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Growth and characterization of nanocrystalline diamond films for microelectronics and microelectromechanical systems

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Title:
Growth and characterization of nanocrystalline diamond films for microelectronics and microelectromechanical systems
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Book
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English
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Jeedigunta, Sathyaharish
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University of South Florida
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Chemical vapor deposition
Nucleation
Mems
Field emission
Pressure sensor
Dissertations, Academic -- Electrical Engineering -- Doctoral -- USF   ( lcsh )
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non-fiction   ( marcgt )

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Summary:
ABSTRACT: Diamond is widely known for its extraordinary properties, such as high thermal conductivity, energy bandgap and high material hardness and durability making it a very attractive material for microelectronic and mechanical applications. Synthetic diamonds produced by chemical vapor deposition (CVD) methods retain most of the properties of natural diamond. Within this class of material, nanocrystalline diamond (NCD) is being developed for microelectronic and microelectromechanical systems (MEMS) applications. During this research, intrinsic and doped NCD films were grown by the microwave plasma enhanced chemical vapor deposition (MPECVD) method using CH₄/Ar/H₂ gas mixture and CH₄/Ar/N₂ gas chemistries respectively. The first part of research focused on the growth and characterization of NCD films while the second part on the application of NCD as a structural material in MEMS device fabrication.The growth processes were optimized by evaluating the structural, mechanical and electrical properties. The nature of chemical bonding, namely the ratio of sp²:sp³ carbon content was estimated by Raman spectroscopy and near edge x-ray absorption fine structure (NEXAFS) techniques. The micro-structural properties were studied by x-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The mechanical properties of the pure NCD films were evaluated by nano-indentation. The electrical properties of the conductive films were studied by forming ohmic as well as schottky contacts. In second part of this study, both free-standing and membrane capped field emitter devices were fabricated by a silicon mold technique using nitrogen incorporated (i.e., doped) NCD films. The capped field emission devices act as a prototype vacuum microelectronic sensor.The field emission tests of both devices were conducted using a diode electrical device model. The turn-on field and the emission current of free-standing emitter devices was found to be approximately 0.8 V/μm and 20 μA, respectively, while the turn-on fields of capped devices increased by an order of magnitude. The emission current in the field emission sensor changed from 1 μA to 25 μA as the membrane was deflected from 280 μm to 50 μm from the emission tip, respectively.
Thesis:
Dissertation (Ph.D.)--University of South Florida, 2008.
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Includes bibliographical references.
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by Sathyaharish Jeedigunta.
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Title from PDF of title page.
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Document formatted into pages; contains 146 pages.
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Includes vita.

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oclc - 319422090
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Growth and Characterization of Nanocrystalline Diamond Film s for Microelectronics and Microelectromechanical Systems by Sathyaharish Jeedigunta A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Electrical Engineering College of Engineering University of South Florida Co-Major Professor: Ashok Kumar, Ph.D. Co-Major Professor: Sh ekhar Bhansali, Ph.D. Stephen E Saddow, Ph.D. Jing Wang, Ph.D. John Bumgarner, Ph.D. Priscila Spagnol, Ph.D. Date of Approval: May 29, 2008 Keywords: chemical vapor deposition, nuclea tion, mems, field emission, pressure sensor Copyright 2008 Sathyaharish Jeedigunta

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DEDICATION To My Family

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ACKNOWLEDGMENTS I would like to thank my co-advisor Prof. As hok Kumar, for giving me an opportunity to pursue my doctoral studies in his group. I w ould also like to thank my other co-advisor Prof. Shekhar Bhansali, for his timely suggestio ns during my researc h. Special thanks are due to Prof. Stephen E Saddow and Prof. Jing Wang for their i nvaluable help and support. I would like to highly acknowledge Dr. J ohn Bumgarner for exte nding support through out this course of research. I thank Dr. Pr iscila Spagnol for her invaluable guidance during this research. I would like to thank Dr. Timothy Shor t and his group members for helping me with the device tes ting. I have learnt a great d eal working with everyone at the MEMS STAR Center. My thanks are due to all the members at COT, SRI International, NNRC, AMRL a nd engineering machine shop. I ha ve to thank office staff in the departments of mechanical, and elec trical engineering and college of marine science for processing all the paper work in time. The NEXAFS study during this research was conducted at the synchrotron ra diation center, Univer sity of Wisconsin– Madison, supported by the NSF under cont ract no. DMR-0084402. The financial support for this research came from NSF NIRT grant # ECS 0404137 and space and missile defense command (SMDC) grant # W9113M06-C-0022. I would like to acknowledge the encouragement and support from my family, friends, and colleagues.

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i TABLE OF CONTENTS LIST OF TABLES.............................................................................................................vi LIST OF FIGURES..........................................................................................................vii ABSTRACT ...................................................................................................................... xv CHAPTER 1. INTRODUCTION..................................................................................1 1.1. Introduction to CVD Diamond...............................................................1 1.2. Phase Diagram of Carbon Based Materials............................................2 1.3. Crystal Structure of Diamond.................................................................4 1.4. Fundamental Properties of Single Crystal Diamond..............................5 1.5. Scope and Outline of the Dissertation....................................................7 CHAPTER 2. BACKGR OUND ON FIELD EMISSION...........................................10 2.1. Introduction...........................................................................................10 2.2. Principle of Field Emission...................................................................11 2.3. Advantages of Nanocrystalline Diamond (NCD) Films for Field Emission Sources..................................................................................15 2.4. Field Emission Based Sensing..............................................................22 2.5. Summary...............................................................................................28

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ii CHAPTER 3. GROWTH AND CHARACTERIZA TION OF NANOCRYSTALLINE DIAMOND FILMS...................................................29 3.1. Introduction...........................................................................................29 3.2. Substrate Pretreatment for the Growth of Diamond Films on Non-Diamond Substrates....................................................................30 3.3. Selective Nucleation of Substrates........................................................32 3.4. Cyrannus I Large Area Microwave Plasma Enhanced Chemical Vapor Deposition (MPECVD) Reactor................................................32 3.5. Growth Mechanism of Nanocrystalline Diamond Films......................34 3.6. Experimental Study on the Nucleation Methods..................................37 3.7. Effect of Nucleation on the Fabrication of MEMS Cantilevers...........44 3.8. Optimized Process Parameters for the Growth of Nanocrystalline Diamond Films......................................................................................46 3.9. Tools and Techniques for the Ch aracterization of Nanocrystalline Diamond Films......................................................................................47 3.10. Raman Spectroscopy...........................................................................47 3.11. Near Edge X-ray Absorption Fi ne Structure (NEXAFS) Spectroscopy.......................................................................................51 3.12. Micro-Structural Analysis of Intrinsic NCD Films............................53 3.13. Nitrogen Doping of Nanocrystalline Diamond Films.........................56 3.14. In-situ Plasma Diagnostics of NCD Films by Optical Emission Spectroscopy.......................................................................................58

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iii 3.15. Structural Analysis of Nitrogen Incorporated Nanocrystalline Diamond Films....................................................................................60 3.16. Electrical Contacts to Nitrogen Incorporated Nanocrystalline Diamond Films....................................................................................70 3.17. Summary.............................................................................................77 CHAPTER 4. FIELD EMITTER DEVICE FABRICATION APPROACH...............78 4.1. Introduction...........................................................................................78 4.2. Fabrication of Vertical Fiel d Emission Source and Sensor..................80 4.3. Testing of Field Emission Source and Sensor......................................82 4.4. Summary...............................................................................................83 CHAPTER 5. DESIGN A ND ANALYSIS OF VERTICAL FI ELD EMITTER DEVICES.............................................................................................84 5.1. Introduction...........................................................................................84 5.2. Design of Diamond Field Emitter Array..............................................84 5.3. Design and Analysis of Anode-on-Membrane Using Generation I Mask.................................................................................................87 5.4. Design and Analysis of Anode-on-Membra ne Using Generation II Mask................................................................................................90 5.5. Summary...............................................................................................94 CHAPTER 6. FABRICATION AND CHARACTERIZATION OF FIELD EMITTER DEVICES..........................................................................96 6.1. Introduction...........................................................................................96

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iv 6.2. Fabrication of Vertical Field Em itter Array by Mold Technique.........96 6.3. Challenges in the Processing of Vertical Field Emitter Devices..........98 6.4. Determination of Tip-Radius by Focused Ion Beam (FIB)................102 6.5. Deposition of Nanocrystalline Diamond Films in the Inverted Pyramidal Molds.................................................................................103 6.6. Processing Issues-Chip-Level Bonding..............................................104 6.7. Electroplating of Gold Film s as Metal Layer for ThermoCompression Bonding.........................................................................111 6.8. Fabrication of Anode-on-Membrane Wafers Using Generation I Mask..................................................................................................113 6.9. Fabrication of Anode-on-Membrane Wafers Using Generation II Mask..................................................................................................116 6.10. Integration of the Anode and the Cathode........................................119 6.11. Wafer-to-Wafer Bonding..................................................................121 6.12. Characterization of Field Emission Devices.....................................122 6.13. Theoretical Analysis of the Field Emission Data.............................126 6.14. Characterization of a Field Emission Sensor....................................128 6.15. Summary...........................................................................................131 CHAPTER 7. CONCLUSIONS................................................................................133 7.1. Summary.............................................................................................133 7.2. Future Work........................................................................................135

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v REFERENCES...............................................................................................................137 ABOUT THE AUTHOR.......................................................................................End Page

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vi LIST OF TABLES Table 1.1 Fundamental Properties of Single Crystal Diamond..........................................5 Table 1.2 Fundamental Properties of S ilicon, Silicon Carbide and Diamond....................7 Table 2.1 Properties of Materials Suitable for Field Emission Applications [33]............15 Table 3.1 Effect of Seeding Method on Nucleation Density............................................31 Table 3.2 Experimental Conditions for Seeding W ith and Without Plasma Exposure............................................................................................................37 Table 3.3 Experimental Conditions for S eeding With and Without Interlayer................40 Table 3.4 XRD Data of Undoped Na nocrystalline Diamond Films.................................69 Table 3.5 XRD Data of Nitrogen Dope d Nanocrystalline Diamond Films......................70 Table 4.1 Comparison Between Piez oresistive and Capacitive Se nsing Mechanisms.......................................................................................................79 Table 5.1 Design Specifications of the Field Emitter Mask.............................................85 Table 5.2 Design Specification of the Cathode................................................................85 Table 5.3 Design Specification of the Anode in the Generation I Mask..........................87 Table 5.4 Design Specifications of the Anode a nd Proof-Mass in Generation II Mask...................................................................................................................91

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vii LIST OF FIGURES Figure 1.1 Phase Diagram of Ca rbon Based Materials [118].............................................3 Figure 1.2 Crystal Structure of Diamond Lattice [11]........................................................4 Figure 2.1 Energy Band Diagram of Diamond Surfaces (a) Positive Electron Affinity (b) Effective Negative Electron Affinity (c) True Negative Electron Affinity [34].....................................................................................13 Figure 2.2 Fabrication of Vertical Field Emitter Array by Mold Technique [41]............18 Figure 2.3 Fowler-Nordheim Characteristics of Vertical Field Emitter Array Device [41]......................................................................................................19 Figure 2.4 Process Steps for the Fabrication of Lateral Field Emission Device [42].......20 Figure 2.5 Field Emission Characteristics of Lateral Field Emission Device [42]..........21 Figure 2.6 Pressure vs. Current Relationship in Field Emission Pressu re Sensor [50]....24 Figure 2.7 Effect of Membrane Thickness and Size on Pressure Domain [48]................26 Figure 2.8 Effect of Anode to Cathode Spacing on Applied Voltage [48].......................26 Figure 2.9 Effect of Membrane Dimensions on Sensitivity of Field Emission Pressure Sensor [48].......................................................................................27 Figure 3.1 Cross-Section Geometry of a Cyrannus I Iplas Plasma Source [69]..............33

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viii Figure 3.2 Cyrannus I Iplas Microwave Plasma Enhanced Chemical Vapor Deposition System..........................................................................................34 Figure 3.3 Growth Mechanism of Nanocrystalline Diamond [58]...................................36 Figure 3.4 Effect of Plasma Treatment on Nucl eation Density After Ultrasonicating for 20 Minutes (a) Without Plasma Exposure in a Mixture of Acetone and Diamond Nanopowde r Slurry (b) Without Plasma Exposure in a Mixture of Methanol and Diamond Nanopowder Slurry (c) With Plasma Exposure in a Mixture of Acetone and Diamond Nanopowder Slurry (d) Mixt ure of Methanol and Diamond Nanopowder Slurry.........................................................................................38 Figure 3.5 Effect of Titanium Interlayer on Nucleation Density After Ultrasonicating for 20 Minutes in (a) Mixture of Acetone and Diamond Nanopowder (b) Mixture of Me thanol and Diamond Nanopowder....................................................................................................41 Figure 3.6 Surface Morphology of the Films After Ultr asonicating for 20 Minutes in (a) Mixture of Titanium Nanopow der, Diamond Nanopowder and Acetone (b) Mixture of Titanium Na nopowder, Diamond Nanopowder and Methanol..................................................................................................42 Figure 3.7 Surface Morphology of the Films After Ultr asonicating for 40 Minutes in (a) Mixture of Titanium Nanopow der, Diamond Nanopowder and Acetone (b) Mixture of Titanium Na nopowder, Diamond Nanopowder and Methanol..................................................................................................42

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ix Figure 3.8 Surface Morphology of the Films After Ultr asonicating for 60 Minutes in (a) Mixture of Titanium Nanopow der, Diamond Nanopowder and Acetone (b) Mixture of Titanium Na nopowder, Diamond Nanopowder and Methanol..................................................................................................43 Figure 3.9 Process Steps for the Fabrication of Cantilevers.............................................44 Figure 3.10 SEM Micrograph of Cantilevers (a) and (b) Due to Poor Nucleation Resulting From Seeding in Diamond Nanopowder in Acetone Slurry (c) and (d) Due to Improved Nucleation Re sulting From Seeding in Mixture of Titanium Nanopowder and Diamond Nanopowder in Acetone...........................................................................................................46 Figure 3.11 Schematic of Raman Spectroscopy...............................................................48 Figure 3.12 Raman Spectra of Ca rbon Based Materials...................................................50 Figure 3.13 NEXAFS Spectra of Single Crys tal Diamond and HOPG Reference Sample [113]...................................................................................................52 Figure 3.14 HERMON Beam Line at the Univ ersity of Wisconsin Madison [117].........53 Figure 3.15 SEM Micrograph of Na nocrystalline Diamond Film De posited Using 1% Hydrogen in the Gas Chemistry (a) Low-Resolution (b) Cross-Section (c) Medium-Res olution (d) High-Resolution....................54 Figure 3.16 AFM Micrograph of Na nocrystalline Diamond Film De posited Using 1% Hydrogen in the Gas Chemistry...............................................................55 Figure 3.17 TEM Micrographs (a) Electron-Di ffraction Pattern (b) Low-Resolution TEM Image (c) High-Resolution TEM Image................................................56

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x Figure 3.18 Energy Band Diagram of Nitrogen Doped Single Crystal Diamond and Nanocrystalline Diamond.........................................................................57 Figure 3.19 Optical Emission Spectra of Plasma Chemistry (a) CH4/Ar/H2 (b) CH4/Ar/N2.................................................................................................59 Figure 3.20 AFM Micrograph of n-NCD Film.................................................................61 Figure 3.21 SEM Micrograph of n-NCD Film (a) Top-View (b) Cross-Section.............62 Figure 3.22 TEM Micrograph of Nitrogen Incor porated Nanocrystalline Diamond Film and Interface Between Silicon Substrate and Diamond Film (a) Low-Resolution (b) High-Resolution (c) High-Resolution Showing the Nanocrystalline Diamond Grains..............................................................63 Figure 3.23 Electron-Diffraction Pattern (a) Silicon Subs trate (b) Nanocrystalline Diamond Film (c) Interface Between Silicon and Diamond (d) Toward the Diamond Surface Near the Interface........................................................ 64 Figure 3.24 Line-Scan Profile Images (a) CrossSection of the TEM Sample (b) Silicon (c) Carbon (d) EDAX Spectra Showing Silicon and Carbon Peaks...............................................................................................................66 Figure 3.25 (a) Raman Spectra and (b) NEXAFS Sp ectra of Nanocrystalline Diamond Films Grown Using 0% and 20% Nitrogen in the Gas Chemistry........................................................................................................67 Figure 3.26 X-ray Diffraction Spectra of Undoped and Nitrogen Incorporated Nancorystalline Diamond Films.....................................................................69

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xi Figure 3.27 Current-Voltage Characteristics of Nitrogen Incorporated Nanocrystalline Diamond Films.....................................................................71 Figure 3.28 Surface Morphology of Nitrogen Incorpor ated Nanocrystalline Diamond Film (a) as-grown (b ) Post Hydrogen Plasma Treated Surface (Low-Resolution) (c) Post Hydr ogen Plasma Treated Surface (High-Resolution)...........................................................................................73 Figure 3.29 AFM Image of Nitrogen Incorporated Na nocrystalline Diamond Film (a) as-grown (b) After the Hydrogen Plasma Treatment................................74 Figure 3.30 Raman Spectra of as-grown a nd Hydrogen Plasma Treated Films...............75 Figure 3.31 Current-Voltage Characteristics of Nitrogen Incorporated Nanocrystalline Diamond Film After the Hydrogen Plasma Treatment........76 Figure 4.1 Process Steps for the Fabricati on of Vertical Field Emission Sensor.............80 Figure 5.1 3D Model of a Free Standing Vertical Field Emitter Array Bonded to a Carrier Wafer...........................................................................................86 Figure 5.2 3D Model of a Capped Field Emitter Device..................................................86 Figure 5.3 Effect of Membrane Dimensions on the Deflection of a 20 m Thick Silicon Diaphragm..........................................................................................89 Figure 5.4 2D Image of Generation II Mask for the Fabrication of Membranes..............90 Figure 5.5 Snapshot of 3D Solid Model of a Single -Cell Using Generation II Mask................................................................................................................91

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xii Figure 5.6 Effect of Membrane Thickness on the Deflection of Membrane (a) 5000 X 5000 m2_8000 X 8000 m2 (b) 5000 X 5000 m2_10000 X 10000 m2 (c) 7000 X 7000 m2_10000 X 10000 m2 (d) 7000 X 7000 m2_12000 X 12000 m2...............................................................................92 Figure 6.1 Process Steps for the Fabricati on of Vertical Field Emitter Array..................97 Figure 6.2 SEM Micrograph of KOH Etched Inverted Pyramidal Mold Containing Iron Particulates.......................................................................... 99 Figure 6.3 EDAX Area Scan of the Pyramida l Mold Containing Iron Particulates.........99 Figure 6.4 SEM Micrograph of the V-groove After Iron Oxide Etch............................100 Figure 6.5 SEM Micrograph of a Diamond Pyramid With Pinholes Resulted From Iron Particulates in the Etched Cavities..............................................101 Figure 6.6 FIB Cross-Section Image of a V-groove in Silicon Substrate.......................102 Figure 6.7 SEM Micrograph of the Surface of Nitrogen Incorporated NCD Film Partially Filled in the Silicon Mold (a) Low-Reso lution Image of the Field Emitter Pattern (b) High-Resolution Image of the NCD Film Deposited in a Single Invert ed Pyramidal Cavity.........................................104 Figure 6.8 Time vs. Temperature Plot of Thermo-Compr ession Bonding Using Anisotropic Conductive Film.............................................................105 Figure 6.9 100 mm Jig for Backside Wa fer Protection During Etching.........................106 Figure 6.10 SEM Micrograph of ACF Bonded Ch ip After Partial Etching in HNA......106 Figure 6.11 Time vs. Temperature Plot for Gold-to-Gold Thermo-Compression Bonding.........................................................................................................107

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xiii Figure 6.12 SEM Micrograph of (a) Sputtered Gold Film (b) Electroplated Gold Film......................................................................................................108 Figure 6.13 AFM Micrograph of (a) Sputtered Gold Film (b) Electroplated Gold Film......................................................................................................109 Figure 6.14 SEM Micrograph of Au-Au Bonded Chip (a) LowResolution (b) HighResolution Showing the Interface.................................................110 Figure 6.15 SEM Micrograph of a Free Standing Field Emitter Array After Successful Silicon Etch.................................................................................111 Figure 6.16 Electroplating Cell for Depos iting Gold Films on 100 mm Wafers............112 Figure 6.17 Process Steps for the Fabrica tion of Anode-on-Membrane Using Generation I Mask.........................................................................................114 Figure 6.18 SEM Micrograph of a Generation I S ilicon Membrane After Anisotropic KOH Etch..................................................................................115 Figure 6.19 Snapshot of a Fully Processed Generation I Silicon Wafer After KOH Etch......................................................................................................115 Figure 6.20 Process Steps for the Fabrication of Silicon Membranes Using Generation II Mask.......................................................................................116 Figure 6.21 SEM Micrograph of a Fully Etched Generation II Membrane After Anisotropic KOH Etch........................................................................118 Figure 6.22 Snapshot of a Fully Proc essed Generation II Silicon Wafer.......................118

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xiv Figure 6.23 (a) Pyrex Wafer After Laser Milling Th rough Holes for the Electrical Contacts (b) Pyrex Wafer Flip-Chip Bonding Diamond Dice to the Gold Bond Pads (c) Snapshot of a 100 mm Wafer After Etching the Top Silicon Mold....................................................................................120 Figure 6.24 Field Emission Device in Diode Configuration..........................................122 Figure 6.25 Field Emissi on Test Chamber.....................................................................123 Figure 6.26 (a) Electric Field vs. Em ission Current Characteristics (b) Fowler-Nordheim Characteristics of Free st anding NCD Emitter Array With a Vacuum Gap of 1000 m...........................................124 Figure 6.27 Electric Field vs. Emission Current Characteristics of Free Standing NCD Emitter Array With a Vacuum Gap of 500 m....................125 Figure 6.28 (a) Electric Field vs. Em ission Current Characteristics (b) Fowler-Nordheim Plot With a Vacuum Gap of 300 m.........................125 Figure 6.29 Fowler-Nordheim Characteristics W ith Approximate Mathematical Fit....127 Figure 6.30 (a) Electric Field vs. Em ission Current Characteristics (b) Fowler-Nordheim Characteristics of Capped Field Emisison Devices...129 Figure 6.31 Pressure vs. Emission Current Characetristics............................................131

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xv Growth and Characterization of Nanocrystalline Diamond Film s for Microelectronics and Microelectromechanical Systems Sathyaharish Jeedigunta ABSTRACT Diamond is widely known for its extraord inary properties, such as high thermal conductivity, energy bandgap and high material hardness and durability making it a very attractive material for microelectronic and mechanical applications. Synthetic diamonds produced by chemical vapor deposition (CVD) methods retain most of the properties of natural diamond. Within this class of material, nanocry stalline diamond (NCD) is being developed for microelectronic and microelect romechanical systems (MEMS) applications. During this research, intrinsic and doped NCD films were grown by the microwave plasma enhanced chemical vapor deposition (MPECVD) method using CH4/Ar/H2 gas mixture and CH4/Ar/N2 gas chemistries respectively. The first part of research focused on the gr owth and characterizati on of NCD films while the second part on the applica tion of NCD as a structural material in MEMS device

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xvi fabrication. The growth processes were optimized by evaluating the structural, mechanical and electrical prope rties. The nature of chemical bonding, namely the ratio of sp2:sp3 carbon content was estimated by Rama n spectroscopy and near edge x-ray absorption fine structure (NEXAFS) techniqu es. The micro-structural properties were studied by x-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electr on microscopy (TEM). The mechanical properties of the pure NCD fi lms were evaluated by nano-i ndentation. The electrical properties of the conductive films were st udied by forming ohmic as well as schottky contacts. In second part of this study, both free-st anding and membrane capped field emitter devices were fabricated by a silicon mold technique using nitroge n incorporated (i.e., doped) NCD films. The capped field emissi on devices act as a prototype vacuum microelectronic sensor. The field emission te sts of both devices were conducted using a diode electrical device model. The turn-on field and the emission current of free-standing emitter devices was found to be approximately 0.8 V/m and 20 A, respectively, while the turn-on fields of capped devices increas ed by an order of magnitude. The emission current in the field emission sensor changed from 1 A to 25 A as the membrane was deflected from 280 m to 50 m from the emission tip, respectively.

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1CHAPTER 1. INTRODUCTION 1.1. Introduction to CVD Diamond “Diamond” has been known to common man as a precious stone in jewelry since biblical times [1]. Even before it was realized as an ornament, diamonds were used in grinding, polishing, and as hard coatings on drill bits cutting tools and dicing saws. Besides the superlative mechanical properties, they po ssess extraordinary electrical, electronic, thermal, and optical properties. Hence, diamon d can be useful in applications such as high power electronics, heat si nks, and radiation de tectors [2]. Diamon d is suitable for operation in harsh environments (except oxygen ambience) as it is hi ghly inert to most acids, bases and other chemicals [3]. The di fficulty in engineering diamond to a desired shape and size has limited its applications to a great extent. Hence, several researchers ha ve investigated suitable alternative methods to produce synthetic diamond. Most of the early grow th techniques failed to deposit good quality diamond films. In 1952, William G. Eversole of the Union Carbide Corporation reported the first successful growth of diamond usi ng a low-pressure chemical vapor deposition method. Later, the low pressure growth tec hnique was confirmed by other groups [1, 2, 4]. This method involved heating the carbon cont aining gaseous mixture in diamond powder

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2 at temperatures ~ 1000 C. Due to the simu ltaneous growth of gr aphitic carbon, the deposition rate on the order of only 0.01 m/hr was achieved [5]. Eversole et. al. suggested periodic injection of diamond powder into hydrogen gas to etch the graphitic carbon and improve the growth rate [6, 7]. In 1974, Russian scientists developed a method of heating the diamond powder by short pulses in a high pressu re gas discharge, which resulted in a higher growth rate of 1m/hr [8]. It was in 1955 that a reproduc ible deposition method using high pressure technique has been reported by the General Electric Company [5]. The initial interest was in the synthesis of single crystal, homo-epitaxial films, and hi gh pressure high temperature (HPHT) diamonds [9]. The high cost of singl e crystal diamond substrates has encouraged researchers to focus on the growth of diamond films on non-diamond substrates. Preliminary research toward this end has pr oduced another fascinati ng class of material known as diamond-like-carbon (DLC) [10]. 1.2. Phase Diagram of Carbon Based Materials Although CVD has been widely accepted as an ideal deposition technique to grow diamond films, the difficulty in the diamond grow th is due to its metastable nature. The deposition conditions favor the nucleation of both diamond and graphite crystals. The phase diagram of carbon based materials is shown in Figure 1.1 [118]. It can be found that the growth regime of CVD diamond is surrounded by the growth of graphitic carbon.

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3 Figure 1.1 Phase Diagram of Ca rbon Based Materials [118] Therefore, the relationship between the diamond and graphite is thermodynamic and kinetic one. At normal temperatures and pressu res, the activation energy of graphite is only few eV higher than diamond which result s in the existence of diamond. Once the diamond crystal is formed, there is no easy mechanism to convert into graphite without completely destroying the lattice.

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4 1.3. Crystal Structure of Diamond The face centered cubic crystal (FCC) lattice of diamond consists of a unique arrangement of carbon atoms with eight corner atoms, six face centered atoms and four other atoms from adjacent interpenetrating lattices offset by one-quarter of the body diagonal as shown in the ba ll and stick model of Figure 1.2 [11]. Each of the carbon atoms is covalently bonded to four nearest neighboring atoms resulting in a strong sp3 character. The (111) planes of the diamond are along the bond direc tion with a lattice constant (a0) of 3.567 and a bond length of 1.54 [12] Due to this unique chemical bonding, and atomic density of 1.76 X 1023 cm-3, diamonds possess several extraordinary material properties. Figure 1.2 Crystal Structur e of Diamond Lattice [11]

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5 1.4. Fundamental Properties of Single Crystal Diamond Table 1.1 summarizes few important properties of diamond [4, 13, 14]. It is evident that diamond is suitable for a wide range of mech anical, thermal and electronic applications. Table 1.1 Fundamental Propertie s of Single Crystal Diamond Property Value STRUCTURAL PROPERTIES Crystal structure FCC( sp3 bonded, tetrahedral) Atomic density 1.76 X 1023cm-3 Lattice constant 3.567 Density 3.52 gmcm-3 MECHANICAL PROPERTIES Young’s modulus 1050 GPa Coefficient of friction 0.03 Knoop hardness 110 GPa THERMAL PROPERTIES Thermal expansion coefficient 1.1 X 10-6 k-1 Thermal conductivity 2500 W/m-k OPTICAL PROPERTIES Optical index of re fraction(at 591 nm) 2.41

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6 Table 1.1 (Continued) Optical transmittivity 225 nm (UV) to long IR (> 25 m), IR absorption band from 2-7 m ELECTRICAL PROPERTIES Intrinsic resistivity 1013-1016 -cm Bandgap 5.45 eV Electron affinity ~ -1 eV Dielectric constant 5.7 Dielectric strength 1.0 X 107 Vcm-1 Electron mobility 2200 cm2(Vs)-1 Hole mobility 1600 cm2(Vs)-1 It can be found that diamond has some of the extraordinary material properties required not only for cutting tools and abrasive coatings, but also for microelectronics and microelectromechanical systems and applica tions. As silicon technology is well matured, most of the present day microelectronic and MEMS devices are fabr icated using silicon. Due to some of its poor material properties, other materials such as SiC and diamond are also currently under investigation especially for the fabrication of MEMS devices. Some of the properties of Si, SiC and diamond are shown in Table 1.2. It can be observed that diamond possess much better properties than both silicon and silicon carbide.

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7 Table 1.2 Fundamental Properties of Silicon, Silicon Carbide and Diamond Property Silicon SiC Diamond Lattice Constant () 5.43 4.358 ( -SiC) 3.567 Hardness (GPa) 10 35 110 Young’s Modulus (GPa) 130 450 1050 Thermal Conductivity (Wcm-1K-1) 1.5 5 25 Thermal Expansion (10-6 C) 2.6 4.7 1.1 Melting Point ( C) 1420 2540 4000 Band gap (eV) 1.1 3.0 5.45 Resistivity ( .cm)-1 103 150 1013-1016 Break down voltage ( X 105 Vcm-1) 3 40 100 Dielectric constant 11.8 9.7 5.7 1.5. Scope and Outline of the Dissertation This dissertation discusses various aspects of nucleation, growth a nd characterization of intrinsic and conductive nanocrystalline di amond films deposited by microwave plasma enhanced chemical vapor deposition (MPE CVD) method. Both intrinsic and conductive NCD films were used as structural material in the fabrication of MEMS devices. The fabrication of vertical NCD based field emission source/sen sor and their field emission characteristics were presented.

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8 Chapter 1 starts with a brief history about CVD diamond films along with some of the interesting properties which make diamond a potential candidate for mechanical, electrical and thermal applications. Chapter 2 provides a background on field em ission, and the advantages of using NCD films for the fabrication of field emission devices. The fabrication procedures and the field emission properties of vertical and late ral field emission devices are discussed. This chapter concludes with an introduction to the principle of field emi ssion based sensing as well as some of the earlier work reported by other researchers. Chapter 3 provides insight into the growth and characterization of intrinsic and conductive nanocrystalline diamond films. A detailed study on nucleation and deposition of NCD films is presented. The electrical prop erties of nitrogen incorporated conductive NCD films conclude this s ection of the dissertation. In Chapter 4, the approach towards the fabric ation of a field emi ssion source and a field emission sensor are presented. An overview on th e fabrication procedur e of vertical field emitter arrays by silicon mold technique is provided. The method of testing the field emission source and sensor for their emission characteristics are discussed in the last section of this chapter.

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9 Chapter 5 provides the design specifications of both field emitter arrays and anode-onmembrane arrays. For the design of anode, tw o types of membranes: free standing and boss type structures have been considered in the current research. The finite element modeling and the mechanical analysis of the membranes were performed in CoventorwareTM. The effect of membrane dimensions and external load on the deflection of the membranes is also discussed. Chapter 6 presents the fabrication details of free-standing NCD field emitter arrays and a fully packaged capped field emission source. The fabrication of NCD field emission sensor involves a three-wafer pr ocess with five levels of lithography and two levels of packaging: a-chip-level and a wafer-level. Th is section also discusses the field emission behavior of both types of devices. Chapter 7 concludes the dissert ation with a summary of the current investigations and suggests few recommendations on the future work of the present research activity.

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10CHAPTER 2. BACKGROUND ON FIELD EMISSION 2.1. Introduction With a revolution in semiconductor field, there was a tremendous growth in the electronics industry during the last decade of the 20th century. Inventions such as cellular phones, palm and laptops, and flat panel disp lays have indicated remarkable progress made in this field. The past 10 years, ther e has been an increasing momentum in the display industry. The evolution of liquid crys tal displays (LCD’s) and plasma displays have replaced the bulky monitors in computer s and televisions with sleek, compact and lightweight flat panel displays Field emission displays (FED’s) can be an alternative and efficient display technology [15]. FED’s provide a flat panel technology that features less power consumption than existing LCD and plas ma display technologies. Besides, they can be cheaper to make as they have fewe r total number of components. Field emission sources are also used in electron micros copes [16], MEMS devices [17] and vacuum microelectronic devices [18]. The principle of field emission is based on the quantum mechanical tunneling of electrons emitted from the surf ace of a material into vacu um under an applied electric

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11 field. If a material possesses sufficiently sm all work function and electron affinity, the barrier height can be small, and electrons can tunnel thr ough the narrow potential barrier. The first field emission devices fabricated us ing molybdenum tips were known as “Spindt type cathodes” or “Spindt FEA's” [19]. In th e meantime, alternative materials such as Si [20], cubic-Boron Nitride(c-BN ) [21], AlN [22], Barium Str ontium Titanate (BST) [23], and nanostructures of Si [ 24], SiC [25], ZnO [26], In2O3 [27], SnO2 [28] and TiO2 [29] were also investigated for field emission a pplications. It was later found that carbon based materials such as carbon nanotubes (C NT's) [30], diamond-lik e-carbon (DLC) [31] and diamond [32] can be efficient electron em itters. The synthesis of vertically aligned CNT's has opened a new direction for the fabr ication of field emission arrays [30]. The fabrication of emitter devi ces with a gate or a collecto r at a micro/nanoscale involves expensive fabrication processes. It will be cost-effective if the material used can compensate for the cost of fabrication in term s of its enhanced performance. For efficient tunneling of electrons, the material should possess low work function, electron affinity, and dielectric constant, and high thermal conductivity, melting point, and chemical and physical robustness [33]. Most of the carbon based materials possess these properties and, hence, are very suitable for field emission and related applications. 2.2. Principle of Field Emission The electrons inside a material are bound clos ely by an electrostatic force of attraction. Due to this force, a potential barrier known as “work function” is established. For any

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12 material to emit these closely bound electrons they have to overcome this potential barrier and escape to the vacuum level. When a sufficient thermal energy is supplied to a material, electrons in the conduction band ga in kinetic energy required to escape into vacuum. In another way, the electrons can be released by applying high electric fields or by applying both thermal and electric fields. Field emission is based on the principle of extracting electrons from the surface of a metal or a highly doped semiconductor to vacuum by applying a potential or an electri c field. The electron affinity is "the amount of energy required for an electron to releas e from the conduction band into the vacuum level". Hence, it is evident that materials with small or negative electron affinity can easily emit electrons from their surfaces. The energy band diagrams of a positive electron affinity (PEA), effective negative electron affinity (NEA) and a true NEA diam ond surfaces are shown in Figure 2.1 (a-c), respectively [34]. The electron affinity of diamond depends on the amount of hydrogen present. If the surface of the diamond is completely free of hydrogen, the net electron affinity is positive. Therefore, the highest level of the conduction band edge is below the vacuum level resulting in a potential barr ier (or electron affin ity) between the diamondvacuum interfaces (Figure 2.1(a)). On the ot her hand, when the surface of the diamond is partially hydrogenated, a positive dipole is i nduced which results in the band bending and correspondingly a smaller electron affinity value than the previous case. In that case, the surface of the diamond will have an effec tive NEA where the highest level of the conduction band edge is above the vacuum leve l while the lowest le vel of the conduction

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13 band edge bends below the vacuum level (F igure 2.1(b)). In Figu re 2.1(c), the energy band diagram of a true NEA surface of diamond is shown. (a) (b) (c) Figure 2.1 E nergy Band Diagram of Diamond Su rfaces (a) Positive Electron Affinity (b) Effective Negative Electron Affinity (c ) True Negative Electron Affinity [34] A true NEA surface in diamond is obtained only when the surface is fully hydrogenated. As the conduction band is above the vacuum level, the net barrier height is negative and therefore, electrons can easily flow from the conduction band to the vacuum level without any potential drop. A true NEA surface is not usually observed in many semiconductors and therefore was not believed to exist. If the emitting surface is a flat film, the corresponding electric field is given by E=V/d where V is the applied voltage and d is the separation between the film and the anode. But if the emitting surface consists of tips, the electric field not only depends on the applied voltage and the separation but also on the geometry of the tip [37]. The field

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14 emission current is governed by the Fowler-Nordheim (F-N) prin ciple [38]. According to Fowler-Nordheim tunneling, the curr ent density is given by Eq 2.1. E B AEJ exp2 (2.1) where J is the current density, E is the electric field in the insulator, A and B are constants given by Bh q m m A83 (2.2) q h m BB 2 3 2 1 2 *2 3 8 (2.3) d V E (2.4) where m, m*, q, h and B are the electron mass, the effective electron mass, the electronic charge, the Plancks constant and the barrier height respectively. is the field enhancement factor, V is the applied potential and d is the thickness of the oxide layer. The field enhancement factor can be expres sed in terms of the geometry of the tip. Accordingly, = L/R, where L is the height of the tip, R is the radius of the curvature of the tip. Substituting all the terms in Eq. 2.1, it can be rewritten as Rd LV B Rd LV aAaJI exp2 (2.5)

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15 As can be observed from the above equations for a fixed thickness of insulator(d) the emission current (I) primarily depends on the em ission area (a), applied bias ‘V’, height of the tip ‘L’, the radius of curvatur e of the tip ‘R’ and the work function ‘ B’. 2.3. Advantages of Nanocrystalline Diamond ( NCD) Films for Field Emission Sources Table 2.1 shows the properties of few materials that were investigated for field emission applications [33]. Table 2.1 Properties of Materials Suitabl e for Field Emission Applications [33] Material Fusion Point (C) Dielectric Constant Band gap (eV) Electron affinity (eV) Thermal Conductivity (W K-1cm-1) Diamond 3550 5.7 5.5 -0.7 20 Cubic-BN 2700 7.1 6 NEA 1 AlN 2400 9.1 6.2 NEA 3 amorphousSiO2 1700 3.9 9 0.6-0.8 0.02 Al2O3 2030 9.4 8.3 0.88 0.46 LiF 870 9.3 14.2 -2.7 0.12 CaF2 1423 8.4 12.1 -0.54 0.08

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16 Of the above materials, diamond is very su itable for the fabrication of field emission devices. The emission current from the tip s is usually on the order of several microamperes. In both hot and cold cathode emission devices, if the emission current from each tip increases, the temperature at the emitter tip increases tremendously due to I2R losses and make the tips blunt and reduce th e device lifetime. In order to circumvent this problem, it will be advantageous if one can fabricate the micro tip array from such a material that can easily dissipate heat, ther eby can enhance the performance. In this context, the tips made from NCD films can effectively dissipate the heat due to high thermal conductivity and emit current for longer periods. The metal emitter tips adsorb the contamin ants from the surrounding environment and increases the surface work function of the materi al resulting in an inconsistent emission current. Hence, intermediate “conditioning the tip surface” becomes mandatory for maintaining constant emission current without a significant increase in the applied voltage [39]. This problem of surface adso rption could be minimized, if not fully be eliminated, by the use of NCD films as they provide a relatively high chemically inert surface. Hence, NCD based field emitters can be operated in harsh environments with almost equivalent performance as can be used in a normal ambience [3]. If the diamond films are completely hydrog enated or partially hydrogenated, the corresponding electron affinity values can be negative or slightly positive [35, 36]. A significant electron emission can be obtained if n-type impuritie s can be incorporated into

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17 the diamond films. The incorporation of n-t ype impurities increases the concentration of electrons in the conduction band, thereby increas es the probability of more electrons to escape to the vacuum. However, the inability to successfully dope n-type diamond has limited diamond field emission devices for seve ral years. With the recent success in obtaining n-type NCD films, di amond field emission devices ha ve gained interest. It is widely known that the field emission from NC D films cannot be comple tely attributed to the NEA surface alone but was found to be al so due to the defective grain boundaries consisting of graphitic carbon [40]. Unlike silicon technology, fabrication procedur es of diamond-based devices are not fully mature. Besides, the growth, etching and pr ocessing of diamond films is time consuming and, hence, may not be cost effective unle ss the current methods are improved. Several reports on the field emission propert ies of diamond films, both as a flat film and also as a vertical and lateral tip shaped emitters can be found in the literature. In the below section, a brief review on the fabrication of vertical and lateral field emitter devices and their field emission properties is presented. In a field emission diode, voltage is applied to the emitter and the current from the tip is collected at the anode (or collector). However, the fabrication of anode structures with sub-micron features may have certain lim itations due to photo-lithography. Hence, alternative methods of self-aligning the ga ted structures have been suggested. The

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18 fabrication steps implemented by W. P. Kang et al., [41] for a self-a ligned structure are shown in Figure 2.2. Figure 2.2 F abrication of Vertical Field Emitter Array by Mold Technique [41] The silicon wafer was anisotropically etched to form V-grooves by the use of standard photo-lithography and etching techniques, followed by the deposition of an insulating SiO2 layer, and finally the diamond films were deposited by plasma enhanced chemical vapor deposition (PECVD) method on the SiO2 inverted sharpened pyramidal mold. The thickness of SiO2 layer determines the gap between the emitter and the anode. A square

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19 window was etched on the backside of the sili con mold to anisotropi cally etch the silicon substrate until the apex of the diamond tips was exposed. Finally, SiO2 near the apex region was etched to expose th e diamond tips (see Figure 2.2). It has been reported that the field emission properties of su ch devices were promising, with a low turn-on voltage of ~ 0.7 V and an emission current of 4 A at a gate voltage less than 5 V (See Figure 2.3). The linear relati on of the inverse of electric field vs. the logarithmic of the square of the current/electric field indicates the Fowler-Nordheim characteristics. The enhanced field emission property in such devices was attributed to the oxide-sharpening effect of the emitter tip. Figure 2.3 Fowler-Nordheim Characteristics of Vertical Field Emitter Array Device [41]

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20 Even today, most of the field emission sources are fabricated in vertical configuration using mold technique. But due to the complexities in the fabrication of vertically-aligned field emitters, the fabrication of lateral field emission devices is also under investigation. The lateral emitters can be fabricated with a fairly simple fabrication procedure. The lateral field emission devices usi ng CVD diamond films reported by W. P. Kang et al., [42] included a single mask pr ocess with a high device yiel d of 80 %. Figure 2.4 shows the schematic of the process steps implemented in the fabrication of lateral field emitters using silicon-on-insula tor (SOI) wafers. Figure 2.4 Process Steps for the Fabricati on of Lateral Field Emission Device [42]

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21 Figure 2.4 (Continued) Boron doped diam ond films (p-type) were grown on patterned SiO2 films to form the lateral field emitter geometry. With an anode to cathode spacing of 2 m, a turn-on voltage of 5 V, and an emission current of 6 A have been achieved (seen in Figure 2.5). The limitation of this approach is the high cost of silicon-on-insulator (SOI) wafers as compared to standard silicon substrates. Figure 2.5 Field Emission Characteristics of Lateral Field Emission Device [42]

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222.4. Field Emission Based Sensing Miniaturizing the sensors and actuators was made possible by MEMS technology. The capacitive [43], piezo-resistive [44], and optical sensors [45] are actively investigated in commercially available MEMS sensors. Th e above sensing met hods have their own advantages and disadvantages and, hence, the choice depends on the end application, manufacturing and integration costs, etc. Field emission based sensors can provide an alternative sensing mechanism to the existing ones. Its specific advantages such as temperature insensitivity, minimal resonance effects, and possibility to integrate with microelectronic circuits put forth this as a highly sensitive techni que. A field emission sensor can operate in a diode or a triode configuration. In a diode configuration, the cathode consists of an emitter array and an anode or a collector is a thin metal plate. In a triode configuration, besides the two terminals of a diode, a control gate is fabricated to limit the emission current. The field emission sensors can be fabricated in two designs: “anode-on-membrane” geometry or “cathode-on-membrane” geometry. In the first one, anode is a flexible diaphragm while the cathode consists of fixed emitter arrays and in the second design, the cathode is fabricated on a flexible diaphragm and anode is a fixed plate. The field emission based sensor can be operate d in two modes: 1) constant current mode and 2) constant voltage mode. In both the modes of operation, the sensor sensitivity depends on the distance between th e emitter tips and the membrane.

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23 According to Fowler-Nordheim equation, the current density is given by the expression Rd LV B Rd LV A J exp2 (2.6) As d d, correspondingly JJ, therefore the above expre ssion can be re-written as d R LV B d R LV A J exp2 (2.7) The sensitivity (Sv or Si) in both the cases is given by Sv = V/ d (I is constant) (2.8) Si = I/ d (V is constant) (2.9) Sv and Si denote the sensitivity in the constant current and constant voltage modes respectively, V and I denote the change in the turn-on voltage and the emission current respectively and d denotes the deflection of the diaphragm. Having briefly discussed the background of fiel d emission sensing, it will be useful to know the previous research carried in this fi eld. Most of the earl ier reports on field emission sensing were based on the modeli ng and simulation work [46-49]. There are only a few reports in the literature that show some preliminary experimental data [50]. The theoretical and the mode ling data provide good backgr ound essential for the device fabrication. The below secti ons will provide a review of the design, fabrication and characterization of field emission sens ors reported by other researchers.

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24 Few materials such as boron nitride, silicon, silicon carbide and DLC coatings on silicon have been used for the fabrication of field emission sensors [51, 52]. N. Badi et al., have reported on the field emission based pressure micro-sensors that consist of an emitter array made with sulphur doped boron nitride fi lms and thin membranes of Ti, Si, Ta and TiN as anodes [50]. The device worked in a di ode configuration with an inter-electrode spacing of ~ 1 m. As the electrodes were cl osely separated, a th reshold field of ~ 50 V/ m and an emission current on the order of few milliamperes was achieved. Figure 2.6 shows the pressure vs. current relationship for various membrane materials. Figure 2.6 P ressure vs. Current Relationship in Field Emission Pre ssure Sensor [50]

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25 It has been reported that for an emitter to ga te separation of 1 m, the magnitude of the current changed by three orders, maintaini ng the linearity. Through simulations, it has been found that the sensitiv ity range of this sensor varies from nA/Pa to A/Pa. It can be found that Si, Ti and Ta membranes showed non-linear pressure-current characteristics while the TiN membranes showed linear response [50]. It has been reported that when thicker TiN membranes were used as anode, the emission current has not varied up to a pressure of 1000 Pa, while thinner membranes showed a significant change in the current up to 3 mA. The linear response in the Ti N membrane characteristics changed to nonlinear behavior when the thickness of the me mbrane was increased from 1 m to 3 m. Dan Nicolaescu et al., [48] have developed a model to an alyze the behavior of a pressure sensor using diamond film as a cathode and a fl exible silicon membrane as an anode. At a constant emission current, the variation in the anode voltage accounted for the amount of pressure applied on the membrane. The effect of membrane dimensions on the pressure domain range is shown in Figure 2.7. In the analytical model, it was shown that thicker and smaller membranes required higher pressu re (107 Pa) for a cons tant deflection of 4 m (Figure 2.7). It can be found that as the thickness of the membrane increased from 5 m to 60 m, the operating pressure range increased by at least three or ders of magnitude.

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26 Figure 2.7 E ffect of Membrane Thickne ss and Size on Pressure Domain [48] Figure 2.8 shows the relationshi p between the operating voltages vs. the vacuum gap. It can be found that a very str ong linear relation exists betw een the applied voltage and the vacuum gap. For a constant emission curren t, the anode voltage changes by more than 300 volts for an approximate change in the inter-electrode spacing of 2 m. This model shows that a field emission based sensor can be highly sensitive even to small changes in the applied pressure or testing conditions. Figure 2.8 Effect of Anode to Cathode Spacing on Applied Voltage [48]

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27 The sensitivity as a function of length of the membrane is shown in Figure 2.9. Figure 2.9 Effect of Membrane Dimensions on Sensitivity of Field Emission Pressure Sensor [48] It has been reported that the sensitivity was inversely proportional to the dimensions of the membrane. If a stress free film can be used as a thin membrane, the sensitivity can be improved by increasing the membrane dimensions. It is evident that the dimensions of the membrane, the design specifications of memb rane and emitter array as well as the spacing between the two are key elements to fa bricate a reliable and a sensitive emission sensor. The limitations due to the photo-lithography ca n be overcome if one can fabricate the membrane and the cathode on two different di ce and bond them efficiently in such a way that the vacuum gap is only on the order of few microns, it will be possible to operate the sensor efficiently at lower operating voltage.

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282.5. Summary A background on the evolution of the field emission devices a nd its applications has been discussed. Though the field emission pheno menon has been observed in various semiconductor materials, the specific advant ages of nanocrystalline diamond thin films have put forth its candidature for fabricating field emission devices in both vertical and lateral device geometries. The field emissi on based sensing has been reported as an alternative sensing technique by detecting the changes in the emission current or the turnon voltages. The field emission based pressu re sensors were fabricated using few materials such as Ti, TiN, and Si. To this e nd, analytical results a nd modeling of the field emission diode sensors have been reviewed.

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29CHAPTER 3. GROWTH AND CHARACTERIZATION OF NANOCRYSTALLINE DIAMOND FILMS 3.1. Introduction It has been widely accepted by late 1970’ s and early 1980’s that chemical vapor deposition (CVD) is the only suitable techni que for growing high quality diamond films. In a CVD process, methane is used as the source of carbon and hydrogen is added to reduce the graphitic content and improve the growth rate For over three decades, research was primarily focused on “micro crystalline diamond” (MCD) films [53]. Microcrystalline diamond films consist of larg e grains (grain size: ~ 5-10 m) and rough surfaces (mean surface roughness: ~ 300-700 nm) thereby limit its application to cutting tools, abrasive coatings and heat si nks [54-56]. The limited applications of microcrystalline diamond (MCD) films have been surpassed by synt hesizing a new class of material known as “nanocrystalline di amond” (NCD) films. The nanocrystalline diamond films can be grown by altering the C VD process [57]. Unlike MCD, NCD films consist of small grains on the order of 2050 nm and a low surface roughness of ~20 nm. Recently, “ultra-nanocrystalline diamond” (UNCD) films having smaller grain size (3-5 nm) than NCD have been developed [58]. The growth of NCD/UNCD films opened a

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30 wide window of applications ranging from tr ibology, MEMS, optics, RF applications and field emission devices [59-62]. 3.2. Substrate Pretreatment for the Growth of Diamond Films on Non-Diamond Substrates Diamond films can be deposited on non-diamond s ubstrates only when the substrates can readily carburize. As all substrates do not form carbide when exposed to the carbon containing precursor gas, a specific sample preparation known as “seeding” is required. Seeding incorporates nucleati on sites on the substrate, thereby promoting the growth of diamond films. It can be done by: 1) mechanic al scratching of the s ubstrates by diamond micropowder [63], 2) ultras onication of the substrates in a diamond nanopowder suspension [63], 3) diamond phot oresist suspension (DPR) [64] and 4) bias enhanced nucleation (BEN) method [65-67] When the surfaces are mechanically abraded, the uniformity of the films is poor correspondingly the deposited films are rough due to the scratches induced during abrasi on. Due to manual scratching, the nucleation densities vary largely across the sample surface and be tween the samples. Ultrasonication of the substrates in diamond nanopowder suspen sion can overcome the problems due to mechanical seeding and can improve the uniform ity. But, it is difficult to implement the ultrasonication method if the underlying substrat e has features such as thin membranes, or high aspect ratio needles and ultra small features that might be damaged during the actual sonication process. Diamond photoresist (DPR) method may suggest an alternative technique to enhance the nucleation density. Bias enhanced nucleation technique is yet

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31 another method of nucleation to produce unifo rm seeding through out the substrate. Unfortunately, BEN cannot be implemented if the substrate is elec trically insulating. James E. Butler et al., at naval research laboratories (NRL) implemented a nucleation process known as “Rotter’s method ” of seeding. In this a pproach, the substrates are initially exposed to CH4/H2 plasma followed by the ultrasonication in a diamond nanopowder suspension. The plasma exposure can clean the silicon surf ace and also etch any native oxide present. Unlike wet etch of silicon in BOE, the in-situ etching provides no subsequent exposure of the wafers to the atmosphere prior to the growth of amorphous carbon layer. It has been re ported that the nucleation de nsities on the order of 1012/cm2 can be achieved by this technique [68]. Tabl e 3.1 shows the nucleation densities obtained from various methods. Table 3.1 Effect of Seeding Method on Nucleation Density Method of Seeding Nucleation Density(/cm2) Mechanical scratching 106-107 [63] Ultrasonic polishing 106-1010 [63] Diamond Photoresist suspension 1011 [64] Bias Enhanced Nucleation 1010-1015 [65-67] Pre-plasma exposure 1012 [68]

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323.3. Selective Nucleation of Substrates In all of the above mentioned methods, seeding is done through out the substrate. Therefore, when the substrates are exposed to the deposition conditions, diamond films grow completely on the substrate surface. But certain applications might require selective deposition of diamond films. As diamond films need an initial nucleation layer to grow on non-diamond substrates, selective nucleation is carried out by first coating the wafer surface with a diamond loaded photoresist, followed by lithography to define the desired pattern. Development of the pho toresist results in leaving the diamond seeds only at the desired regions on the wafer surface. Therefor e, diamond films grow at these nucleation sites and forms a continuous film representi ng the pattern on the substrate. Selective nucleation can also be carried out by fully seeding the wafer in diamond slurry by ultrasonication followed by lithography and et ching of the diamond seeds from unwanted regions to define the desired features. When those wafers are exposed to the growth conditions, diamond films can be deposited se lectively only in th e seeded regions. 3.4. Cyrannus I Large Area Microwave Plasma Enhanced Chemical Vapor Deposition (MPECVD) Reactor MPECVD is the most widely used technique to grow diamond films. Typically all microwave CVD reactors are equipped with a 2. 45 GHz generator head. Microwaves have shorter wavelengths or higher frequencies a nd therefore can produce high density plasma. In a MPECVD reactor, the growth of high quality diamond films are ensued at relatively high pressure and operating temperatures. Th e cross-section of the commercial plasma

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33 source is used in this research is shown in Figure 3.1. Being an electrodeless deposition, the films produced can be free of contaminati on. The plasma generated by microwaves is stable for a long time. Emax = (qe) 2/8 2f2m (3.1) where Emax maximum ion energy, qion charge, mmass of ion and ffrequency Figure 3.1 Cross-Section Geometry of a Cyrannus I Iplas Plasma Source [69] In this research, Nanocrystalline diamond films have been grown by the microwave plasma enhanced chemical vapor deposition (MPECVD) method in a Cyrannus I Iplas reactor. The Cyrannus I Iplas system (shown in Figure 3.2) has a 6" plasma source equipped with a 6 kW microwave generato r that can produce high density uniform plasma up to 100 mm in size. A manual E-H tuner is used to adjust the plasma density. A graphite heater assembled inside a circular molybdenum substrate holder can be used to heat the substrates up to 800 C. The substrate temperature can be controlled (1 to 2) C

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34 using a closed-loop heater contro ller while the process pressure is controlled by a throttle valve. Figure 3.2 Cyrannus I Iplas Microwave Plasma Enhanced Chemical Vapor Deposition System 3.5. Growth Mechanism of Nanocrystalline Diamond Films The drastic reduction in the grain size of th e diamond films from several microns to few nanometers by changing the gas chemistry suggests that the growth mechanism of nanocrystalline diamond films is differe nt from conventional CVD diamond films. Methyl radicals (CH3) and acetylene molecules (C2H2) are the dominant species in the

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35 growth of conventional CVD films using CH4 and H2 gas chemistries [70]. NCD films are typically grown in 1% CH4, with or without 1% H2 and 98% or 99% Ar. Reduction in the hydrogen concentration from 99% to 1% reduces the grain size of the diamond from several microns to few nanometers. The pha se-pure nanodiamond films were grown from a gaseous mixture of C60/Ar in microwave plasma CVD with a total argon pressure of 98 Torr, C60 partial pressure of 0.01 Torr, a total flow of 100 sccm, and at a microwave power of 800 watts. A C2 dimer-based growth mechanism that would result in nanocrystalline structure was proposed [ 71]. In the films deposited using 5% CH4 and 95% Ar, the C2 dimers resulted in the inclusi on of an amorphous carbon or graphitic carbon [72-74]. Such non-diamond form of carbon was due to the homogenous nucleation resulted from high ratio of hydrocarbon to carbon dimers. But on the other hand, during the deposition of nanodiamond f ilms, heterogeneous nucleation rate (>1010 cm2sec-1) increases due to highly reactive C2 species, resulting in the smaller grain size of the diamond films [71]. Figure 3.3 shows the schematic of the growth mechanism of nanocrystalline diamond [58]. A ccording to this model, the feed gases methane and argon disassociate and favor the formation of (C2H2)+ at a low ionization potential. The positively charged acetylene radical attracts an electron to form a highly reactive carbon dimer and hydrogen. Hydrogen is then desorbed away while the carbon dimers nucleate at the reconstructed surface. As the reaction continues, the number of carbon dimers in the plasma increase and they join the previous ly hybridized carbon atoms. In this way, a closely hybridized sp3 network of carbon atoms forms a continuous film of nanocrystalline diamond.

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36 Figure 3.3 Growth Mechanism of Nanocrystalline Diamond [58]

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373.6. Experimental Study on the Nucleation Methods Through out this research, th e seeding was implemented by ultrasonic polishing of the samples in diamond nanopowder slurry. During u ltrasonication, fine powders (4-5 nm) of diamond suspended in acetone or methanol scratch the surface of the wafer. These diamond seeds are held to the substrate by weak Vander walls forces of attraction. Typically, the ultrasonic polishing was carried for 20 minutes, followed by an ultrasonic cleaning in methanol for 40 minutes. In this st udy, some of the samples were exposed to the growth conditions for 30 minutes prior to the ultrasonication. Th e process conditions for the growth were: CH4-0.5%, H2-1%, Ar-98.5%, total gas flow800 sccm, M. W. power-1.8 kW, pressure-135 Torr, substrate temperature-750 C, and deposition time-0.5 hrs. The experimental conditions implemented during the seeding are shown in Table 3.2. Table 3.2 Experimental Conditions for S eeding With and Without Plasma Exposure Acetone + diamond nanopowder Methanol + diamond nano powder Nucleation Density (cm-2) Without Plasma Exposure 20 min seeding 20 min seeding 108 With Plasma Exposure (30min) 20 min seeding 20 min seeding 108 A batch of four samples with two of them exposed to the plasma for 30 minutes, and the remaining two without any plasma treat ment, were ultrasonicated in diamond nanopowder slurry. The plasma treated sample s were exposed to the above indicated

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38 growth conditions. The ultrasonicatio n was carried for 20 minutes in a Branson ultrasonic generator followed by the ultras onic cleaning. SEM micrographs of the samples with and without the plasma exposure and after a 30 minute growth are shown in Figure 3.4. A nucleation density on the order of ~108 cm-2 was achieved on the samples. Figure 3.4 Effect of Plasma Treatment on Nucleation Density After Ultrasonicating for 20 Minutes (a) Without Plasma Exposure in a Mixture of Acetone and Diamond Nanopowder Slurry (b) Without Plasma E xposure in a Mixture of Methanol and Diamond Nanopowder Slurry (c) With Plasma Exposure in a Mixture of Acetone and Diamond Nanopowder Slurry (d) Mixture of Methanol and Diamond Nanopowder Slurry Unlike the Rotter’s nucleation process [75] which can yield higher nucleation densities after the plasma treatment, in this study, it was observed that the plasma exposure of the (a) (b) (d) (c)

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39 samples prior to the growth has not enhanced the nucleation densities. This could be due to several reasons such as th e type of gas chemistry, the gas mixture and the process parameters. In Rotter’s method of seeding, both deposi tion of NCD films and plasma exposure were carried in CH4/H2 gas chemistry. The plasma exposure was carried at relatively higher methane content than the actual deposition, which results in the deposition of a thin amorphous carbon film. After the plasma exposure and during the subsequent ultrasonic seeding, the diamond s eeds embed themselves into this amorphous matrix thereby increase the nucleation density In the current study, the deposition and plasma treatment were conducted in hydroge n poor gas chemistries with only 0.5% methane and excess argon in the gas mixture. The difference in the process chemistry and the experimental conditions explain the difference in the nucleation densities of this experimental study from Rotter’s method. It has been reported by Nevin N. Naguib [76] et al., that thin metal interlayer (~ 10 nm) of tungsten can enhance the nucleation density and thereby reduce the surface roughness of the films. The improved nucleation density mi ght be due to the formation of a tungsten carbide layer on the surface. Experiments we re conducted to observe if the nucleation density could be improved by depositing a thin interlayer of Titanium (~ 10 nm). Like tungsten, Titanium can also form carbide when exposed to carbon-containing gas mixtures. In another experiment, new slurry was prepared by first combining a homogenous mixture of titanium nanopowde r and diamond nanopowder. The slurries were prepared both in meth anol and acetone by adding the mixture of two powders.

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40 Table 3.3 shows the experimental matrix using titanium metal as an interlayer, and also with the addition of titanium nanopowde r and diamond nanopowder to methanol and acetone. Table 3.3 Experimental Conditions for Seeding With and Without Interlayer Acetone + Diamond Nanopowder Methanol+ Diamond Nanopowder Nucleation Density(cm-2) With Titanium Interlayer 20 min seeding 20 min seeding 108 20 min seeding 20 min seeding 40 min seeding 40 min seeding With a mixture of Titanium nanopowder and diamond nano powder 60 min seeding 60 min seeding 1010 Figure 3.5 shows the SEM micrographs of th e samples deposited with a thin titanium interlayer. Although, there was no significant improvement in the nucleation density as compared to the previous experimental re sults, it can be observed that the samples ultrasonicated in acetone susp ension show smaller and a higher number of nuclei than those ultrasonicated in methanol. Unlike tungsten, titanium readily reacts with oxygen in the atmosphere and forms titanium oxide [77]. The formation of oxide does not favor the improvement of the nucleation density.

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41 Figure 3.5 Effect of Titanium Interlayer on Nucleation Density Af ter Ultrasonicating for 20 Minutes in (a) Mixture of Acetone and Diamond Nanopowder (b) Mixture of Methanol and Diamond Nanopowder It has been reported earlier on the pre-nuc leation methods for improving the nucleation density of UNCD film s deposited in CH4/Ar gas chemistry at low substrate temperature (~ 400 C) [78]. It was shown that addi tion of titanium micropowder during the seeding has resulted in the nucleation densities on the order of 5 X 109 cm-2. The role of titanium powder in improving the nucleation density wa s not addressed clearly. In the present study, it was observed that the addition of titanium nanopowder enha nces the nucleation density of the samples ultrasonicated in both acetone and methanol. The SEM micrographs of the samples are shown in Figure 3.6 through Figure 3.8. It can be observed from these SEM images that a cont inuous film of nanodi amond was formed on the surface after seeding for 20 minutes when the slurry was prepared using acetone. Few voids can be observed on the samples treated in methanol slurry. By increasing the ultrasonic polishing time to 1 hour, the films cover the voids and re sults in a continuous film. (a) (b)

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42 Figure 3.6 Surface Morphology of the Films Af ter Ultrasonicating for 20 Minutes in (a) Mixture of Titanium Nanopowder, Diamond Nanopowder and Acetone (b) Mixture of Titanium Nanopowder, Diamond Nanopowder and Methanol Figure 3.7 Surface Morphology of the Films Af ter Ultrasonicating for 40 Minutes in (a) Mixture of Titanium Nanopowder, Diamond Nanopowder and Acetone (b) Mixture of Titanium Nanopowder, Diamond Nanopowder and Methanol (a) (b) (b) (a)

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43 Figure 3.8 Surface Morphology of the Films Af ter Ultrasonicating for 60 Minutes in (a) Mixture of Titanium Nanopowder, Diamond Nanopowder and Acetone (b) Mixture of Titanium Nanopowder, Diamond Nanopowder and Methanol It can be observed from all the SEM microgra phs (Figure 3.4 through Figure 3.8) that the samples treated in slurry consisting of titanium and diamond nanopowder provided a higher nucleation density (at least 1010 cm-2). Continuous films were observed on all the samples polished in diamond slurry made using acetone. In general, ultrasonication in acetone suspension has provided slightly higher nucleation sites than the ones seeded in methanol suspension. A slight improvement in the density can be due to the higher dispersion efficiency of the diamond seeds in acetone than methanol [79]. Such a higher nucleation density was not observed on the samples grown with a thin titanium interlayer (~ 10 nm). The higher nucleation densit y resulted by seeding in the new slurry was due to a physicochemical surface reaction [ 80]. The titanium powde r activates the surface of the substrate for the incoming carbon dime rs to nucleate at the diamond seeds and coalesce into a continuous thin film. (a) (b)

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443.7. Effect of Nucleation on the Fabrication of MEMS Cantilevers The effect of nucleation de nsity on the fabrication of MEMS cantilevers using nanocrystalline diamond films as structural ma terial is discussed in this section [113]. Fabrication of the cantilevers is done by the standard photolithography techniques. The process flow is shown in Figure 3.9. Figure 3.9 Process Steps for the Fabrication of Cantilevers

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45 The fabrication steps involve th e growth of a sacrificial SiO2 layer (2 m) by LPCVD, followed by a photo-lithography step to form a dimple in the oxide which defines the anchor region. NCD films of thickness ~1 m were deposited on the patterned substrates. After the growth of NCD f ilms, Al alloy (hard mask) of thickness ~ 300 nm was deposited by sputtering. A photo-lithography step followed by the Al alloy wet etch exposes the unwanted portions of the NCD film s. After the reactive i on etch of NCD, the sacrificial oxide was etched in buffered oxide etch (BOE) to release the nanodiamond cantilevers. The SEM micrographs of the can tilevers fabricated using the nanodiamond films treated in a mixture of diamond na nopowder and acetone slurry as well as the mixture of titanium nanopowder and diamond nanopowder in acetone slurry during seeding are shown in Figure 3.10 (a-d), respec tively. Due to the lower nucleation density, the nanodiamond films have not deposited unifor mly on the anchor pads when the wafers were pretreated in the diamond nanopowder and acetone slurry (Figure 3.10 (a) and (b)). Therefore, during the subsequent sacrificial ox ide etch, the structures have detached from the substrate and the cantilevers were f ound separated from the underlying silicon substrate. The successful fabrication and re lease of the cantilev ers was possible after improving the seeding method. The samples pr etreated in a mixt ure of titanium and diamond nanopowder suspended in acetone has resulted in a uniform and a continuous growth. So, during the subsequent sacrificia l oxide etch, the adhesion to the underlying silicon substrate has improved and the cantileve rs could hold to the substrate during the release (Figure 3.10 (c) and (d)). It was observed that the Young’s modulus and the hardness of the films were ~ (740 80 ) GPa and (82 12) GPa respectively.

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46 Figure 3.10 SEM Micrograph of Cantilever s (a) and (b) Due to Poor Nucleation Resulting From Seeding in Diamond Nanopowder in Acetone Slurry (c) and (d) Due to Improved Nucleation Resulting From Seeding in Mixture of Titanium Nanopowder and Diamond Nanopowder in Acetone 3.8. Optimized Process Parameters for th e Growth of Nanocrystalline Diamond Films The optimized process recipe for the growth of nanocrystal line diamond films has been achieved by systematically adjusting the process gas compositions, operating pressures, substrate temperatures, and ra tio of individual gases. The amount of hydrogen present during the growth affects the properties of the films such as the grain size, grain boundaries and surface roughness. Therefore, the ratio of [Ar]/[H] in the gas composition

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47 has to be precisely controlled. The optimized process parameters for the growth of intrinsic NCD films were found to be: CH40.5%, H21%, Ar98.5%, total gas flow800 sccm, M. W. power1.8 kW, pressure135 Torr, substrate temperature750 C. 3.9. Tools and Techniques for the Characteri zation of Nanocrystalline Diamond Films The structural properties of NCD films were studied by seve ral analytical and metrology techniques including x-ray diffr action (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission elect ron microscopy (TEM), energy dispersive x-ray analysis (EDAX), Raman spectroscopy, and near edge x-ray absorption fine structure (NEXAFS) studies. Most of these t echniques are widely used in characterizing the semiconductor materials. Their operating pr inciples, physics and applications have been extensively covered in the literature [116]. Of the above techniques, Raman and NEXAFS are two spectroscopic methods excl usively employed in characterizing the properties of carbon based materials. As this research is primarily focused on diamond films, it will be useful to understand the operating principles and basics of Raman and NEXAFS Spectroscopy. 3.10. Raman Spectroscopy Raman spectroscopy is a powerful technique to determine the chemical and structural properties of liquid or solid materials by a simple non-destru ctive and non-contact

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48 method of measurement. It possesses seve ral advantages over other conventional structural characterization techniques such as no sample preparation, rapid and wide measurement range (50 cm-1 to 4000 cm-1). It is based on the principle of change in the polarization or electron moveme nt in the material when a laser of a specific wavelength strikes the surface of the sample. When a la ser beam strikes the sample, the molecular vibrations result in two types of scattering mechanisms: 1) Rayleigh scattering (elastic) and 2) Raman scattering (in-elastic). Due to Rayleigh scattering, there is no change in the energy. On the other hand, due to Raman scatte ring, change in the energy results in antistokesh ( 0+ n) and stokesh( 0n). The change in energy due to the molecular vibration is collected in the Raman spectrum. Figure 3.11 shows the schematic of Raman spectroscopy. Figure 3.11 Schem atic of Raman Spectroscopy In this research, micro-Raman meas urements have been carried in a Renishaw 1000 Raman spectrometer using an argon laser (514. 5 nm) at a laser power of 24.8 mW and a spot size of 1 m. The carbon based materials are chemically bonded in sp, sp2 or sp3 states depending on the type of structure. It is well known that the chemical bonding of

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49 graphite and diamond are predominantly sp2 and sp3 respectively [81, 82]. As Raman spectroscopy depends on the long-range order of atoms and also due to the wide range Raman scattering for the sp2 bonded carbon to sp3 carbon, quantitative information cannot be obtained [86]. The typical Raman spectrum of a single crystal diamond, highl y ordered pyrolytic graphite (HOPG), microcrystalline diamond and nanocrystalline diamond are shown in Figure 3.12 (a-d) respectively. It is we ll known that the Raman spectrum of a single crystal diamond is characterized with a sharp single band at 1332 cm-1 corresponding to the sp3 hybridization of diamond. The Raman spectrum of graphite has two bands representing the dis-ordered carbon (D-band) and graphitic carbon (G-band) at 1350 cm-1 and 1580 cm-1 respectively. Both the ba nds are the signatures of sp2 bonded carbon. A highly ordered pyrolytic graphite (HOPG) consists of hexagonal sp2 network. The Raman signatures of a microcrystalline diamond ha s a sharp well distin ct feature at 1332 cm-1 and a mild background at ~ 1480 – 1550 cm-1, representing the sp3 and sp2 characteristics respectively (Figure 3.12 (c)). As MCD consists of large grains, the intensity of the peak at 1332 cm-1 is predominant. However, in NCD, as the grain size decreases, a completely different Raman characteristic is observed.

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50 Figure 3.12 Raman Spectra of Carbon Based Materials Visible Raman being more sensitive (~ order of 50 times) to the sp2 bonded carbon than is for the sp3, results in a broader Raman scattering for the former [83]. Hence, the spectra for the NCD films predominantly show the scattering due to sp2 carbon, even though the majority of the film is sp3 bonded [84]. Figure 3.12 (d) shows a typical Raman spectrum of NCD film grown in presence of 1% hydrogen in the gas chemistry. The

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51 Raman spectra of NCD films consist of three features: D-band, G-band, and a feature at 1140 cm-1 indicating the presence of trans-polyacetylene (TPA) states [87-89]. 3.11. Near Edge X-ray Absorption Fine Structure (NEXAFS) Spectroscopy Near edge x-ray absorption fi ne structure technique was de veloped in 1980’s to study the surface chemistry of lowZ molecules. This is a powerful technique to study the nature of chemical bonding in molecules such as hydrogen, carbon, nitrogen and oxygen. NEXAFS specifically selects the atomic species through its K -edge and probes its bonds to intramolecular and surface atoms. It can efficiently detect the existence of various bonds such as hydrocarbons (C-H), C-C, C=C and C C bonds. It provides information related to the inter-molecular bond length, orientation, and f unctional groups attached to the surfaces. The NEXAFS spectra of carbon base d materials are dominated by and resonances [85]. The measurements are made in two modes: 1) Total el ectron yield (TEY) mode and 2) Photon yield (PY) mode. It is based on th e principle of absorption of an x-ray photon by a core level atom and a corresponding emi ssion of a photoelectron. The resulting core hole is filled with an electron from a different shell or through an Auger emission process. In TEY mode, the current is monitored thr ough an ammeter while in the PY mode the photon count is monitored. Typical NEXAFS spectra of a single-crysta l diamond and a HOPG reference sample are shown in Figure 3.13. The NEXAFS spectra of HOPG sample is characterized by a peak at 284.5 eV corresponding to resonance. A single crystal diamond is characterized by

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52 a peak at 289.5 eV corresponding to resonance and a second order band gap of diamond at 302.5 eV. Figure 3.13 NEXAFS Spectra of Single Crys tal Diamond and HOPG Reference Sample [113] Figure 3.14 shows the HERMON beam line at the Syncrotron Raidation Center, University of Wisconsin, Madison and the test chamber [117] for conducting the NEXAFS measurements.

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53 Figure 3.14 HERMON Beam Line at the Un iversity of Wisconsin Madison [117] 3.12. Micro-Structural Analysis of Intrinsic NCD Films The top-view and the cross-sectional SEM im ages of the NCD films are shown in Figure 3.15 (a) and (b), respectively. The medium and high-resolution SEM images are shown in Figure 3.15 (c) and (d) respectively, where the surface morphology and the grain distribution can be noticed. The top-view SE M micrographs show that the surface of the film is uniform and continuous without any di sparities. The SEM images taken at several locations on a 4 wafer showed similar surface morphol ogy. The growth rate of the films deposited using standard conditions (CH40.5%, H21%, Ar98.5%, total gas flow800 sccm, M. W. power1.8 kW, pressure135 Torr, substrate temperature750 C, deposition time-3 hrs) was approximately 0.17-0.2 m/hr. Such a low growth rate is reported on all NCD films grown using same gas chemistry in similar CVD reactors.

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54 Figure 3.15 SEM Micrograph of Nanocrystal line Diamond Film Deposited Using 1% Hydrogen in the Gas Chemistry (a) Low-Re solution (b) Cross-Section (c) MediumResolution (d) High-Resolution The average surface roughness (Ra) of the films was estimated by atomic force microscopy (AFM). Figure 3.16 shows the AFM micrograph on a 5 X 5 scan size, where the average surface roughness (Ra) was found to be ~ 12 nm. The surface roughness measured at several locations on a 4 sample area was consistent. On a 25 X 25 scan size, the average surface roughness was below ~ 18 nm. (b) (c) (d)

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55 Figure 3.16 AFM Micrograph of Nanocrystal line Diamond Film Deposited Using 1% Hydrogen in the Gas Chemistry In order to gain understanding of the grain st ructure at atomic level, the transmission electron microscopy (TEM) studies were c onducted. TEM samples were prepared by focused ion-beam (FIB) technique. Figure 3.17 shows the TEM image of intrinsic NCD sample. The electron diffraction pattern is shown in Figure 3.17 (a). The diffraction spectra shows that the film is polycrystalli ne in nature with (111), (220) and (311) reflections corresponding to the diamond cr ystal lattice. The low-resolution image (Figure 3.17 (b)) shows dark and bright re gions on the sample i ndicating the contrast between the grains and the gr ain boundaries. The average grai n size was estimated to be ~10 nm. The variation in the grain size wa s between 10-15 nm. The high-resolution image taken at the same spot clearly s hows the atomic planes of the carbon.

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56 Figure 3.17 TEM Micrographs (a) Electron-Di ffraction Pattern (b) Low-Resolution TEM Image (c) High-Resolution TEM Image 3.13. Nitrogen Doping of Nanocrystalline Diamond Films The intrinsic diamond films are electrically insulating with resistivity on the order of 10131016 ohm.cm. Electrical conductivity can be achieved by doping the films during the deposition. The diffusion of dopants into the diamond films is not a practical method of doping as the surface is not diffusive to most of the impurities. Though there are few reports on the ion-implantation of diamond film s [90], it is an expensive technique and can damage the surface. Therefore, dopa nts such as boron (p-type), nitrogen, 111 220 311

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57 phosphorous and sulphur (n-type) are incorporated in the gas chemistry during the growth [87]. The most widely used dopa nts are boron (ptype ) and nitrogen (n-t ype), as these are readily soluble with diamond. It was observed that the quality of the films improve with the incorporation of trace am ounts of boron by reducing the point defects. On the other hand, excess concentration of boron promotes graphitization due to the incorporation of boron interstitial sites. Incase of single crys tal or microcrystalline diamond, p-type conductivity can be easily achieve d. But, it is difficult to obtain n-type conductivity at room temperature in these film s as nitrogen forms a deep donor (~1.7 eV). On the other hand, nitrogen forms a shallow donor level (~ 0.4 eV) [91] in NCD/UNCD films and results in high n-ty pe conductivity (~143 -1 cm-1). The energy band diagrams of nitrogen doped single crystal diamond and NCD is shown in Figure 3.18. Figure 3.18 Energy Band Diagram of Nitrogen Doped Single Crystal Diamond and Nanocrystalline Diamond The electron affinity ( ) in both single crystal diam ond and NCD is ~ -0.7 eV. As nitrogen forms a donor level at 1.7 eV in single crystal diamond, the resulting work function ( ) is ~ 1 eV. On the other hand, nitrog en in NCD forms a shallow donor level

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58 at 0.4 eV and therefore results in a small and a negative wo rk function. The conduction in the NCD films is via grain boundaries by hopping mechanism [91] The electrically conductive NCD films have gained special interest in field emission, high power devices and diamond electrodes for biosensing applications [91-93]. In the current research, nitrogen incorporat ed NCD (n-NCD) films have been grown on n-Si (100) substrates using CH4/Ar/N2 gas chemistry. The optimized process parameters are: CH41%, Ar79%, N220%, total gas flow200 sccm power800 watts, pressure100 Torr, and substrate temperature750 C. For a deposition time of 3 hours, the thickness of the films was estimated to be approximately 2.7 m. The electrical contacts were formed by sputtering and patterning a bi-l ayer of Ti/Au or Cr/Au on n-NCD films. The room temperature electr ical conductivity measurements were made in plane by Jandel manual multi-position four-point probe tool. The electrical conductivity (measured at 25 C) of these films deposit ed under standard conditions was found to be approximately 100 (ohm.cm)-1. 3.14. In-situ Plasma Diagnostics of NCD F ilms by Optical Emission Spectroscopy Optical emission spectroscopy (OES) provides information about the chemical species present in the plasma environment. The relative intensities of the species in the emission spectra suggest the dominant chemical constitu ents present in the gas chemistry. It has been mentioned earlier that carbon dimers (C2) are the prominent chemical species in hydrogen poor CH4/Ar gas chemistries. The color of th e plasma is intense lilac green due

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59 to the excitation of the argon gas. As th e emission spectra of different chemical constituents vary, this technique is useful to detect any impurities present in the reaction chamber. Moreover, significant changes in the spectra can be observed if impurities or dopants are intentionally in troduced during the growth. The OES spectra of CH4/Ar/H2 and CH4/Ar/N2 plasma chemistries are shown in Fi gure 3.19 (a) and (b) respectively. Figure 3.19 Optical Emission Spectra of Plasma Chemistry (a) CH4/Ar/H2 (b) CH4/Ar/N2 (a) + * (b) + ** +

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60 All the peaks (466, 516 and 556 nm) in Figure 3.19 (a) correspond to the C2 bands, indicating the high concentration of these sp ecies [94]. A trace am ount of nitrogen is present in the gas ambience. Hence, a small feature at 388 nm indicating the presence of CN species is observed. When 20% nitrogen (Figure 3.19 (b)) is in troduced in the gas chemistry, plasma turns milky white and the intensity of the CN peak increases by several orders, as compared to othe r peaks. The increase in the CN/C2 ratio affects the structural morphology and the electri cal properties of n-NCD films. 3.15. Structural Analysis of Nitrogen In corporated Nanocrystalline Diamond Films A systematic study has been conducted by first growing nitrogen incorporated NCD films with varying nitrogen compositions between 5% and 40% in the total gas mixture. The room temperature electrical conduc tivities of all the films have been measured and it was observed that the highest electr ical conductivity of 100 (ohm.cm)-1 was achieved in the films grown using 20% nitrogen in the gas chemistry. Therefore, further study was conducted only on those films. The below sec tion presents AFM, SEM, TEM, and EDAX analysis of nitrogen incorporated NCD films. Figure 3.20 shows the AFM image of nNCD film. The average surface roughness on a 5 m X 5 m scan size was found to be ~ 12 nm. Such a smooth surface indicates the un iform growth of the NCD film deposited by the present technique.

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61 Figure 3.20 AF M Micrograph of n-NCD Film The top-view and cross-section SEM microgr aphs of n-NCD films are shown in Figure 3.21 (a) and (b), respectively. It can be obser ved from the top-view image that the surface of the film is uniform and homogenous. After 15 hour deposition, the thickness of the film was found to be ~ 13.8 m. It was found that the growth rate of the nanodiamond increased from 0.2 m/hr to 0.9 m/hr when 20% nitrogen is added to the gas composition. The CN species produced by the addition of nitrogen do not actively contribute in the growth; instead, they redu ce the heterogeneous secondary renucleation rate, and increase the grain size and the growth rate [95].

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62 Figure 3.21 SEM Micrograph of n-NCD Fi lm (a) Top-View (b) Cross-Section The TEM analysis was conducted on n-NCD f ilms for determining the grain size and crystal orientation. The TEM specimens of thickness ~ 100-120 nm were prepared by FIB milling. The high-resolution images of the sample are shown in Figure 3.22 (a-c) respectively. The dark regions of the diamond area in Figure 3.22 (a) indicate the grains, and the bright regions indicat e the grain boundaries. It can be noticed from Figure 3.22 (a) (b)

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63 (a) that the interface between the underlying silicon subs trate and the diamond film is ~ 10 nm. From the high-resolution images, the grain size was estimated to be approximately on the order of 15-20 nm. Figure 3.22 TEM Micrograph of Nitrogen Inco rporated Nanocrystalline Diamond Film and Interface Between Silic on Substrate and Diamond Film (a) Low-Resolution (b) HighResolution (c) High-Resolution Showing the Nanocrystalline Diamond Grains

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64 The electron diffraction pattern at different regions on the sample, including silicon, diamond, and the interface between silicon and diamond areas are shown in Figure 3.23 (a-d) respectively. Figure 3.23 Electron-Diffracti on Pattern (a) Silicon Subs trate (b) Nanocrystalline Diamond Film (c) Interface Between S ilicon and Diamond (d) Toward the Diamond Surface Near the Interface

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65 The well defined point reflections in Figure 3.23 (a) indicate the planes of silicon lattice. On the other hand, in Figure 3.23 (b) the circ ular pattern with point reflections on the Debye fringes indicate the polycrystalline na ture of the diamond film. The first two fringes from the center of the electron beam represent the (111) and (220) planes of the diamond crystal. Figure 3.23 (c) shows the electron diffraction pattern of the interface between silicon and diamond, where the point re flections of the silicon and the circular patterns of the diamond can be observed. Fi gure 3.23 (d) shows the electron diffraction pattern of the diamond areas away from the in terface, hence well defined circular fringes can be observed prominently. Figure 3.24 (a) shows the cross-section s canning profile image of the TEM sample. Figure 3.24 (b) and (c) show th e line scan images of silicon and carbon spectra. In the EDAX spectra of Figure 3.24 (d), the signa tures of elemental s ilicon and carbon were observed. All these results show that by the addition of nitrogen, the crystalline nature of the NCD is not damaged and the films cons ist of nanodiamond grains with an average grain size ~ 20 nm.

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66 Figure 3.24 Line-Scan Profile Images (a) Cros s-Section of the TEM Sample (b) Silicon (c) Carbon (d) EDAX Spectra Show ing Silicon and Carbon Peaks In this section, the visible Raman, NEXAFS and XRD results of the nitrogen incorporated NCD films are disc ussed. For comparative purposes, the structural analysis of pure NCD film (0% N) is also discusse d. The Raman and the NEXAFS spectra of the samples are shown in Figure 3.25 (a) and (b ) respectively. The Raman spectra of both intrinsic and conductive NCD f ilms show three important features. The assignment of each of the features has been di scussed earlier. It can be obse rved that when nitrogen is introduced in the gas chemistry, a significant change in the G-band can be observed. As the sp2 content in the films determine the mech anical and electrica l properties, it is important to estimate the amount of sp2 present.

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67 Figure 3.25 (a) Raman Spectra and (b) NEXAF S Spectra of Nanocrystalline Diamond Films Grown Using 0% and 20% Nitrogen in the Gas Chemistry In the NEXAFS spectra, three features, corresponding to *, and the second order band gap of diamond, can be observed at ~ 284.7 eV, ~289 eV and ~302 eV, respectively. The and features correspond to the sp2 and sp3 bonded carbons, respectively. From the NEXAFS spectra of intrinsic films, a sh arp and well defined peak corresponding to bonding can be observed. The peak is weak and the FWHM of second-order band gap of diamond is narrow. With the addition of 20% nitrogen in th e gas chemistry, the area under the peak increased, peak is not as distinct as intrinsic diamond film, and FWHM of the second order band gap increas ed. These changes in the NEXAFS spectra suggest that the sp2 content increases with the add ition of nitrogen. The fraction of sp2/sp3 was estimated by using the following expression. E I E I I ISAMPLE REFERENCE REFERENCE SAMPLE spF * 2 (3.2) (a) (b)

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68 where 2 spFrepresents the fraction of sp2 in the films,*SAMPLEI, *REFERENCEI represent the areas of the peak of the sample and the HOPG reference. E IREFERENCE and E ISAMPLE represent the remaining area under the spectrum that is contributed by the sp3 bonded carbon [68]. It was estimated that th e films grown without any nitrogen have approximately 7% sp2 and this fraction has increased to ~ 14% when 20% nitrogen was added in the total gas mixture. Therefore, the nitrogen doped NCD films consists of a considerable fraction (~86%) of sp3 bonded carbon. With the addition of the deliberate amount of sp2, the electrical conductivity of thes e films increases by four orders. The x-ray diffraction spectra of pure and nitrogen incorporat ed NCD films are shown in Figure 3.26. It can be observed that the spectra of both the films represent the diffraction lines of (111), (220), and (311) peaks resp ectively. The strong match in the diffraction data of both the films furthe r support that by the addition of nitrogen, the damage incurred to the crystal latti ce of the diamond is minimal. Table 3.4 and Table 3.5 shows the diffraction data of undoped and nitrogen incorporated nanocrystalline diamond films respectively.

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69 Figure 3.26 X-ray Diffraction Spectra of Undoped and Nitrogen Incorporated Nancorystalline Diamond Films Table 3.4 XRD Data of Undoped Nanocrystalline Diamond Films Peak Position(2 )Counts d-spacing() (111) 43.9929 32.8 2.058 (220) 75.3532 55.72 1.26 (311) 91.5874 2.95 1.15

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70 Table 3.5 XRD Data of Nitrogen Doped Nanocrystalline Diamond Films Peak Position(2 )Counts d-spacing() (111) 43.5068 39.83 2.08017 (220) 75.592 21.75 1.25691 (311) 91.5674 1.63 1.13 3.16. Electrical Contacts to Nitrogen Incorporated Nanocrystalline Diamond Films The electrical propert ies of nitrogen incorporated NCD films were measured by forming suitable electrical contacts to the films. A bi-layer of Ti/Au film s were deposited by sputtering and later patterned to form metal electrodes on n-NCD films. The currentvoltage properties were measur ed by making electrical contacts to the front and back side of the dice. Figure 3.27 shows the currentvoltage response when the bias voltage was varied from -8 V to + 8 V. The strong linea r I-V characteristics i ndicate that the high quality ohmic contacts were formed on these fi lms [112]. It has been reported that Ti/Au metal electrodes on the boron doped polycrysta lline diamond films failed to form ohmic contacts without any post depos ition annealing due to the ab sence of the formation of a carbide layer [96]. Ohmic contacts to phosphor ous doped films could not be formed even after annealing the Ti metal contacts; on the c ontrary they have shown rectification [97].

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71Before Hydrogen Plasma Treatment Figure 3.27 Current-Voltage Characteristics of Nitrogen Incorporated Nanocrystalline Diamond Films In this research, low resistive ohmic c ontacts could be formed easily on conductive nitrogen doped NCD films, ev en before annealing. The a nnealing was done to understand the effect of temperature on the electrical cont acts and the contact resistivity. It suggests that the conduction mechanism in the nitrogen and phosphorus doped diamond films is different. The introduction of nitrogen in the diamond films increases the defect density

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72 of states in the Fermi level [98]. It can be st ated that the defects in these films play an important role in the form ation of ohmic contacts. To further study the role of defects on the type of electrical contac t and the electrical conductivity, the as-gro wn samples were exposed to hydrogen plasma for about 30 minutes prior to the deposition of metal elect rodes. It is well know n that the surface conductivity of polycrystalline diamond films can be increased by the hydrogen plasma treatment due to “surface transfer doping” mechanism [113, 92, 93, 114]. As polycrystalline diamond films have large gr ains, higher amount of hydrogen termination and consecutively a greater reduc tion in the electrical resistivity can be achieved by the plasma treatment. But, the electrical propertie s of NCD films are diffe rent as they have small grains and tight grain boundaries. In f act, it has been reporte d that the electrical conductivity of pure NCD film s has changed only from 10-6 -1cm-1 to 10-8 -1cm-1 after hydrogenation [114]. As the elect rical conductivity of the n-t ype NCD films is expected to be due to sp2 bonded carbon, defects and other impurities [91, 115], the surface conductivity induced by the pl asma treatment may be diff erent. The hydrogen plasma treatment is also known to re duce the electron-affinity of the diamond surfaces; thereby the electron emission characteristics can be im proved. As one of the applications of the conductive NCD films in this research is to fabricate NCD based field emission devices, therefore, this experimental study is important to know if th e field emission properties of nitrogen incorporated NCD films could be improved. Figure 3.28 shows the top-view

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73 SEM of the as-grown (20% N doped) NCD film s before and after they were exposed to the hydrogen plasma. Figure 3.28 Surface Morphology of Nitrogen In corporated Nanocrystalline Diamond Film (a) as-grown (b) Post Hydrogen Plasma Treated Surface (Low-R esolution) (c) Post Hydrogen Plasma Treated Surface (High-Resolution) A “cauliflower-like” structure was observed on the as-grown sample (Figure 3.28 (a)), which later disappeared after a subsequent exposure to the hydrogen plasma (Figure 3.28 (b)). After the hydrogen plasma treatment, th e crystals of the nanodiamond on the surface of the film were revealed due to the etching of a part of sp2 bonded surface carbons. The AFM images of the as-grown and hydrogen plasma treated n-NCD films is shown in Figure 3.29 (a) and (b) respectively. The av erage surface roughness of the as-grown films

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74 was before and after the hydrogen plasma tr eatment was found to be ~ 13 nm and ~ 24 nm respectively. The increased surface roughness is due to the uneven surface morphology, resulting from etch ing of the surface carbon (sp2). Figure 3.29 AFM Image of Nitrogen Incorporat ed Nanocrystalline Diamond Film (a) asgrown (b) After the Hydrogen Plasma Treatment Figure 3.30 shows the visible Raman spec tra of both as-grown and hydrogen plasma treated nitrogen incorporated NCD films. The signatures of the Raman bands are identical in both the curves. The important features in the spectra are at ~1145 cm-1, 1348 cm-1, 1486 cm-1 and 1564 cm-1, respectively. The features at ~1145 cm-1 and ~1480 cm-1 indicate the presence of trans-polyacetylene states in the grain boundaries of NCD films [108]. In the as-grown film s, the shoulder at ~ 1486 cm-1 is more predominant than the plasma treated ones. The amount of TPA states present in the film s is related to the presence of hydrogen [109]. As TPA states are unstable at high temperatures, these features disappear when the films are heated to ~1200 C [110]. But in this study, the

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75 films were exposed to the hydrogen plasma at relatively lower temperatures (~700C750C). Therefore, the TPA states have not comp letely disappeared; inst ead, their relative intensities only decreased. Figure 3.30 Raman Spectra of as-grown and Hydrogen Plasma Treated Films The shift in the position of the G-band (1564 cm-1) to a higher wave number (1577 cm-1) suggests the possibility of changes in the bonding configuration of the sp2 bonded carbon. It can also be observed that after the plas ma treatment, the relative intensity of the Gband decreased than D-band. We have also observed from the NEXAFS measurements that the sp2 content in the films reduced after the plasma treatment. These changes in the bonding characteristics of the plasma-treated films have shown significant influence on the electrical properties of these films. In Figure 3.31, the I-V response of nitrogen doped

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76 NCD films after they were subjected to the hydrogen plasma and subsequent metal contact annealing is shown. After Hydrogen Plasma Treatment Figure 3.31 Current-Voltage Characteristics of Nitrogen Incorporated Nanocrystalline Diamond Film After the Hydrogen Plasma Treatment The type of electri cal contact formed between a meta l and semiconductor depends on the properties of the interface. In all the cases, it can be observed that Ti/Au, which originally showed linear I-V characteristics on the as-gro wn films, changed their behavior after the plasma treatment. The same metal contacts do not form a true ohmic or a Schottky junction. Further, the post deposition anneal ing (PDA) of these cont acts has not improved

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77 the linearity in their current-voltage respons e. It is known that the hydrogen treatment passivates the surface and compensates the de fects present in a semiconductor. In a similar way, exposure of the diamond films to the hydrogen plasma could passivate the defects containing graphitic car bon and alter the electr ical properties of these films. It has been reported that at room temperature, hydr ogenation of HPHT synthetic type IIA (001) diamond substrates created a defective surface [ 93]. But, in this study, the samples were exposed to the plasma at temperatures (~700-750 C) which removed the graphitic content [99]. It suggests that the formation of ohmic contacts in thes e films is definitely due to the presence of defects and graphitic carbon. It further suppor ts that th e electrical conduction in the films is through the def ects in the grain boundaries [100]. 3.17. Summary With a brief introduction on the nucleation, growth of intrinsic and conductive NCD films, and growth mechanism of the nanoc rystalline diamond films, this section has discussed the MPECVD growth reactor used for this research, along with various experiments performed to study the nucleation and optimization of the process chemistry for the growth of both intrin sic and nitrogen incorporated NCD films. The structural properties of both intrinsic and conductive NC D films have been evaluated. Several analytical techniques have been used to ch aracterize the films and optimize the growth chemistry. The electrical properties of th e conductive NCD films have been evaluated by forming suitable ohmic contacts and the e ffect of the surface plasma treatment on the nature of the electrical co ntacts have been discussed.

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78CHAPTER 4. FIELD EMITTER DEVICE FABRICATION APPROACH 4.1. Introduction The current research is aimed at the app lication of nanocrystal line diamond films as a structural material for the fabrication of micro-electromechanical system (MEMS) devices and sensors. In spite of the extrao rdinary properties of diamond films, only a few diamond based MEMS applications have been reported due to the limitations in micromachining of complex MEMS structures The fabrication and testing of MEMS devices such as cantilevers, and beams etc, have been reported [53, 101]. Other devices such as resonators, accelerometers and pressu re sensors [102] have been fabricated as well. The inability to successfully grow n-type diamond films has limited the MEMS applications to use only intrinsic NCD or boron doped polycryst alline diamond [103]. The capacitive type pressure sensors and accelerometers using intrinsic diamond films were also under investigation. Though bot h capacitive [43] and piezo-resistive mechanisms [44] have drawn much attention due to their potential advantages, there are certain limitations with the current sensing technologies that should be paid attention.

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79 Table 4.1 summarizes some of the fundamental differenc es and limitations with the capacitive and piezoresistive sensing mechanisms. Table 4.1 Comparison Between Piezoresistive and Cap acitive Sensing Mechanisms Piezoresistive sensing Capacitive sensing Good linearity More sensitive Low temperature sensitivity Highly accurate and repeatable Consumes less power Highly prone to temp erature instability Requires on-chip complex circuitry Due to the above limitations, other sensing m echanisms have attracted. Field emission based detection can be an alternative and e ffective method for sensing in terms of its temperature insensitivity, minimal resonance effects, and possibility to integrate with microelectronic circuits. The negative electron affinity (N EA) nature of diamond is an important property for its use in the both field and thermion ic emission sources [62]. In addition to its NEA, the high thermal conduc tivity, robustness and chemical inertness play a significant role in enhancing the performance of diamond based devices. Both vertical and lateral field emission devices were fabricated using nitrogen doped NCD films [42, 51]. Field emission based magnetic sensors have been dem onstrated previously [104]. The proposed work is aimed to develo p vertical field emission devices using nitrogen doped NCD films as a structural material. Both free st anding and capped field

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80 emission devices are fabricated and tested. The capped field emission source can act as a microelectronic field emission sensor. 4.2. Fabrication of Vertical Fiel d Emission Source and Sensor The processing steps involved in the fabricatio n of a vertical field emission sensor are shown in Figure 4.1. Figure 4.1 Process Steps for the Fabricat ion of Vertical Field Emission Sensor

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81 Figure 4.1 (Continued) The first step involves patterning and etching the silicon (100) star ter wafer in KOH to form the patterns for the emitter tips, followed by the deposition of a layer of conductive NCD film. A blanket film of metal is deposit ed on NCD as an intermediate metal layer for thermo-compression bonding. After bonding, the silicon mold is etched back to obtain free standing diamond emitters. This completes the fabrication of free standing field emitter array, which forms the cathode in th e emission source. To fabricate the capped device, the next step is to form an anode on top of the cathode for collecting the electrons. Several techniques, such as self-aligned gates [42], silicon-on-insulator (SOI) wafers [105] etc., for the formation of gated structur es have been reported. However, most of

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82 these methods involve complex fabricati on steps due to the limitations of the photolithography. Fabricating the anode independently and inte grating it with the cathode array is another technique to form the capped devices. In order to accomplish this, a nitride layer is thermally grown on another silicon wafer and later patterned to open square windows and etched to form a thin membrane. This wafer containing membranes is bonded to th e bottom wafer to form a complete field emission structure. Both free standing and cap ped field emission devices are tested for their emission characteristics in a high vacuum test chamber. By finite element modeling and simulations using CoventorwareTM, the deflection of the membranes are determined as they are subjected to external load. This da ta is useful to determine the inter-electrode separation and safe operati ng range of the sensor. 4.3. Testing of Field Emission Source and Sensor The field emission testing of the devices is conduc ted in an ultrahigh vacuum test chamber equipped with high voltage feed-thr oughs for electrical pr obing. The substrate holder is custom made with a conductive metal plate through which the emitter is made contact and the collector is connected to anothe r thin metal plate isol ated from the emitter plate. Both free standing and capped field emission devices are tested for their field emission characteristics in diode configuration after the test chamber is pumped-down to a base pressure of 5 X 10-8 Torr. In free standing devices, the emitted electrons are collected at the metal plate. In the capped devi ces, the field electrons are collected at the

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83 membrane which forms the anode. A sufficien tly high voltage is applied to the cathode for the emission to initiate, and the emi ssion current is recorded. Once the emission current is stabilized, the voltage is increased gradually and the corresponding emission current is measured. A low turn-on voltage and a stable emission current indicate desirable field emission propert ies. Once the field emission s ource is optimized to operate under stable conditions, the emission current at any instance should be constant at a constant applied voltage. At this instance, the membrane can be deflected. The variation in the field emission current can provide an es timate of the deflection which in turn can be correlated to the applied pressure. This type of pressure or a vibra tion sensor will be a proof of concept for detecting the changes in the vacuum level. 4.4. Summary To summarize this section, the principle of field emission based sensing has been discussed along with the fabr ication details of both free standing and capped field emission sources. Method of testing the field emitter device in a high vacuum environment using diode configuration is men tioned. The research on the vertical field emitter array acting as a cathode with a thin silicon membrane as an anode is proposed. The possibility of testing the capped field em ission source as a sensor is discussed.

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84CHAPTER 5. DESIGN AND ANALYSIS OF VERTICAL FIELD EMITTER DEVICES 5.1. Introduction A diode model of a field emission source/sensor consists of an array of emitter tips as cathode and an electron collector that acts as an anode. In capped field emission devices, the anode is made of silicon membranes which can function as a mechanical element (for deflection) in the sensor. In this section, the design of both the components is discussed and the finite element modeling and simula tions were conducted us ing MEMS simulation software (CoventorwareTM). 5.2. Design of Diamond Field Emitter Array The process flow for the fabrication of the field emitter array was shown in Figure 4.1. The details of the featur es on the emitter mask are listed in Table 5.1. The mask consisted of arrays of circular patter ns with varying radii from 2 m to 5 m. The length of the trapezoidal base is equal to the diameter of th e circular pattern; the height (L) of the tip can be calculated from the base width (W) by the expression W= 2 X L. Therefore, with the existing mask set, the height of the tips vary between 5.6 m and 14 m as the base

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85 width changes from 4 m to 10 m respectively. The dimensions of the field emitter dice are listed in Table 5.2. Table 5.1 Design Specifications of the Field Emitter Mask Size(m) Pitch 4 5 6 7 8 10 Array # 15 11 17 21 25 2C(Pitch-20) 15 Array # 35 12 1C 22 27 57(Pitch-20) 15 Array # 75 13 37 51 55 15 Array # 31 3C 52 15 Array # 32 15 Array # 33 15 Array # 71 15 Array # 72 15 Array # 73 15 Table 5.2 Design Specification of the Cathode Cell Area ( m2) A 5000 X 5000 B 7000 X 7000 C 10000 X 10000

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86 A 3D solid model generated in CoventorwareTM (not to the scale) of a single cell demonstrating the field emitter array is illustrated in Figure 5.1. Figure 5.1 3D Model of a Free Standing Vertical Field Emitter Array Bonded to a Carrier Wafer A 3D solid model representing the capped fiel d emission device is shown Figure 5.2. The mechanical simulations have been conduc ted by parametric study in the MEMmech module of the CoventorwareTM. Figure 5.2 3D Model of a Capped Field Emitter Device

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875.3. Design and Analysis of Anode-onMembrane Using Generation I Mask Anode of the field emission sensor is fabricat ed using thin silicon membranes flexible to external pressure and/or acceleration. Displacement of the anode depends on the magnitude of the applied pressure, and the dimensions of the membrane. As per the dimensions of the cathode (listed in Table 5. 2), the anodes were designed to have square patterns of dimensions listed in Table 5.3. Table 5.3 Design Specification of th e Anode in the Generation I Mask Cell Area ( m2) D 8000 X 8000 E 10000 X 10000 F 12000 X 12000 G 14000 X 14000 The dimensions of the square patterns have been chosen to account for the undercut in the Si (100) plane during the anisotropic KOH etch. The boundary conditions applied for conducting the simulations on the diaphragms include fixing the deflection of the diaphragm frame along x, y and z directions while the load was applied gradually on the top face. For a fixed length and breadth, the effect of membrane thickness on the deflection was estimated to be a function of pressure. The thickness of the membranes was varied from 10 m to 40 m.

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88 As the dimensions of the membranes were on the order of few millimeters, the deflection of the membranes was large and the respons e was non-linear when the pressure exceeded 0.2 MPa. Taking into account the processi ng limitations, loads exceeding 0.2 MPa cannot be exerted on these membranes. By using th e design I masks, the anode and the cathode can be fabricated with a maximum separation of 260-270 m depending up on the thickness of the diaphragm. To be realistic, if the deflection exceeds a safe value of 250 m, there is a high probability for the anode and cathode to form a closed circuit and result in the device failure. Therefore, consid ering a 3% variation in the thickness of the membranes resulting from wet etching, th e maximum deflection was limited to 250 m. Thinner and larger membranes deflect more than 250 m, and so pressures beyond 0.17 MPa (for cell # E, F, and G) cannot be ap plied. Devices with thinner and larger membranes as anode can be more sensitive to the changes in the pressure while the thicker and smaller membranes can be useful to increase the operating range. Figure 5.3 shows the effect of membrane dimens ions on the deflection as an external load up to 0.25 MPa was applied gradually. Thou gh simulations have been conducted on membranes with thickness between 10 m to 40 m, the anode of the final device was fabricated with a 20 m thick silicon membra ne. Therefore, the simulation results of only those membranes are documented here.

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89 Figure 5.3 Effect of Membrane Dimensions on the Deflection of a 20 m Thick Silicon Diaphragm From the above graph, it can be noticed th at devices with largest membranes as anodes can be operated below 0.15 MPa while pressu res exceeding 0.2 MPa can be exerted on the smallest membranes (cell # D). Due to the above limitations, the devices fabricated using the generation I mask can function only as low pressure field emission sensors. In order to increase the operating range, s econd generation masks which include a boss structure in the center were designed. The in corporation of the center mass increases the net weight of the membrane thereby increases the measurement range.

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905.4. Design and Analysis of Anode-onMembrane Using Generation II Mask The second generation design included a center boss structure with the membrane acting as four suspended springs. The first layer c onsists of the generation I mask, which defines the complete areas of the anode, and the second layer in the mask was used to define the regions of the center boss and form the membranes. The dimensions of the center boss and the outer membrane are listed in Table 5. 4. A shallow etch after the exposure of the first mask determines the separation between the emitter tips and the anode. A deep etch followed after the exposure of the sec ond mask determines the thickness of the membranes. A snapshot of the photo-masks used in the generation II membrane design is shown in Figure 5.4 (a) and (b) respectively. Figure 5.4 2D Image of Generation II Ma sk for the Fabrication of Membranes

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91 Table 5.4 Design Specifications of the A node and Proof-Mass in Generation II Mask Cell Area of Proof-mass( m2) Total Area ( m2) D 5000 X 5000 8000 X 8000 E 7000 X 7000 10000 X 10000 F 9000 X 9000 12000 X 12000 A snap shot of the 3D solid model showing one individual cell is shown in Figure 5.5. Figure 5.5 Snapshot of 3D Solid Model of a Single-Cell Using Generation II Mask The simulations for the second generation membranes were carried with identical boundary conditions as the first generation. The depth of the shallow etch was approximately 50 m. Therefore, the maximum safe operating deflection was limited to 40 m. It can be observed that the linear rang e of operation is extened by incorporating a silicon boss structure within the membrane. As the anode and the cathode are seperated by ~ 50 m, if pressures exceeding 0. 8 MPa were exerted on the smaller membranes, the deflection exceeds the safe operating limits. Theref ore, these devices can operate only up Etched Cavity Proof mass

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92 to 0.8 MPa i.e., four times increase in the full-s cale range as compared to the first generation devices. Figure 5. 6 shows the effect of thic kness of the membranes on the deflection as the pressure was varied. Wh en the dimensions of the proof mass are increased and the net diaphragm area was ke pt constant, the operating range can be increased (Figure 5.6(b) and (c)). On the othe r hand, if the dimensions of the proof mass are kept constant and the total area of the dia phagm is increased, the converse appeared to be true (Figure 5.6 (a) and (b)). Figure 5.6 Effect of Membrane Thickness on the Deflection of Membrane (a) 5000 X 5000 m2_8000 X 8000 m2 (b) 5000 X 5000 m2_10000 X 10000 m2 (c) 7000 X 7000 m2_10000 X 10000 m2 (d) 7000 X 7000 m2_12000 X 12000 m2

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93 Figure 5.6 (Continued)

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94 Figure 5.6 (Continued) 5.5. Summary In this chapter, the design aspects of the field emitter arrays and the anode consisting of silicon membranes were discussed. The emitt er mask has features with base widths varying between 4 m and 10 m. The height of the tips varied between 5.6 to 14 m respectively. For the anode, two designs Generation I consisting of free standing membranes and Generation II consisting of free standing membranes with center bossed structure were presented. The simulation re sults and the mechanical analysis of the membranes were done using CoventorwareTM. The effect of thickne ss and length of the membrane edges on the deflection were studie d as a function of external pressure. The safe operating range of the devices was found by simulations. It was found that the

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95 operating range can be increased by at least f our times on some of the devices when the anode consists of boss structure.

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96CHAPTER 6. FABRICATION AND CHA RACTERIZATION OF FIELD EMITTER DEVICES 6.1. Introduction This chapter describes the process technique s and characterization methods implemented in the fabrication of a field emission source and a sensor. The first part of the chapter discusses the fabrication of a vertical field emitter array by mold technique. The second part of the chapter presents the process impl emented for the fabrication of anode array, and the third part details the integration of the cathode array and the anode to form a capped field emission device. Th e last part of the chapter provides the characterization results of the field emission devices. 6.2. Fabrication of Vertical Field Em itter Array by Mold Technique The process flow for the fabrication of a vertical field emitter array by mold technique is shown in Figure 6.1. Single-side polished Si (100) wafers (n-type, thickness: 500-550 m, resistivity: 5-10 ohm.cm) were used as substr ates for this application. Prior to the fabrication, the wafers were cleaned in 30:1 BOE to etch native oxide. A SiNx layer of thickness ~120 nm was grown by low-pressure chemical vapor deposition (LPCVD)

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97 method. The SiNx layer acts as a good hard mask for the subsequent KOH etch. A positive resist PR1 2000 was spun on the wafers, followed by a subsequent pre-bake at 120 C for 60 seconds. The photoresist ac ts as hard mask during the SiNx etch in a reactive-ion etcher (RIE) a nd the pattern on the silicon s ubstrate is exposed for the subsequent KOH etch. The Si (100) wafers are etched by anisot ropic KOH etch (30% KOH (269.5 ml) in DI water (229.5 ml), 80C, 450 rpm) to form the inverted pyramidal grooves with an angle of 54.7 betw een the (100) and (111) planes. Figure 6.1 Process Steps for the Fabrication of Vertical Field Emitter Array

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986.3. Challenges in the Processing of Vertical Field Emitter Devices Unlike the fabrication of lateral field emission devices, the fabrication of vertical field emission devices involves many processing step s. As the number of fabrication steps increase in a process, the corresponding yiel d will be low unless the process is extremely controlled. Anisotropic KOH etching is very widely implemented in most silicon MEMS fabrication methods. It was observed that afte r the KOH etch, the surface of the patterned pyramidal structures was covered with some pa rticulates that appear as “sugar-crystals”. The size of these particulates varied between 50-200 nm. The scanning electron micrograph of a pyramidal pattern after the KOH etch is shown in Figure 6.2. It was found that the source of contamination was not from the experimental set-up or the glassware but from the KOH pellets used fo r preparing the commercial KOH solution. A similar observation reported by another group st ated that the particulate precipitation occurs only after th e removal of the wafers from th e KOH bath and was found to be independent of the durat ion of etching [106]. It was confirmed by energy dispersive x-ray (E DAX) analysis that the crystals inside the etched cavities are indeed the precipitates of iron particles. The particles were present only in the etched cavities and not on the SiNx hard mask confirming that the source of the particles was not from the experimental set-up but instead the dissolved impurities in the KOH solution which adhere only to th e silicon surface. The area scan EDAX spectrum obtained is shown in Figure 6.3.

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99 Figure 6.2 SEM Micrograph of KOH Etched Inverted Pyramidal Mold Containing Iron Particulates Figure 6.3 EDAX Area Scan of the Pyrami dal Mold Containing Iron Particulates Iron crystals

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100 The EDAX spectrum represented the peaks corresponding to elements-Be, C, O, Fe, Cu and Si. The intensity of the peaks correspond ing to Be, C and Cu were negligible and their source could not be identified. On the other hand, the intensities of K, L, K and L lines corresponding to the elemental Fe and K line of elemental O were pronounced. The presence of this unwanted precipitat e in the pyramidal etched cavities had detrimental effects during the nucleation and the growth of diamond films. Consequently, after the KOH etch, the wafers were soaked in iron-oxide etchan t for about 15 minutes. The SEM micrograph of a sample after iron-oxide etching is shown below in Figure 6.4. Figure 6.4 SEM Micrograph of the V-groove After Iron Oxide Etch

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101 It can be noticed that the ir on-precipitates have been su ccessfully removed from the etched cavities. As mentioned earlier, th e presence of iron particles in the grooves resulted in several problems during the post wafer processing. It was observed that the presence of iron particles has resulted in a discontinuous film and rough morphology of the diamond tips after they were etched a nd released. The SEM image shows the rough morphology of the diamond tip with several voi ds due to the embedded iron particles (Figure 6.5). Figure 6.5 SEM Micrograph of a Diamond Py ramid With Pinholes Resulted From Iron Particulates in the Etched Cavities

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1026.4. Determination of Tip-Radius by Focused Ion Beam (FIB) It is important to determine the radius of th e emitter tip as it determines the geometrical field enhancement factor ( ). It is widely known that as the tip becomes sharper the factor is improved, thereby the field emission properties ca n be enhanced. In order to determine the tip radius, a single inverted pyramid in the pattern was milled by the focused ion beam. The sample preparation wa s made by depositing a layer of Pt followed by the milling or thinning down with Ga+ ions. During ion milling, as the sample thins down from the edge of a pattern to inside, th e depth of the mold gradually increases until the geometrical center plane of the square base is reached and then decreases with further milling. The actual depth and the radius of th e tip are obtained at the geometrical center plane. The scanning electron micrograph of a tip at the geometric center plane is shown in Figure 6.6. Figure 6.6 FIB Cross-Section Image of a V-groove in Silicon Substrate

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103 It was estimated that the tip radius in the patterned wafers was ~ 100 nm. The radii can be further reduced by sharpening the tips by growi ng an oxide layer in the etched cavities [107]. 6.5. Deposition of Nanocrystal line Diamond Films in the Inverted Pyramidal Molds It is known that that the KOH et ch of patterned Si (100) subs trates results in intersecting (111) planes with an angle of 54.7 betw een (100) and (111) pl anes. As high density nucleation is important to grow uniform films, it is essential to achieve high density nuclei along all the planes of the patterned substrate. The seeding is initiated by ultrasonic scratching in a mixture of titanium nanopowder and diamond nanopowder suspended in acetone. NCD films have been de posited for different durations-3 hours, 6 hours and 12 hours to determine the uniformity of the growth. The SEM micrograph of an emitter pattern filled with n-NCD film is shown in Figure 6.7. The low-resolution SEM image (Figure 6.7(a)) shows an array of individual emitter cell, while the highresolution SEM image (Figure 6.7(b)) shows the growth of the nanodiamond film in the etched cavity. It can be noticed that after a 6 hour deposition, the diamond film did not completely fill the silicon mold. For longer durations (samples grown for 12 hours), NCD films fill the pyramidal molds in the substr ate. Continuous and uniform film along the inner walls of the pyramid, i.e. along the (111) planes, indicat es the uniform nucleation in the cavity.

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104 Figure 6.7 SEM Micrograph of the Surface of Nitrogen Incorporated NCD Film Partially Filled in the Silicon Mold (a) Low-Resoluti on Image of the Field Emitter Pattern (b) High-Resolution Image of the NCD Film Deposited in a Single Inverted Pyramidal Cavity 6.6. Processing Issues-Chip-Level Bonding After depositing NCD films on the silicon substrates, the wafers are diced and the individual dice are bonded to a carrier wafer for etching the silicon mold, and thereby exposing the emitter tips. The chip-level bonding is performed by flip-chip method using anisotropic conductive film (A CF) and by gold-to-gold th ermo-compression bonding. Figure 6.8 shows the temperatur e-time profile used in this method of bonding. A bi-layer of Cr/Au or Ti/Au was deposited by sputtering onto Pyrex or silicon carrier wafers. Prior to the bonding, ACF was cured for 20 to 30 minut es at room temperature. The flip-chip bonding was carried by applying an external load of 80N at a temperature of 175C for 30 minutes. ACF melts under external heat a nd load to form a bond between the die and the carrier wafer.

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105 Figure 6.8 Time vs. Temperature Plot of Thermo-Compression Bonding Using Anisotropic Conductive Film The next step in the device fabrication is to etch the silicon mold and expose the emitter tips. The bulk etching of silicon was carried ei ther by HNA (mixture of Hydrofluoric acid, Nitric acid and water (instead of Acetic acid ) in 2:1:2 ratio) or by hot KOH (30% KOH in H2O, 80C). The choice of the wet etchant depended upon the type and size of carrier wafer, and the bi-metal stack. It was observed th at the etch rate of Si (100) planes in both KOH and HNA solution was approximately 1 m/min. In order to protect the edges of the carrier wafers from etching, the wafers were loaded into custom-made jigs (Figure 6.9) that can protect the back and the sides of the wafer dur ing etching the samples in the HNA solution.

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106 Figure 6.9 100 mm Jig for Backside Wafer Protection During Etching ACF consists of metal particles in the polymer matrix that allows the current conduction along one direction. The meta l particles present in ACF were attacked by HNA during etching thereby the adhesion between the bond ed die and the carrier wafer became poor. The SEM micrograph of a sample bonded us ing ACF is shown in Figure 6.10. Figure 6.10 SEM Micrograph of ACF Bonde d Chip After Partial Etching in HNA

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107 The poor adhesion between the ACF bonded die and the carrier wafer can be observed. For prolonged HNA etching, the die detached from the carrier wafer. Hence, the use of ACF for bonding was abandoned and alternative bonding was carried. To avoid the problems during etching, th ermo-compression bonding was done with AuAu as intermediate metal layers. A layer of Ti (100 nm)/Au (120 nm ) or Cr (100 nm)/Au (120 nm) was deposited on both the patterned emitter wafer and the carrier wafer. In thermo-compression bonding, the individual die was bonded to the carrier wafers by applying an external load of 80 N at a te mperature of 320 C for 4 minutes. Figure 6.11 shows the temperature vs. time profile used in thermo-compression bonding. Time (seconds) Figure 6.11 Time vs. Temperature Plot for Gold-to-Gold Thermo-Compression Bonding

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108 It was observed that the bondi ng was not strong and the di e was separated from the carrier wafer at the Au layer. By varying the thickness of Au films, and experimental conditions during bonding, there was no impr ovement. An electro-plated gold layer served as a very good interm ediate metal layer for thermo-compression bonding. The details on the electro-plating are provided in the below section. The difference in the bond strengths of the sputtere d and electro-plate d gold films was attributed to the differences in the surface morphology of both the films. The SEM micrographs of sputte red and electro-plated gold films are shown in Figure 6.12 (a) and (b) respectively. It can be obser ved that the sputtered gold films consist of well defined and closely packed grains. On the other hand, the su rface of the electroplated gold is non-homogenous. These differe nces in the morphol ogy resulted in the different bond strengths duri ng the chip-level bonding. Figure 6.12 SEM Micrograph of (a) Sputtered Gold Film (b) Electroplated Gold Film

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109 Figure 6.13 AFM Micrographs of (a) Sputtered Gold Film (b) Electroplated Gold Film The poor bonding between the sputtered gold fi lms can also be due to extremely smooth surfaces of the films. The atomic force microscopy images of both sputtered and electroplated gold films are shown in Figure 6. 13 (a) and (b) respectively. It can be found that the average surface roughness of the spu ttered films was approximately 8 nm where (a) (b)

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110 as the electroplated films have a r ough surface morphology with a mean surface roughness of approximately 30 nm. The difference in the surface roughness, along with the density and the mechanic al properties of the films can cause poor mechanical interlock when the sputtered gold films were used as an intermediate metal layer. The low-resolution and high-resolution SEM mi crograph of a thermo-compression bonded die is shown in Figure 6.14 (a ) and (b) respectively. Figure 6.14 SEM Micrograph of Au-Au Bonde d Chip (a) Low-Resolution (b) HighResolution Showing the Interface (b) (a)

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111 After bonding, the silicon mold was completely etched either by HNA or by KOH without any adhesion problems at the Au -Au interface. SEM micrograph of a free standing emitter array on a thin NCD film is successfully released from the buried silicon mold (Figure 6.15). Figure 6.15 SEM Micrograph of a Free Standi ng Field Emitter Array After Successful Silicon Etch 6.7. Electroplating of Gold Films as Metal Layer for Thermo-Compression Bonding Electroplating is a very conve nient method for depositing thick metal layers, which may not be practical to deposit by other conve ntional deposition tech niques. The electro-

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112 plating of copper and gold are widely im plemented in semiconductor manufacturing for interconnects, vias and trenches. In this re search, electroplating was used to deposit the gold films as intermediate metal layers for bonding. Prior to the deposition of gold, a seed layer of Ti/Au or Cr/Au was deposited by spu ttering. A simple electroplating setup is shown in Figure 6.16. Figure 6.16 Electroplating Cell for De positing Gold Films on 100 mm Wafers Like any other electroplating cell, the cathode is connected to the sample and the anode to a platinum mesh. A commercially availabl e TG25 RTU consisting of gold sulphate solution (Technic Inc.) is used as an electrolyte. The solution is continuously stirred and heated to 58 C. A constant cu rrent density between 1-4 mA/cm2 is passed to the cell by a

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113Kiethley 2400 current source meter. Typically for a 100 mm wafer, an approximate current of 162 mA/cm2 is applied to electroplate a film of thickness ~ 4 m. 6.8. Fabrication of Anode-on-Membrane Wafers Using Generation I Mask The double side polished (DSP) Si (100) wafers (n-type, thickness; 300-350 m, resistivity: 5-10 ohm.cm) were used as starter wafers for the fabrication of membranes. After the standard cleaning, a thin SiNx layer was grown by low-pressure chemical vapor deposition (LPCVD). A negative resist NR 9 1500 was spun on the wafers followed by a subsequent pre-bake at 150 C for 60 sec onds. After photo-lithography, the wafer is post baked at 100C for 60 seconds and developed in RD6 for 40 seconds. After the nitride layer from the patterned regions is etched, the wafers were loaded in KOH bath (30% KOH (269.5 ml) in DI water (229.5 ml)) to etch the exposed silicon surface. As there was no etch-stop layer, a timed KOH etch wa s done until the desired thickness of the membrane was achieved. The process flow for the fabrication of silicon membranes using the generation I mask set is shown in Figure 6.17.

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114 Figure 6.17 Process Steps for the Fabricati on of Anode-on-Membrane Using Generation I Mask The SEM micrograph of a membrane after the pa ttern is fully etched is shown in Figure 6.18. The thickness of the membrane as estimated using an optical microscope was between 10-20 m. Figure 6.19 shows the picture of a fully processed 100 mm wafer with various membranes.

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115 Figure 6.18 SEM Micrograph of a Generation I Silicon Membrane After Anisotropic KOH Etch Figure 6.19 Snapshot of a Fully Processed Generation I Silicon Wafer After KOH Etch

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1166.9. Fabrication of Anode-on-Membrane Wafers Using Generation II Mask In addition to the process step s as shown in Figure 6.17, an additional nitride layer is grown after the first KOH step. The modified process flow for the fabrication of generation II membranes is shown in Figure 6. 20. After the shallow etch, a 120 nm thick nitride film was grown by lo w-pressure chemical vapor deposition (LPCVD) method. A photo-lithography step defines the regions of th e center boss structur e. After etching the nitride from the developed regions on the wa fer, the negative photor esist is stripped and the wafers were then loaded in the KOH ba th for bulk etching of the exposed silicon surface to form thin silicon membranes. Figure 6.20 Process Steps for the Fabrication of Silicon Membranes Using Generation II Mask

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117 Figure 6.20 (Continued) For a typical wafer thickness of 300-350 m, and for an etch rate of 1 to 1.2 m/min, the wafer is etched for 4.5 to 5 hours to obtai n membranes of thickne sses on the order of 10 to 20 m. The SEM micrograph (sample tilted) of a membrane formed after etching Si (100) wafer using second generation mask is shown in Figure 6.21. The under cut in the center boss structure is due to the anisotro pic KOH etching. As the dimensions of the membranes are in several millimeters, the full size of the membrane could not be captured. Figure 6.22 shows the snap-shot of a fully processed 100 mm wafer with various membranes prior to the nitride etch.

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118 Figure 6.21 SEM Micrograph of a Fully Et ched Generation II Membrane After Anisotropic KOH Etch Figure 6.22 Snapshot of a Fully Pr ocessed Generation II Silicon Wafer

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1196.10. Integration of the Anode and the Cathode To integrate the anode and the cathode, both the parts were fabricat ed on 100 mm wafers. The earlier version of the devices was bonde d to a silicon wafer using gold as an intermediate metal layer. But due to the difficulty in protecting the silicon (carrier) wafer from unwanted etching, the pr ocess has been changed by implementing Pyrex wafer as a carrier substrate for bonding the individual field emitter dice. A bi-layer of Cr/Au was sputtered onto the Pyrex wafer followed by th e deposition of a thick electroplated gold film. The wafer is then pattern ed to define the contact pads to bond the dice in a flip-chip bonder. In order to make the electrical cont act, vias are opened in the bond pad regions by laser milling and through wafer interconnects we re achieved. Prior to the testing, the vias were filled using a conductive silver epoxy. Figure 6.23 (a-c) shows the processing steps at the wafer level. In the next step, the silicon mold was comple tely etched in hot KOH solution with similar composition as described earlier. By changing the process from silicon to Pyrex, the device fabrication was more re liable and reproducible. Afte r 6 to 8 hours of KOH etch, the silicon mold was completely etched and the wafer consisted of an array of field emitter devices. The snapshot of a wafer after etching the silicon mold is shown in Figure 6.23 (c). The next step in th e fabrication is bonding of th e anode wafer to the cathode wafer in a wafer-to-wafer bonder. As the ther mal expansion coefficients of Pyrex and silicon wafers are similar, thermal mismatch is minimal and therefore the bonding can be better.

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120 Figure 6.23 (a) Pyrex Wafer After Laser Milling Through Holes for the Electrical Contacts (b) Pyrex Wafer Flip-Chip Bonding Diamond Dice to the Gold Bond Pads (c) Snapshot of a 100 mm Wafer Afte r Etching the Top Silicon Mold (a) (b) (c)

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1216.11. Wafer-to-Wafer Bonding The integration of anode and the cat hode wafers was carried out in an EVG EV501 wafer bonder by vacuum anodic bonding technique. Prior to the bonding, the hard mask (SiNx) on the silicon wafer was etched and the silicon wafer with the membranes is bonded to the Pyrex carrier wafer. The process conditions used for th e vacuum anodic bonding are: Force-500 N, Voltage-1000 V, Temperature400C, Bond time-20 minutes. As the thickness of the membranes was between 10 and 20 m, some of the membranes were not mechanically strong to withstand bonding. When the si licon wafer consisted of the generation I membranes, the gap between th e apex of the tips and the surface of the membrane was ~ 260-270 m and therefore, the vacuum gap between the anode and the cathode could be achieved. But when the wafe rs consisted of generation II membranes, the spacing was reduced to ~ 50 m and during wafer-to-wafe r bonding; it was observed that the generation II silicon wafers and the Pyrex wafers were completely bonded. Ideally, during this step, it is anticipated to have a separation between the center proof mass regions and the cathode so that the vac uum gap can be obtained. A batch of wafers have been fabricated with different etch depths (20 m to 100 m) to find out the optimum depth at which the vacuum gap can be prevailed without bonding completely. But, the problem during the wafer-to-wafer bonding was not fully solved. When the initial etch depth was ~ 100 m, the transfer of patterns fr om the second layer of mask was difficult due to non-unifo rm photoresist thickness at th e edges of the first pattern. Therefore, the final version of the device s consisted of anodes fabricated using the generation I mask. After the wafers were bonded, a top metal contact (Cr/Au) was

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122 deposited by sputtering and the wafer stack wa s diced to individual chips for carrying the field emission testing. 6.12. Characterization of Fi eld Emission Devices The field emission characteristics of both free standing and capped devices were tested in a diode model (Figure 6.24). Figure 6.24 Field Emission Devi ce in Diode Configuration The field emission test chamber shown in Figu re 6.25 is equipped with a turbo-molecular pump that can pump down to low pressure of 10-8 Torr. The D.C bias to the cathode can be applied through the high voltage electrical feed throughs and the emission current can be measured. Manipulation of the sample can be done by a linear mo tion feed through. In free standing devices, a mol ybdenum plate was used as an anode. The anode and the cathode are separated by an insu lator. The field emission propert ies were tested at a base pressure of 5 X 10-8 Torr.

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123 Figure 6.25 Field Emission Test Chamber Initially, when the bias is applied, the emission current can be unstable due to the impurities at the tip surface. To overcome this problem, the tips are conditioned by thermal annealing or by appl ying a constant bias or by v acuum thermal electric (VTE) treatments. Of these three methods, VTE has been reported to st abilize the emission current by desorbing the impurities and reducin g the surface work function [34]. In this study, a constant voltage (300-400 V) was app lied for ~ 1 hour to condition the tip. After the emission current was stabilized, the voltage was gradually increased and the emission current was recorded. The emission current vs. electric field (I-F) characteristics of free standing emitter tips as collect ed at a molybdenum anode is shown in Figure 6.26 (a). The Fowler-Nordheim (F-N) characteristic s are shown in Figure 6.26 (b). The

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124 exponential I-F in the forward bias and the linear F-N charact eristics indicate the field emission behavior. Figure 6.26 (a) Electric Field vs. Emission Current Characteristics (b) Fowler-Nordheim Characteristics of Free standing NCD Em itter Array With a Vacuum Gap of 1000 m The turn-on field is defined as the electric fi eld at which there is a stable emission current of at least 1A. In free standing devices with an inter-elect rode spacing of ~ 1000 m, the turn-on electric fiel d was found to be ~ 3 V/ m. As the applied voltage was increased, the emission current increased to only few microamperes. Due to the large separation between the anode and the cathode, the emission does not occur under low applied voltages. The gap between the anode and the cathode was gradually reduced after improving the fabrication methods. With a vacuum gap of 500 m, the turn-on fields reduced to ~ 1.5 V/ m. The emission current vs. electric field characteristics of these devices at the turn-on voltage is shown in Figure 6.27. (a) (b)

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125 Figure 6.27 Electric Field vs. Emission Curre nt Characteristics of Free Standing NCD Emitter Array With a Vacuum Gap of 500 m By reducing the inter-electrode spacing furthe r to 300 m, a lowest tu rn-on field of ~ 0.8 V/ m and highest emission current of 20 A was achieved from free standing devices. The field emission characteristics of the device are shown in Figure 6.28. Figure 6.28 (a) Electric Field vs. Emission Current Characteristics (b) Fowler-Nordheim Plot With a Vacuum Gap of 300 m (a) (b)

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126 In all the above cases, it can be observed th at under forward bias, the emission current increases exponentially (Figure 6.28 (a)) representing a simple diode characteristics. The electric fields of lines are highly concentr ated at the apex and their density decreases away from tip. Therefore, as the inter-elec trode spacing is change d, the turn-on field correspondingly changes. Besides, the linear trend in the Fowler-Nordheim plots (Figure 6.28 (b)) of the devices confirms that the el ectron flow in the devi ces is by tunneling. 6.13. Theoretical Analysis of the Field Emission Data The field emission data is primarily examin ed by Fowler-Nordheim equation given by: Rd LV B Rd LV aA aJ I exp2 (6.1) The F-N plot shows a linear ch aracteristics between ln(I/E2) vs. 1/E. Accordingly, the linear mathematical equation can be written as y = mx + c (6.2) m = (B 3/2)/ (6.3) = (B 3/2)/(L/R) (6.4) c = ln((aA 2)/ ) (6.5) A (constant) = 1.54 X 10-6 (6.6) B (constant) = 6.83 X 107 (6.7)

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127 The slope and intercept of the F-N equation includes three unknown factors ( , and area (a)). It is not possible to deduce th ree unknown quantities from two equations. Therefore, one of the three parameters has to be determined using another technique. The emission from the individual tips takes place onl y from few atomic sites at the tip apex. Therefore, it is difficult to precisely estimate the net emissi on area. Hence, the emission area is omitted for the calculations. The geometrical field enhancement factor ( ) can be estimated by SEM. The value of in the emitter arrays varied between 56 to 140 for tips with height of 5.6 m and 14 m respectively. For the field emission characteristics of freestanding devices shown in Figure 6.28 (b), the approximate linear mathematical fitting (F igure 6.29) is done to calculate the field emission parameters. Figure 6.29 Fowler-Nordheim Characteristic s With Approximate Mathematical Fit

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128 The variation in the slopes across the high fi eld and the low field regions is due to the image force effect [34]. The calculated value of from the slope of the over-all curve was found to be 0.036 eV. Though the theoretica l models showed that the nitrogen forms a shallow donor level with a ne t negative value of the electr on affinity and work function, the true work function is sl ightly positive. Therefore, the diamond surface possesses an effective negative electron a ffinity instead of a true negative electron affinity. 6.14. Characterization of a Field Emission Sensor The field emission sensors were fabricated with cathode and anode areas listed in Table 5.2 and Table 5.3 respectively. As discussed in chapter. 4, higher pressures or loads can be exerted on smaller anode membranes, while in the larger membranes, the full scale pressure range was reduced by at least one-half to operate the sensor in a linear and safe operating range of the device. The field emi ssion sensor was tested in the same test chamber as the field emission source. Due to certain limitations in the system, the pressure inside the test chamber could not be increased through a leak valve in a controlled way. Hence, a linear motion feed through with an end effecter was employed to contact the anode and disp lace the membrane. Therefore, during this measurement, the actual change in the current is a function of the deflection of the center of the membrane. A proof-of concept of a field emisison sensor is demonstrated while further investigation is necessary to completely optimize the performance.

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129 The dimensions of the cathode and the anode for the tested field emission sensor were 7000 X 7000 m2 and 10000 X 10000 m2 respectively. In this version of devices, the anode and the cathode are se parated by at least ~ 270-280 m. Unlike the free standing devices, where the anode was a thin molybdenu m plate, the anode of the capped devices has a thin silicon membrane. It was observe d that the field emission properties of the capped devices were different than the free standing emitters. The turn-on voltage of the capped devices was found to be ~ 2 kV and th e corresponding turn-on electric field was ~ 8V/ m. For the same emission current and inter-electrode spacing of ~ 280 to 300 m, the turn-on electric field of free standing devices was ten times less than the capped devices. The turn-on electric field vs. emission current characteristics and the corresponding Fowler-Nordheim plot of the capped devices is shown in Figure 6.30 (a) and (b) respectively. Figure 6.30 (a) Electric Field vs. Emission Current Characte ristics (b) Fowler-Nordheim Characteristics of Capped Field Emisison Devices (a) (b)

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130 In case of free standing devices, the anode is made of highly conductive metal plate, and therefore, the current losses within the metal plate are minimal. Besides, the devices are tested at a true base vacuum of 5 X 10-8 T. As the emission current directly depends up on the vacuum level in the test chamber, higher emission curr ent and lower turn-on fields can be obtained at higher vacuum level. On the other hand, in the capped field emissi on devices, the anode is made of silicon membrane. The electrical conductivity of the silicon membranes will be very much lower than the molybdenum plate. Moreover, Si (100 ) wafers used for the fabrication of the membranes are not heavily doped. Therefore, si gnificant loss in the emission current can take place within the anode be fore it is collected. In the capped field emission devices, though the integration of anode and cathode is done under vacuum conditions, true vacuum inside the cavity cannot be exactly determined once they are bonded. Therefore, the voltage required for the emission of equivalent current is higher. The field emission sensor is tested under constant voltage mode. After collecting a stable emission current of 1 A, the end effecter of the linear feed thr ough was brought in contact with the anode to push the membrane toward the cathode. The emi ssion current values were recorded as the relative position of the anode was changed fr om its previous position. The emission current increased from 1 A (at 280 m separation) to 25 A (at 50 m separation) at a constant turn-on field of ~ 8V/ m. A linear response was obtai ned between the change in the current vs. change in the relative position of the membrane. The corresponding pressure exerted on the membrane due to th e deflection is obtaine d theoretically. The

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131 emission current is a linear function of the applied load (Figure 6.31). As the maximum travel of the linear motion feed through was limited to ~ 280 m, the calculated value of the maximum load was below 40 psi. The cu rrent design of the field emission sensor limits its operation only to a low pressure regime. Pressure vs Emisison Current 0 5 10 15 20 25 30 010203040 Pressure(psi)Emission Curren t (microamperes) Figure 6.31 Pressure vs. Emi ssion Current Characetristics 6.15. Summary This chapter has discussed the process details for the fabrication of field emitter arrays by mold technique and anode-on-membranes using standard silicon wafers. The challenges in the fabrication of a vertical field em ission device were presented and alternative process techniques have been developed for the successful device fabrication. Though the

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132 two types of anodes were fabricated, the di fficulty in the integr ation of the second generation anode mask was a limitation. Two t ypes of emitter devices were tested for their field emission characteristics in a di ode configuration. The capped field emission devices were tested as a field emission s ource and a possible fiel d emission sensor. Low turn-on electric fields (0.8 V/m) were obt ained in the un-capped emission devices while the turn-on electric field incr eased by an order of ten times (8 V/m) in an integrated anode device. The field emission paramete rs of free standing and capped emission devices were different due to the different anode materials and relatively lower vacuum level during the test in the capped devices.

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133CHAPTER 7. CONCLUSIONS 7.1. Summary The primary aim of this research was to develop the process for the application of CVD diamond thin films as a structural ma terial in the microelectronics and microelectromechanical system (MEMS) devi ces. The research was two-folded with much of the focus on the growth of NCD thin films by microwave plasma enhanced chemical vapor deposition (MPECVD) techniqu e and their characterization. Toward the growth of the NCD films, this research has presented three aspects. Firstly, the nucleation densities have been evaluated and improved method of seeding has been experimentally found. The next part of the research was focused on the de velopment of process recipes for the growth of NCD films using hydrogen poor gas chemistry in presence of excess argon and a trace amount of methane at high microwave discharges. The structural and mechanical properties of the films were eval uated by the use of analytical and metrology tools. Finally, the optimized process recipe s were used to grow high quality nanodiamond films on large area substrates. The next part of the research discussed on the development of conductive NCD films using nitrogen in th e gas chemistry. The growth process has been optimized to obtain electrically conduc tive NCD films. The structural and the

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134 electrical properties of the conductive films were evaluated and the process has been optimized for its application in the device fabrication. The second part of the resear ch focused on the fabrication of field emission devices using conductive NCD films. The benefit of using NCD as a structural material for the fabrication of “Spindt-type” devi ces is recognized and the deta ils on the fabrication of the vertical field emission devices by silicon mold technique were discussed. Both free standing, and capped field emitter devices were fabricated. A proto-type field emission sensor was fabricated using “anode-on-me mbrane” geometry. The fabrication of integrated field emission sensor includes th ree wafer processes with six levels of lithography and two levels of packaging. The anode-on-membrane configuration was designed to have thin silicon membranes a nd the cathode is made of nanodiamond field emitter arrays. The anode of the membrane forms the mechanical component of the sensor and the electrical co mponent or as an electron co llector in the field emission device. The integration of individual components was done by anodic bonding technique. The challenge in the fabrication of capped vertical emitter device was to achieve the vacuum gap between the anode and the ca thode with optimum separation for the emission characteristics to be reliable. The fi eld emission device in this configuration can be represented as a diode and their emission characteristics we re tested in a high vacuum test chamber to determine the electric fields and the emission current densities. Low turnon field of 0.8 V/ m and an emission current of 20 A were obtained on the free standing devices while the turn-on fields in the capped devices increased by an order of magnitude.

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1357.2. Future Work Much of the work in the gr owth and characterization of both intrinsic and conductive NCD thin films has been reported in the li terature. Nevertheless, a fundamental study toward the nucleation and growth of the films is essential to fully exploit the material properties for its application in microelect ronic device processing. During this entire research, the seeding has been initiated by ultrasonic scratch method. Alternative methods such as bias enhanced nucleation n eeds to be fully unders tood and the process has to be optimized for its application in Cyrannus I IPLAS reactor. As the growth rate of the nanodiamond films is less, through-put can be increased by in-sit u seeding using bias enhanced nucleation technique. Though extens ive characterization te chniques have been used to analyze the structural, mechanical a nd electrical properties, the thermal properties of the films have to be evaluated as the future microelectronics industry needs high efficiency heat sinks. During the growth of conductive NCD films, it was observed that the addition of nitrogen in the reactor drasti cally changes the behavi or of the microwave plasma discharge. Therefore, the process has to be further optimized to address this issue and grow conductive films on 100 mm wafers. In terms of the device fabrication, vertical field emitter devices have been fabricated using diamond and several other materials. Th e current research focused primarily on the application of conductive NCD films for electron emitters. As mentioned in chapter. 5, the dimensions of the trapezoidal base determ ine the tip height which in turn determine the geometrical field enhancement factor. Therefore, for smaller base width and pitch, the

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136 density of the emitter tips can be increa sed, and correspondingly the field emission current can be increased. Therefore, the next generation devices have to be fabricated using improved device designs. Further, the method of tip sharpening by oxidation can be included in the fabrication process to reduce the radius of the tip and improve the field emission properties. The fabrication of a proto-type field emission sensor has been described in this research. In the current test method, the load ex erted and the corresponding deflection on the membrane is a relative position of the memb rane displaced by the motion feed through. The current design of the field emission sensor limits its operation only to a low pressure regime. It is important to increase the net anode area for efficiently collecting the field electrons, but on the other hand, as the anode is made of thin membranes, the larger membranes tend to deflect more than the safe operating range and therefore limit the operation of the sensor to a narrow pressure scale. Though an improved design for the fabrication of the anode membranes was aimed, the difficulties in the integration of the individual components have limited its applic ation. Therefore, a lternative fabrication procedures have to be implemented to addr ess these issues. The ne xt generation devices can be fabricated by improving the designs of the anode and the cathode masks.

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137REFERENCES 1. S. Tolansky, (1962). The History and Us e of Diamond, London: Shenval Press. 2. Davies, (1984). Diamond. Bris tol: Adam Hilder Ltd. 3. W. P. Kang, J. L. Davidson, K. Subraman ian, B. K. Choi, and K. F. Galloway, (2007). Nanodiamond Lateral VFEM Technol ogy for Harsh Environments, IEEE Transactions on Nuclear Science, 54, 4. 4. L. S. Pan and D. R. Kania, (1995) Diamond: Electronic Properties and Applications, Boston: Kluw er Academic Publishers. 5. General Electric Company, (1955). Press release. 6. W. G. Eversole, (1962 a) U.S. Patent 3, 030,187, (1962 b) U.S. Patent 3,030, 188. 7. J. C. Angus, H. A. Will, and W. S. Stanko, (1968). Growth of Diamond Seed Crystals by Vapor Deposition, J. of Appl. Phys. 39, 6, 2915. 8. B. V. Deryagin, and D. V. Fedoseev, (1976). Sci. Am, 233, 102. 9. F. P. Bundy, H. T. Hall, H. M. Strong, and R. J. Wentorf Jr., (1955). Man Made Diamonds, Nature, 176, 51. 10. S. Aisenberg, R. Chabot, (1971). Ion-Beam Deposition of Thin Films of Diamond like Carbon, J. of Appl. Phys. 42, 2953. 11. Electronic-reference: http://newton.ex.ac.uk/research/qsyst ems/people/sque/diamond/structure/ 12. R. F. Davis, (1993) Diamond Films and Coatings. 13. M. A. Prelas, G. P. Popovici, and L. K. Bigelow, (1998). Handbook of Industrial Diamonds and Diamond Films, New York: Marcel Dekker. 14. Paoletti and A. Tucciarone, (1997). The physics of Diamonds, Amsterdam, IOS Press.

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ABOUT THE AUTHOR Sathyaharish Jeedigunta has received Bachelor of Technology in Electrical and Electronics engineering from Jawaharlal Nehru T echnological University (JNTU), India in 2001. He has received Master of Sc ience in Electrical Engineering with microelectronics as specialization from Univer sity of South Florida, Tampa in 2004. He has continued working toward the doctoral de gree in Electrical engine ering from the year 2004. During the doctoral work, he has work ed on the development of thin films, nanostructures and fabrication of microelect ronic devices. His dissertation work is focused on the development of the diamond thin films for microelectronics and MEMS applications. The author’s other resear ch interests include: one-dimensional nanostructures, photo-voltaics, and thin film s. He has been affiliated with Materials Research Society (MRS) and The Metals Society (TMS).


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Growth and characterization of nanocrystalline diamond films for microelectronics and microelectromechanical systems
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ABSTRACT: Diamond is widely known for its extraordinary properties, such as high thermal conductivity, energy bandgap and high material hardness and durability making it a very attractive material for microelectronic and mechanical applications. Synthetic diamonds produced by chemical vapor deposition (CVD) methods retain most of the properties of natural diamond. Within this class of material, nanocrystalline diamond (NCD) is being developed for microelectronic and microelectromechanical systems (MEMS) applications. During this research, intrinsic and doped NCD films were grown by the microwave plasma enhanced chemical vapor deposition (MPECVD) method using CH/Ar/H gas mixture and CH/Ar/N gas chemistries respectively. The first part of research focused on the growth and characterization of NCD films while the second part on the application of NCD as a structural material in MEMS device fabrication.The growth processes were optimized by evaluating the structural, mechanical and electrical properties. The nature of chemical bonding, namely the ratio of sp:sp carbon content was estimated by Raman spectroscopy and near edge x-ray absorption fine structure (NEXAFS) techniques. The micro-structural properties were studied by x-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The mechanical properties of the pure NCD films were evaluated by nano-indentation. The electrical properties of the conductive films were studied by forming ohmic as well as schottky contacts. In second part of this study, both free-standing and membrane capped field emitter devices were fabricated by a silicon mold technique using nitrogen incorporated (i.e., doped) NCD films. The capped field emission devices act as a prototype vacuum microelectronic sensor.The field emission tests of both devices were conducted using a diode electrical device model. The turn-on field and the emission current of free-standing emitter devices was found to be approximately 0.8 V/m and 20 A, respectively, while the turn-on fields of capped devices increased by an order of magnitude. The emission current in the field emission sensor changed from 1 A to 25 A as the membrane was deflected from 280 m to 50 m from the emission tip, respectively.
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