamond film.
102 (a) NNCD/Si (b) NNCD/NCD/Si (c) NNCD/SiO 2 /Si Figure 4 9 SEM image s of NNCD films for (a) NNCD/Si, (b) NNCD/NCD/Si, (c) NNCD/SiO 2 /Si Figure 4 10 a shows the atomic force microscopy image of the NNCD/Si structure The dark brown area corresponds to the boundary region in the SEM which is rich with charge carriers. Figure 4 10 b i s the AFM image of the NNCD/NCD/Si structure, withi n the same scanning area of 10m10m The surface feature indicates that the partial diffusion appears at the interface layer of intrinsic nanocrystalline diamond films while the concentration of charge carriers was not observed to change as much as in the case of the NNCD/Si structure. In f ig ure 4 10 c for the NNCD/SiO 2 /Si structure, the
103 nanocrystalline diamond display s identical grain size The average roughness of ~ 29 nm is the smallest amo ng these three layer structures. This suggests that the SiO 2 thin laye r prohibited the charge migration toward the p type silicon substrate and formed an identical density on the surface. Figure 4 10 AFM topograghic images of NNCD films for (a) NNCD/Si, (b) NNCD/NCD/Si (c) NNCD/SiO 2 /Si
104 The XRD diffraction pattern ( f igure 4 11 ) reveals that the major phase in as deposited films is diamond (111). The highest intensity peak occurs in the NNCD/SiO 2 layer structure, indicating high phase concentration in this structure. analytical software was e mployed to obtain the data such as intensity, space distance, half width at full maximum. Grain size can be calculated by the Scherrer formula t = 0.9 / (Bcos ) Eq. 2 where t = = Scherrer constant, = the wavelength which is 1.54 , and B = full width at half maximum (FWHM). Table 6 presents th e calculated results of the grain size. As seen in t able 6 NNCD/SiO 2 /Si has the lowest roughness, smallest crystallite and highest growth rate. Table 6 Structural features of diamond for different layer structures Structure Roughness Crystallite d spac ing() Growth rate NNCD/Si 76nm 2.079nm 2.04 NNCD/NCD/Si 46nm 1.038nm 2.06 NNCD/SiO 2 /Si 29nm 0.3163nm 2.05 From this it can be concluded that the diffusio n has a great influence on the growth of diamond films, and the SiO 2 layer blocks the migration of charge carriers toward the p type Si substrate. It should be pointed out that diamond at (111) plane appeared highest in the NNCD/SiO 2 /Si structure and lowest in the NNCD/Si structure It can be explained that the SiO 2 layer successfully blocks hydrogen and electron diffusion toward the p type silicon substrate, and causes high a concentration of hydrogen on the diamond film surface in the form of H + H H 0 as well as electrons, resulting in negativ e
105 affinity of substra te surface This leads to the enha n cement of nucleation and formation of sp 3 bond ings and further develops into a higher concentration of diamond structure Figure 4 11 XRD p attern of NNCD films for different layer structures In a brief summary, d iffusion of charge carriers during the CVD process has great influence on the topographic feature as well as crystallites of nanoc r ystalline diamond film. Three different layer structures NNCD/Si, NNCD/NCD/Si and NNCD/SiO 2 were especially designed to stu dy this phenomenon. D iffusion does not chang e the d spacing of diamond but greatly affects the morphology size of diamond crystallite, and the growth rate of diamond film. The SiO 2 layer underneath the diamond film prohibited the migration of charge carri ers toward the silicon substrate. Understanding this phenomenon is significant in mak ing higher quality nanocrystalline diamond films. Figure 4 12 indicate s the Raman sp ectra of three layer structures NNCD/Si, NNCD/NCD/Si and NNCD/SiO 2 It is evident from figure 4 12 that the NNCD/Si has no 33 35 37 39 41 43 45 47 Angles (2 ) Si (100) D (111) Intensity(counts/sec) NNCD/Si NNCD/SiO 2 /Si NNCD/NCD/Si
106 nanopeak, whereas, both NNCD/NCD/Si and NNCD/SiO 2 /Si contains the nanopeak This can be explained as the SiO 2 layer and NCD layer successfully prevented the electrons and negative atomic hydrogen from migrat ing downward to Si substrate, and consequently the higher density of negative charges on the surface enhanced the growth of nanosized diamond. The identical shape and feature of the Raman spectrum of the NNCD/NCD/Si and NNCD/SiO 2 /Si structures indica ted th at the SiO 2 layer and NCD layer played a similar role in blocking electron diffusion in the CVD deposition process. Figure 4 12 Raman spectra of three layer structures Figure 4 13 is the Raman spectrum of nitrogen doped nanocrystalline diamond film on quartz ; quartz can be usually considered as SiO 2 bul k material. The gas chemistry i s similar to the one d escribed earlier i.e., H 2 /CH 4 /Ar/N 2 with a 2.5sccm/2.5sccm/165sccm/30sccm flow rate ratio which contain s 15% N 2 As evident from the Raman spectra the nanopeak was sharp and high and the HWFM wa s broaden ed indicating that the concentration of nanosized diamond account ed for a large 1000 1200 1400 1600 1800 2000 Intensity Wavenumber cm 1 4hrs NNCD 2hrs NNCD/2hrs NCD 4hrs NNCD 2hrs NNCD/2hrs NCD 2hrs NNCD/SiO2
107 percent age of film and the grain size was rather small. This figure further ed the conclusion that substrate with differ ent materials can greatly alter the feature, quality and property of diamond film and the reason for that might be the variation of density of charge particles such as electrons, atomic hydrogen caused by atomic diffusion toward the substrate. Fig ure 4 13 Raman spectrum of nanocrystalline diamond film on quartz 4 .8 Summary Nitrogen doped nanocrystalline diamond films with three different concentrations of 5% N 2 10% N 2 and 15 % N 2 were deposited at 120HPa. I n the 10% N 2 and 15% N 2 samples there are no nano peaks due to the increase in diamond grains. The SEM image s further proved the growth of diamond crystallites accompanying the increase of N 2 flow 1000 1200 1400 1600 1800 Intensity Wavenumber cm 1 D quartz 03
108 rate. The s ample of 5% N 2 presented elongated grains due to 10HPa decrease of pressure, and similar symptoms were observed in the sample of 10% N 2 and 15% N 2 with the increase of 10HPa and 20HPa respectively. The XRD patterns showed that the major crystalline structures were (111) and (311) planes for all of the three N 2 concentrations. Each of (111) and (311) accounted for a half percent in the 5% N 2 sample; whereas in the 10% N 2 and 15% N 2 samples, the (111) planes occupied the majority of the films, and the (311) planes only acc ounted for 1/3 of the total area. Ohmic contact of silver on nitrogen doped nanocrystallline diamond films was successfully achieved using the PLD method, and I V characteristics between diamond film and Si substrate were also obtained. Nanocrystalline diamond films of 15% N 2 grown on different layer structures NNCD/Si, NNCD/NCD/Si and NNCD/SiO 2 were especially designed to study the surface change. It turned out that different structural substrates would greatly influence the surface morphology due to the charge atomic particle variation resulting from the diffusion. Characterization of SEM, AFM, Raman spectra and XRD illustrated that the SiO 2 layer would enhance the growth of nano scaled diamond crystallites, decrease the roughness and increase the gro wth rate.
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114 CHAPTER 5 : POLYCRYSTALLINE DIAM OND FILMS SYNTHESIZED BY MICROWAVE AND HOT FI LAMENT CVD 5.1 Introduction to HFCVD T echnique Hot filament chemical vapor deposition (HFCVD) is another popular alternative method of diamond syn thesis because of it s low capital cost and simple operation. Similar to microwave chemical vapor deposition, it also consists of four major subsections: reactant gas supply, power energy supply, vacuum exhaustion system, and heating and cooling components. Power energy supply distinguishes these two systems by different energy source s to activate the gas chemistry Microwave CVD utilizes microwave radiation generated by a magnetron to dissociate the reactant molecular gas es whereas HFCVD uses a tungsten (W) filament glowed by dc or ac current to about 2000 2300 C to decompose carrier molecul e s into atomic ions  It should be noted that the temperature of deposition on the substrate surface is between 700 1300C which is much lower than the temperature of filament and the gas activation temperature. By changing the length, shape and size of the tungsten filament, like a function of tuning, the deposited diamond in various area s with different qualit ies can be achieved. However, the glowed tungsten wire is readily being oxid iz ed by oxygen and some other oxidant gases ; a highly vacuumed ambient environment is necessary for the HFCVD process. In addition, the tungsten filament was discovered to react with methane and result in carburization of the
115 tungsten wire during deposition process; this phenomenon will absolutely induce contamination s on the deposited diamond surface [ 1 6 ] A s ilicon wafer was cut into several pieces. The dimension of each piece will be kept at 2 .5 cm 2 .5 cm a piece and ea ch piece will be treated according to the standard seeding process described in chapter 2.1.2. For HFCVD, t he gas recipe is CH 4 /H 2 at 5sccm/60scccm, the temperature is 730 C, and the pressure is 20 HPa The experiment was carried out by an HFCVD 008 system manufactured by Blue Wave Semiconductors Inc. F igure 5 1 is the outlook of the system. Fig ure 5 1 Image of HFCVD 008 system
116 For microwave CVD, the gas chemistry was CH 4 /H 2 at ratio of 1 0sccm /100 0sccm as illustrated in table 1 of chapter 2.1.2. The depo sition temperature was 650C, and the pressure was 50 HPa 5.2 Raman S pectra and S tress E valuation of P olycrystalline D iamond F ilm Figure 5 2 displays the R aman peaks at four corners of the specimen. As can be observed, the upper left and bottom left corner s have the same micro diamond peak at 1330.15 cm 1 The upper right and bottom right corner s have the same peak at 1331.69 cm 1 The Raman shift is 1.54 cm 1 indicating that a residual stress gradient exists on the diamond film surface because of th e different thermal coefficient of diamond and Si substrate. Diamond film was grown at 730C in this case, during cooling process, compressive thermal residual stress was yielded on the diamond surface because diamond has higher thermal conductivity than that of Si substrate, thus diamond cooled faster than Si substrate If the Ram an peak shifts to a higher frequency, it indicates compressive stress ; if the Raman peak shifts to a low wave number it represents tensile stress 
117 Fig ure 5 2 Raman peaks of HFCVD poly crystalline diamond at four corners of specimen Figure 5 3 shows the R aman peak in the center of the specimen at 1 3 3 3.23 cm 1 Compared with the peaks at the four co r ners, the center of t he specimen had the compressive stress. In this case, the lowest stress is at the right side with 133 1.69 cm 1 ; w ith respect to the point of center to the edge, t he biggest difference of the Raman shift is 1.54 cm 1 toward a higher frequency, hence the residual stress at the center is supposed to be compressive.
118 Fig ure 5 3 Raman peaks of HFCVD poly crystalline diamond at center and corner s of specimen Figure 5 4 indicates the Raman peaks of poly crystalline diamond film by the MPECVD method. The peaks were measured at the four corners as well as the center of the sample As can be observed, the corner peaks are positioned at 1337.85cm 1 ; the center peak is located at 1333.23cm 1 The between the corners an d the center equals 4.62cm 1 If the center peak is considered to be moving toward to the lower frequency, the surface stress in the center is supposed to be in a tensile status compared with those of the corners. 1000 1200 1400 1600 1800 Intensity Wavenumber /cm center 1333.23 center 1333.23
119 Fig ure 5 4 Raman peaks of MP ECVD poly crystalline diamond films The residual stress of CVD diamond films can be categorized into two different parts; one is the thermal stress resulting from the different thermal expansion between diamond and substrate; another one is the intrinsic stress due to the growth process, for instance, grain protrusion. The stress reflected by the Raman shift is the resultant stress of thermal and intrinsic stress . There are several different models to evalua te stress by the Raman peak shift, but the results of them contradict each other due to complexit ies such as local microstructure, crystalline orientation, film thick ness and boundary conditions  A reference to the Raman peak of stress free diamond film is necessary to calculate the exact value of surface residual stress. In consideration of (111) plane s in polycrystalline diamond films a ccording to model in [ 9 1 1 ] the stress free Raman peak is defined at 1332cm 1 0 ). The stress can be calculated by the equation, 0 ) GPa Eq. 3 1000 1200 1400 1600 1800 Intensity Wavenumber /cm center MPECVD Poly Diamond 1337.85 1333.23
120 T he constant is evaluated by biaxial stress of diamond films on Si substrate as a plate bending model [7 11, 12 ]. S tress distribution can be calculated from the Raman peak in the follow ing way : f or HFCVD, the stress es at the corner s are 1.049GPa (tensile) and 0.175GPa (tensile); the stress at center is 0.663GPa (compressive). For MPECVD the stress at the corner is 3 .316GPa (compressive); the stress at the center is 0.697GPa (compressive). The stress deviat ion from its true value is attributed to the impact of several complicated factors. It is impossible to find a model that can exactly describe the CVD process and diamond film on Si substrate contraction during the cooling process. In this case, the constant for calculating surface residual stress was derived based on plate bending theory; actually, in the structure of thin diamond film adhering on the thick solid Si substrate, the Si foundation will not shrink or extend along with diamond thin film, the stress will mostly be relax ed and absorbed within the bulk of diamond material. As a result, the calculated stress distribution is nonlinear and does no t completely obey a simple plate bending model. The reference peak at 1332cm 1 is also dependent on CVD growth techniques, conditions and texture orientation [13 17]. In addition, Raman spectra of MP E CVD samples exhibite d a smaller sp 2 carbon peak than th e HFCVD samples grown under analogous conditions Upon comparison, it is evident that high quality of diamond is obtained by the MPECVD technique The G bands in the HFCVD present sharp triangle peaks rather than broad band with a range of wavelength, illu strating the presence of graphitic crystalline rather than the scattered amorphous carbon phase of sp 2 bonds.
121 5.3 SEM of P olycrystalline D iamond F ilm SEM image s of MPECVD polycrystalline diamond in f ig ure 5 5 may be helpful in explain ing residual stress distribution results by the peak shift of Raman spectra. Fig ure 5 5 MPECVD polycrystalline diamond growth evolution From the image s, many facets with different orientation s can be observed. The presence of c rystal twin, tetrahedron and pentahedron (pyramid like grain) which are formed by facets such as (111) (220), (311) and (001) planes are also visible in the SEM images. Since different orientation s ha ve different growth rate s in the crystal growth, it is impossible to grow a fl at surf ace without any post deposition treatment With the grow th of diamond grains in the limited space crystal planes with different orientation s will meet
122 and merge into crystal twin, causing the intrinsic stress on the surface. T he intrinsic stress is easily relaxed at the boundary rather than in the center of the samp le due to the free space On the other hand, because of the irregular shape and size of diamond crystallites, several cavities are formed inside of the diamond bulk, and these spaces are beneficial in relax ing the thermal stress when the cool ing process happen s ; and grains at the boundary have less bondage than those in the center of specimen. This can partly explain why at the center point of the sample, residual stress is always compressive and is always higher than points at the boundary ; stresses at boundaries sometimes present tensile behavior. 5.4 AFM of P olycrystalline D iamond F ilm AFM provides information on the roughness of samples utilizing its unique technique of three dimensional imaging. Figure 5 6 present s the profile of a three 2 scanning area of HFCVD polycrystalline diamond film As evident from the two dimensional imag e many tiny particles considered second nucleation embed around the big diamond crystallites resulting in triangle peaks of G band in Raman spectra ( f ig ure 5 3 ) The average roughness is 35.5 nm in this image
123 Fig ure 5 6 T wo and three dimensional AFM images of HFCV D poly crystalline diamond film Figure 5 7 display s two and three dimensional images of MPECVD polycrystalline diamond. As demonstrated in the two dimensional image, it present s fewer small graphitic particles among b ig diamond crystallites than that of HFCVD diamond ( Fig. 5 6) resulting in the disappearance of G band in Raman spectra ( Fig. 5 4 ) With the same scanning area t he average roughness in the MPECVD sample is 55.4nm, about 20nm higher than that of HFCVD sample.
124 Fig ure 5 7 T wo and three dimensional AFM images of MPECVD poly crystalline diamond film 5.5 XRD of P olycrystalline Diamond F ilm Figure 5 8 displays the XRD patter n of MPECVD diamond film grown with a mixture of CH 4 /H 2 at a ratio of 10sccm/1000sccm under 650C and 50 HPa The deposition time was 10 hours. As can be seen the (220) peak was three times as high as that of (111) peak, i ndicating (220) texture was dominantly consisted of the diamond
125 Fig ure 5 8 XRD patter n of MPECVD diamond film 5.6 Summary In conclusion both polycrystalline diamond films by the HFCVD and MPECVD method s presented a faceted surface. Raman spectra indicated that the HFCVD polycrystalline diamond film contained a certain percentage of crystal graphite, whereas the MPECVD polycrystalline diamond did not have the same The MPECVD polycrystalline diamond has bigger crystallites and a rough er surface than the HFCVD diamond. The MPECVD diamond also presents bigger compressive residual stress on the whole surface of the sample than that of the HFCVD diamond, the (220) plane accounts for mainly texture structure in the as deposited polycrystalline diamond film. 42 52 62 72 82 Intensity Angles (2 ) D (111) Si (400) D (220) POLY 10HRS
126 5.7 List of References  J J Gracio, Q H Fan and J C Madaleno, J. Phys. D: Appl. Phys. 43 (2010) 374017  J E. Yoheda, Diamond Films Handbook edited by J. Asmussen and D. K. Reinhard, ISBN: 0 8247 9577 6 2002 127.  I rfan Ahmed, Growth of texture d diamond coatings on multifunctional substrates using the hot filament chemical vapor deposition method, The sis (master) -University of South Alabama, 1997.  Mushtaq Ahmad Dar, The nucleation and growth of diamond and non diamond coatings, Dissertation (Ph.D.) Chonbuk National University, 2006.  Ganesh Sivananthan, Effects of surface treatment and gas ambient on growth of hot filament chemical vapor deposited diamond coatings, Dissertation (Ph.D.) University of South Alabama, 2000.  Anirudha V. Sumant, Some studies on nucleation and growth aspects of HF CVD diamond films, Dissertation (Ph.D.) -Univ ersity of South Pune, 1998. [7 ] Q. H Fan, A. Fernandes, E. Pereira, J. Grcio, Diamond Relat Mater 8 ( 1999 ) 645 [8 ] C A. Taylor, M F. Wayne and W K. S. Chiu Thin Solid Films 429 (2003) 190 [9 ] D.S. Knight, W.B. White, J. Mater. Res. 4 (1989) 385. [10 ] J.W. Ager, M.D. Drory, Phys. Rev. B 48 (1993) 2601 [ 11 ] V.G. Ralchenko, A.A. Smolin, V.G. Pereverzev, E.D. Obraztsova, K.G. Korotoushenko, V.I. Konov, Yu.V. Lakhotkin, E.N. Loubnin, Diamond Relat. Mater. 4 (1995) 754. [ 12 ] Q H. Fan, J. Gracio, E. Pereira Diamond Relat Mater ., 9 ( 2000 ) 1739 [ 13 ] P. R. Chalker, A. M. Jones, C. Johnston, I. M. Buckley Golder, Surf. Coat. Technol. 47 ( 1991 ) 365. [ 14 ] H. Windischmann, G. F. Epps, Y. Cong, R. W. Collins, J. Appl. Phys. 69 ( 1991 ) 2231. [ 15 ] J. A. Baglio, B. Thin Solid Films, 212 ( 1992 ) 180. [ 16 ] N. S. Van Damme, D. C. Nagle, S. R. Winzer, Appl. Phys. Lett. 58 ( 1991 ) 2919.
127 [ 17 ] K. H. Chen, Y. L. Lai, J. C. Lin, K. J. Song, L. C. Chen, C. Y. Huang, Diamond Relat. Mater. 4 ( 1995 ) 460.
128 CHAPTER 6 : FABRICATION OF CMUTS 6 .1 I ntroduction to CMUTs CMUTs is the abbreviation for capacitive micro machined u ltrasonic transducers, which is essentially a capacitor with one vibrating electrode serv ing as a membrane Figure 6 1 is the basic schematic structure of CMUTs, which is primarily a cavity formed by a membrane and substrate serving as positive and negative electrodes. The working principle of CMUTs is that when an alternating voltage is applied to both electrodes, the charges on the electrodes change caus ing the variation of an attractive force between the two electrodes, and thus one flexible electrode will deflect and bend following the alternating voltage, therefore, the ul trasonic vibration is generate d. Fig ure 6 1 Schematic structure of a CMUT unit cell
129 As evident from f ig ure 6 1, a CMUT unit cell, serv ing as an individual capacitor, consists of three main elemen ts : th e cavity, the membrane, and the electrode Using a general integrated circuit (IC) fabrication process, a capacitor cell appears as a membrane with a metallic contact (top electrode) suspended above a heavily doped silicon substrate (botto m electrode) An insulating layer is necessary to prevent the two electrodes from an electrical short in case of contact, especially when the patterned bottom electrodes are made of metallic materials such as aluminum, chromium, and gold; whereas when heavily doped silicon substrate serves as the bottom electrode, the is olation layer is not required. CMUTs are constructed b y organizing many units in a series or parallel way with different geometries and array of shape s as illustrated in f ig ure 6 2. Fig ure 6 2 A CMUT constructed by connecting units in series and parall el way CMUTs cause extensive attention and interest as an alternative method to conventional piezoelectric transducers because they are small and cheap to produce in bulk quantities using photolithographic fabrication techniques with good quality and rep roducibility. CMUTs have advantages over piezoelectric transducers such as high frequency ultrasonic, wide bandwidth, CMOS compatibility, a very high level of integration and batch fabrication (low cost). Ultrasonic vibration, similar to RF and surface acoustic wave (SAW) technology, can be utilized in almost all walks of life such as pressure sensor, fluid flow meter, non destructive material diagnosis, distant
130 measurement, medical imaging, wireless commu nications, infrared cargo screening and airport security etc [ 1 7]. 6.2 CVD Diamond Film Grown for CMUTs D iamond film becom es the material of choice for the CMUTs design due to its extraordinary properties such as chemical inertness and acoustic wave t ransportation property. The chemical inert ness make s diamond membrane CMUTs perform properly in harsh environments T he acoustic wave transportation velocity in diamond can reach 17. 5km/s  which make s diamond CMUTs generate high er frequency signals tha n other materials CMUTs expect a membrane made of insulating materials, but CVD diamond films with diverse structural forms such as nanocrystalline, polycrystalline and diamond like carbon (DLC) contain a different concentration of sp 2 bonds which make d iamond low conductive with different levels. It is necessary to deposit different form s of CVD diamond films to make the best quality for CMUTs application. Nanocrystalline, polycrystalline and DLC CVD diamond films were deposited with the following descri ptions. Nanocrystalline diamond film depositions were carried out by the Astex CVD system in gas chemistry CH 4 /H 2 /Ar at a flow rate ratio of 2sccm/2sccm/196sccm. The working pressure was 135 HPa and the deposition temperature was 720C. Deposition time was one hour for each patterned wafer. Polycrystalline diamond films were synthesized in the same Astex system with the gas chemistry CH 4 /H 2 at a flow rate ration of 15sccm/1500sccm. The working
131 pressure was 40 HPa and the deposition temperature was 600C. Growth time was one Table 7 Experimental conditions of diamond deposition for CMUTs Phases CH 4 H 2 Ar Temperature Pressure Nano Dia 2sccm 2sccm 196sccm 720 C 135 HPa Ploy Dia 15sccm 1500sccm 0 600 C 40 HPa 6.3 Structure Design and Fabrication Process L Edit is the popular software used to design the masks and layout of micro device s that are fabricated by the IC process technique. It has a unique feature to provide a cross sectional view of a designed multiple layer structure. All the figures of masks and structures of CMUTs used in the micro fabrication were drawn by the L Edit software, excluding the schematic demonstrating figures presented in this chapter. All the process steps, including spin coating photoresist, mask alignment exposure, wet etching, plasma dry etching, electrode metallization, were carried out in the Nanotechnology Research and Education Center (NREC) in University of South Florida (USF), except the CVD diamond film depositing. Only nanocrystalline diamond form was selected as an example in the subsequent description.
132 6.3.1 First Mask and Unit Cell Pattern First SiO 2 s were deposited on two inches o f Si wafer respectively by a thermal oxidizing method. Second, the HMDS was deposited on the SiO 2 surface in order to increase the adhesion of the positive photoresist. After that, the positive photoresist coatshipley 1813, was spin coated on the SiO 2 layer a Laurell spinner. Both the HMDS and coatshipley 1813 followed the same procedure programmed on the Laurell spinner as follows: 10 seconds at 700rpm with an acceleration parameter of 4rpm/sec ; 40 seconds at 3000rp m with an acceleration of 40rpm/sec ; then 10 sec ond s at 700rpm with an acceleration of 4rpm/sec The spinned photoresist wafer was then soft baked for one minute at 115C, and then it was moved and settled on the stage of a Karl Suss mask aligner, expos ed to ultraviolet ray s under the first mask for 4 seconds at 25mv. T he exposed photoresist w as stripped away in MF 319 developer for 70 seconds A rectangular pattern with four anchors was formed on the surface. Figure 6 3 illustrates the one unit cell of pattern by the first mask. Figure 6 4 is the real SiO 2 pattern with the removal of exposed photoresist. As evident in f ig ure 6 4, the substrate was SiO 2 the pattern was the unexposed photoresist left on the SiO 2 The subsequential process was to etch off the SiO 2 in the buffered oxide etchant (BOE) which was comprise d of a standard mixture of hydrofluoric acid and water ( HF:H 2 O ) at a ratio of 6:1 in volume. The SiO 2 etching rate is about 750/min; so for the 2 thickness, the approximate etching time was 13 minutes and 20 seconds. At last, the unexposed photoresist on the soft bake hardened positive
133 coatshipley 1813 was washed away in acetone Now the wafer was only left with the SiO 2 pattern on Si substrate, and ready to deposit diamond f ilm. Fig ure 6 3 One unit cell of SiO 2 pattern by first mask Fig ure 6 4 F irst mask pattern after removal of exposed photoresist
134 6.3.2 Diamond Deposition and Second Mask for Opening Window Nanocrystalline diamond film was deposited in the gas chemistry CH 4 /H 2 /Ar at a film on the SiO 2 pattern. Similar to the description s in 6.3.1, the sample was again spin coated with HMDS for adhesion and positive photoresist (coatshipley 1813), and after soft baked, then was exposed to UV under second ma sk for the opening window on the diamond film. After the removal of exposed photoresist in developer MF 319 the sam ple was ready to etch an opening on the diamond film. Figure 6 5 demonstrates the schematic diagram of the unit cell with an opening by se c ond mask. Figure 6 6 illustrates the real unit cell after the removal of photoresist on the opening window. As eviden t in f ig ure 6 6, the photoresist still remained on the membranes, whereas the anchor areas were SiO2 with the removal of photoresist. Fig ure 6 5 Unit cell with an opening window by second mask
135 Figure 6 6. U nit cell with removal of photoresist on the anchors 6.3.3 Diamond Etching and M embrane R elease Diamond etching to open a window for the SiO 2 sacrificial layer removal is the most challenge step in CMUTs fabrication process due to the extraordinary structural and compositional characteristics of CVD diamond. Because of chemical inertness of diamond, wet chemistry etching of diamond is not available. Plasma dry etching of diamond has been repo rted with a low etching rate due to the high strength of diamond. S everal different techniques generally combined with biased voltage substrate have been employed to diamond dry etching such as plasma assisted reactive ion etching (RIE), ECR (electron cycl otron resonance) plasma etching, Xe + ion beam etching (also referred as milling) i nductively coupled plasma etching and microwave plasma enhanced CVD etching. These etchings usually activate O 2 H 2 ,Ar, F 4 CF 4 SF 6 NO 2 Cl 2 gases as plasma chemistry, either individual ly or in a mixture with different recipe s and flow rate
136 ratio s taking place in the range of pressure 0.4torr~ 65torr, and the etching rate reported from 10nm/min to 220nm/min[9 18]. Diamond etching was carried out by a plasma dry etching method in NREC USF. The nanocrystalline diamond film was deposited one hour. Based on the gas recipe and in which t he section over SiO 2 is thicker than that of Si substrate in accordance with the results of chapter 4.7. The RIE etching was implemented only by O 2 at an 80sccm flow rate under 65mtorr for three five minutes stop checking. Because of the low etching rate, the plasma etching chemistry gas was changed to a mixture of O 2 / CF 4 at 50sccm/1sccm under 50mtorr for two three minutes. The diamond was completely etched away under the observation of a microscope. Fig ure 6 6 is the unit cell with released diamond membr ane. The membrane was still covered by diamond film; the anchor was exposed by Si substrate. Some damaged defects caused by plasma etching can be observed on the diamond membrane. Using the argon chlorine based plasma will not damage the diamond surface and sub surface regions can also avoid the formation of undesirable etch pits [ 18].
137 Figure 6 7 T he unit cell with released diamond membrane The sample was immersed into acetone to clean the residual photoresist remains on the membrane. Condensed buff ered oxide etchant mixed by hydrofluoric acid and water with HF :H 2 O at 10:1 in volume was employed to etch away SiO 2 to form the gap of membrane. Polycrystalline diamond film was then deposited with the gas chemistry CH 4 /H 2 at a flow rate ratio of 10sccm/1 000sccm. The reason for selecting polycrystalline diamond deposition to finally form the diamond membrane rather than nanocrystalline diamond is because the deposition pressure of polycrystalline diamond was 40 HPa whereas the pressure of nanocrystalline d eposition was usually 100 HPa For safety consideration, to avoid the collapse of the diamond membrane during the second diamond deposition, polycrystalline diamond was deposited for one hour so that a 0.75
138 6.3.4 Electrode M etallization The commonly used metals for CMUTs electrode metallization are aluminum, gold, silver, titanium and copper. Aluminum was selected and sputtered 200nm thick by thermal evaporator equipment in the thin film lab of NREC. To produce t he metal electrode by the third mask, spin coating photoresist process was carried out again. Because this time positive photoresist coatshipley 1813 was coated on aluminum, the adhesion HMDS coating was skipped. The spinner program was also changed as fol lows: 10 seconds at 5 00rpm with an acceleration parameter of 6 rpm/sec ; 40 seconds at 2500rpm with an acceleration of 3 0rpm/sec ; then 10 sec onds at 500rpm with an acceleration of 6 rpm/sec The s u bsequential processes were the same as the descriptions in chapter 6.3.1. The aluminum etchant was lab made with a mixture of H 3 PO 4 :HNO 3 at a proportion of 80%:20%, and the etching time was 20 minutes. Figure 6 7 displays the patterned aluminum electrode of the as fabricated CMUTs. In f ig ure 6 7a, t he substrate was covered by aluminum; the metallic contact on the membrane was covered by photoresist. In f ig ure 6 7b, the aluminum on substrate and photoresi s t on the membrane were all etched away, aluminum contact was formed on the membrane, and all of t he unit cell s were connected by an aluminum electrode
139 a Aluminum and photoresist pattern b. Final aluminum contact on unit cell Figure 6 8 T he patterned aluminum electrode 6. 4 Summary In summary, CVD diamond films were successfully integrated into CMUTs by the IC processing technique. The advantages of CVD diamond were considered in two aspects : first, CVD diamond was chemically inert which allows diamond CMUT to work under harsh environment s ; second, the acoustic wave transportation velocity was believed to be the highest (17.5km/s) in the current materials T his merit makes diamond CMUT generate high frequency output signals which is significant for medical imaging applications. During the fa brication process, several technical challenges were overcome by many times of repeated experiments. For instance, the plasma etching of diamond opening ; two times of deposition of diamond to seal the anchor and form the cavity, and, in the meantime, avoid membrane collapse under the CVD deposition pressure; photoresist spin coating on the aluminum emboss; aluminum electrode center positioned on the membrane. It proved that t he two step CVD diamond depositing solutions was simpl y designed and operational to form a cavity within the lower pressure ambient circumstances inside the CVD chamber. Various selection s of diamond forms leave potential to improve the performance of diamond membrane CMUTs.
140 6.5 List of References  Rasim O. Guldiken, Dual electrode capacitive micromachined ultrasonic transducer for medical ultrasound applications, Dissertation (Ph.D.) Georgia Institute of Technology, 2008  Stanford University, Ultrasonics Group, http://www kyg.stanford.edu/khuriyakub/opencms/en/research/cmuts/general/index.html  Tyndall National Institute, CMUTs Research Group, http://www.tyndall.ie/research/mems/cmuts.html  Georgia Institute of Technology, Micromachined Sensors and Transducers Laboratory, http://www.me.gatech.edu/mist/cmut.htm  http://en.wikipedia.org/wiki/Capacitive_Micromachined_Ultrasonic_Transducers  O Oralkan, A. S. Ergun, J A. John son, M Karaman, U Demirci, K Kaviani, T H. Lee, and B T. Khuri Yakub, IEEE Transactions on Ultrasonics, F erroelectr ics, and Frequency Control, 49 ( 2002 ) 11.  Y Huang, E O. Hggstrm, X Zhuang, A S. Ergun, and B. T. Khuri Yakub IEEE Ultrasonics Symposium, Rotterdam, Netherland, Sep. 2005, 589  P. W. May, Phil. Trans. A ., 358 (2000) 473  X. W. Zhu, D M. Aslam Diamond Relat Mater ., 15 (2006) 254  J C. Zhang, J W. Zimmer, R T. Howe, R Maboudian, Diamond Relat Mater ., 17 (2008) 23  X. D. Wang, G. D. Hong, J. Zhang, B. L. Lin, H. Q. Gong, W.Y. Wang Journal of Materials Processing Technology 127 (2002) 230  G. F. Ding, H. P. Mao, Y. L. Cai Y. H. Zhang, X. Yao, X. L. Zhao Diamond Relat Mater ., 14 (2005) 1543  T Yamada, H Yoshikawa, H Uetsuka, S Kumaragurubaran, N Tokuda, S. Shikata Diamond Relat Mater ., 16 (2007) 996  C. L. Lee, E. Gu, M. D. Dawson, I. Friel, G. A. Scarsbrook, Diamond Relat. Mater., 17 (2008) 1292  D. T. Tran, C. Fansler, T. A. Grotjohn, D. K. Reinhard, J. Asmussen Diamond Relat Mater 19 ( 2010 ) 778.
141  D. S. Hwang, T. Saito, N. Fujimori Diamond Relat Mater 13 ( 2004 ) 2207  M. Bernard, A. Deneuville, L. Ortega, K. Ayadi, P. Muret Diamond Relat Mater 13 ( 2004 ) 287  F. Silva, R. S. Sussmann, F. Bndic, A. Gicquel Diamond Relat Mater 12 ( 2003 ) 369
142 CHAPTER 7 : CONCLUSIONS AND FUTURE WORK S 7.1 Conclusions The quality of diamond films synthesized by the MPECVD and HFCVD method s are greatly affected by the experimental parameters such as the reactant gas H 2 carrier gas Ar, depositing pressure ; they can also be characterized by Raman spectra to determine the electronic structure of surface in terms of sp 2 and sp 3 bondings by XRD patterns to investigate crystalline orientation, content and texture structure, by AFM to investigate average roughness and three dimensional morphology and by SEM to display the texture structure and grain size and boundaries Micro structures d etermine the properties, and properties determine the applications. By utilizing characterization techniques, the deposited diamond qualities were optimized to better serve specific application s In this dissertation, a systematic research on the influential factors such as H 2 Ar volume, and depositing pressure on the quality of diamond films were investigated by metrological and analytical techniques largely including SEM, AFM, XRD, Raman spectroscopy etc For polycrystalline diamond synthese s, H 2 volumes less than 500sccm generate a cauliflower like surface ; more than 1000sccm H 2 volumes yield a faceted surface; mainly crystal planes include (111) and (220) (220) planes occupy more than (111) For intrinsic nanocrystalline diamond film growth the change of Ar volume will
143 vary the surface morphology such as cauliflower like or ear of wheat like texture structure; (111) and (220) planes exhibit the same amount of content in the nanocrystalline diamond films. Similar symptoms accompany the change of deposition pressure. For nitrogen doped nanocrystalline diamond films, the grains grow with the increase of N 2 volume, and the preferential growth plane is (111). 15% N 2 doped diamond films grown on different substrate structure s present morphology, ro ughness and growth rate change due to atomic charge particle diffusion. MPECVD and HFCVD polycrystalline diamond display similar surface feature s different roughness and residual stress. CMUTs built up by a d iamond membrane successfully fabricated utilizing IC processing technique s, and overcoming several technical challenges such as diamond window etching, two stage diamond depositing, vacuumed cavity formation and alignment of electrode of metalli zation with the pattern. T he outstanding properties of CVD diamond films are expected to enhance the potential development s and applications of micro devices in the semiconductor industry 7.2 Future Works The resultant surface residual stress of CVD diamond films derived from lattice mismatch and different thermal coefficient s between diamond and substrate is a big challenge that prevent s the practicable application of diamond films. Current evaluation s of residual stress of diamond films are mostly calculated by XRD curvature measurement and Raman spectra peak shift. Due to the complexity of the CVD proce ss and simpli ci ty
144 of the simulation model, the calculated results are greatly deviat ed from the rea l situation. Further in depth investigation is n eeded to develop a new meth od that can reach a consistent result for both XRD and Raman method and closely describe the real stress field of diamond surface. Grow i ng CVD diamond films on commonly applied MEMS layer such as SiO 2 Si 3 N 4 polysilicon, metal and metal oxide will generate different surface structure features which are significant to the fabrication and performance of micro device s This surface change is most probably caused by the electron s a tomic H density on the surface and their correlations with atomic charge particles diffusion toward substrate. This phenomenon has not been given extensive attention and the knowledge about the functioning mechanism under it is even more lacking It is necessary to make a systematic in vestigation on this phenomenon and clarify the principles in order to better serve the CVD diamond film applications. Other interests also include m etal particle modified extrinsic diamond electrode on the electrochemical applications d iamond film integr ated micro device fabrications simulations, measurements and applications
ABOUT THE AUTHOR Qiang Hu majored in mechanical engineering in his bachelor degree and received a n M.S. in solid mechanics from North ea s tern University in 199 8, China. H e started research on fatigue and fracture, transformation behavior of TiNi shape memory alloys based on c yclic stress and strain in the Institute of Metal R esea rch (China) H e entered the Ph.D. program at the Department of Me chanical Engineering in the University of South Florida in fall 2006 While in the Ph.D. program at the University of South Florida, M r Hu was very active in academic research ; he published two papers as first author, and two papers as coauthor in peer v iewed prestigious journals, and also has several papers in review He attended five national academic conferences with poster exhibition s He was granted the Graduate Multidisciplinary Scholars (GMS) fellowship by the Graduate School of USF from the fall semester of 2007 to the summer semester of 2009 based on his o utstanding academic performance