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Micro-electro-discharge machining of carbon-nanotube forests and its application Dahmardeh, Masoud 2014

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Micro-Electro-Discharge Machining of Carbon-Nanotube Forests and Its Application  by Masoud Dahmardeh  B.Sc., Amirkabir University of Technology, 2006 M.Sc., Amirkabir University of Technology, 2009  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY in The Faculty of Graduate and Postdoctoral Studies (Electrical and Computer Engineering)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  March 2014  ? Masoud Dahmardeh, 2014 ii  Abstract Carbon nanotubes (CNTs) are relatively new materials with exceptional properties which have attracted significant interest in the past two decades. The ability to grow arrays of vertically aligned carbon nanotubes, so called CNT forest, opened up opportunities to develop different types of novel devices enabled by the material. A key in facilitating micro-electro-mechanical systems (MEMS) applications of the material is the ability to pattern the material in a batch mode with high precision and high reproducibility. Patterning CNT forests prior to, during or after the growth is reported. The mentioned techniques are, however, primarily for the formation of two-dimensional types of patterns (with uniform heights). Laser micromachining is reported to shape CNT forests for different applications while exhibiting its inherent limitations including tapered sidewalls, lack of high-precision depth control, and thermal damages. Hence, there is a need to develop machining techniques to fabricate CNT forests in any shape for MEMS and other applications.  This thesis is based on the idea that a powerful micromachining technique is a path that should be taken to reach a successful integration of smart materials such as nanotubes and MEMS (and other) devices to achieve more complex and improved devices. This work develops an effective micromachining technique based on dry micro-electro-discharge machining (?EDM) to produce free-form, three-dimensional (3D) patterns out of CNT forests with high precision (~2-?m machining tolerance), high-aspect-ratios (of about 20), high reproducibility, and at very small machining voltages (~10 V) which corresponds to several orders of magnitude smaller discharge energy (0.5 nJ compared to 15 ?J). The machining mechanism has been found to be different from the one in typical ?EDM. Furthermore, techniques to achieve high removal precision with tighter tolerance are investigated. Also, elemental and molecular analysis of the iii  machined structures is carried out to observe the level of cross-contamination of the process. To demonstrate an application of the processed nanotubes, high-power MEMS switches that integrate micropatterned CNT forests as electrical contact have been developed. Micropatterned CNT forests as field emitters and atomic force microscopy (AFM) probe tips are also demonstrated. iv  Preface This thesis is based on the contributions that have been reported in the following papers: Journal papers: 1. W. Khalid, M. Sultan Mohamed Ali, M. Dahmardeh, Y. Choi, P. Yaghoobi, A. Nojeh, and K. Takahata, ?High-aspect-ratio, free-form patterning of carbon nanotube forests using micro-electro-discharge machining?, Diamond and Related Materials, 19(11), 1405, 2010. (Part of Chapter 2) 2. M. Dahmardeh, A. Nojeh, and K. Takahata, ?Possible mechanism in dry micro-electro-discharge machining of carbon-nanotube forests: A study of the effect of oxygen?, Journal of Applied Physics, 109(9), 093308, 2011. (Part of Chapter 2) 3. T. Saleh, M. Dahmardeh, A. Bsoul, A. Nojeh, and K. Takahata, ?Field-emission-assisted approach to dry micro-electro-discharge machining of carbon-nanotube forests?, Journal of Applied Physics, 110, 103305, 2011. (Part of Chapter 3) 4. T. Saleh, M. Dahmardeh, A. Nojeh, and K. Takahata, ?Dry micro-electro-discharge machining of carbon-nanotube forests using sulphur-hexafluoride?, Carbon, 52, 288, 2013. (Part of Chapter 3)  5. M. Dahmardeh, M. S. Mohamed Ali, T. Saleh, T. M. Hian, M. Vahdani Moghaddam, A. Nojeh, and K. Takahata, ?High-power MEMS switch enabled by carbon-nanotube contact and shape-memory-alloy actuator?, physica status solidi (a), 210: 631?638, 2013 (Front Cover). (Part of Chapter 4) 6. M. Dahmardeh, M. Vahdani Moghaddam, T. M. Hian, A. Nojeh, and K. Takahata, ?The effects of three-dimensional shaping of vertically aligned carbon-nanotube contacts for v  micro-electro-mechanical switches?, Appl. Phys. Lett., 103, 231606, 2013. (Part of Chapter 5) 7. Z. Xiao, M.S. Sarwar, M. Dahmardeh, M. Vahdani Moghaddam, A. Nojeh, and K. Takahata, ?Cone-shaped forest of aligned carbon nanotubes: An alternative probe for scanning microscopy?, Appl. Phys. Lett., 103, 171603, 2013. (Appendix C) 8. T. Saleh, M. Vahdani Moghaddam, M. S. Mohamed Ali, M. Dahmardeh, C. A. Foell, A. Nojeh, and K. Takahata, ?Transforming carbon nanotube forest from darkest absorber to reflective mirror?, Appl. Phys. Lett., 101, 061913, 2012.  Conference papers: 1. M. Dahmardeh, W. Khalid, M.S. Mohamed Ali, Y. Choi, P. Yaghoobi, A. Nojeh, and K. Takahata, High-aspect-ratio, ?3-D micromachining of carbon-nanotube forests by micro-electro-discharge machining in air?, Proceeding IEEE MEMS, Mexico, Jan 23-27, 2011, pp. 272 - 275?. (part of Chapter 2) 2. T. Saleh, M. Dahmardeh, A. Bsoul, A. Nojeh, and K. Takahata, ?High-precision dry micro-electro-discharge machining of carbon-nanotube forests with ultralow discharge energy?, Proceeding IEEE MEMS, Paris, Jan 29-Feb2, 2012, pp. 259-262. (Part of Chapter 3) 3. M. Vahdani Moghaddam, M.S. Sarwar, Z. Xiao, M. Dahmardeh, K. Takahata, A. Nojeh, ?Field-emission from carbon nanotube cones fabricated by micro-electro-discharge machining?, 26th Int'l Vacuum Nanoelectronics Conf. (IVNC 2013), Roanoke, USA, 2013. (Appendix B) vi  4. M. Chang, M. Vahdani Moghaddam, A. Khoshaman, M. S. Mohamed Ali, M. Dahmardeh, K. Takahata, A. Nojeh, ?High Temperature Gradient in a Conductor: Carbon Nanotube Forest under the ?Heat Trap? Condition?, 57th Int?l Conf. on Electron, Ion, and Photon Beam Technology and Nanofabrication (EIPBN 2013), Nashvile, USA, 2013.  In addition, exact quotations from Journal papers 1-7 and Conference paper 3 may appear in Chapters 2-5 and appendices B and C. Necessary permissions have been obtained from the respective publishers to reproduce the reported results in this thesis. Here, I clarify that I am the principle researcher and main author in Journal papers 2, 5, 6 and Conference paper 1. I conducted the literature survey, design, process development and fabrication. I also prepared the experimental setup and performed all of the measurements. Dr. Mohamed Sultan Mohamed Ali assisted me with the fabrication work of Journal paper 5 while Dr. Tanveer Saleh helped me with developing the concept. Min Hian Tee assisted me in collecting data using LabVIEW for Journal papers 5 and 6. All the manuscripts were co-authored by my supervisor, Prof. Kenichi Takahata and my co-supervisor, Prof. Alireza Nojeh, who have guided me in each and every aspect of my research. Prof. Takahata and Prof. Nojeh kindly provided me with the general idea for this research, and with guidance and continuous support throughout the project. Prof. Takahata also assisted me in writing the manuscripts and Prof. Nojeh commented on them and helped me with editing. I am the co-author for Journal papers 1, 3, 4, 7, 8 and Conference papers 2-4. For Journal paper 1, Dr. Waqas Khalid initiated the project. I characterized machining CNT forests with ?EDM at different machining conditions, such as energy, feed rate, electrode size, rotation speed. I also characterized the machined structures by capturing and discussing SEM images and vii  elemental analysis. During the joint experiments of Dr. T. Saleh and me for Journal papers 3 and Conference paper 2, Anas Bsoul suggested the enhanced field emission properties of the CNT forests to be the reason for higher discharge current in the reverse mode of ?EDM. Experiments were conducted by Dr. T. Saleh and myself together. Immediate discussions and explanations of the collected data were done the same way and later with Prof. Takahata and Prof. Nojeh. The experimental setup for Journal paper 4 was set up by Dr. Tanveer Saleh and myself together. The results and outcomes of the experiments were immediately discussed and interpreted the same way and later by Prof. Takahata and Prof. Nojeh. Dr. Zhiming Xiao led the project of Journal paper 7 and the main study was done by himself. I helped him with preparing patterned catalyst to grow CNT forest, growing CNT forest, developing the machining code, and machining with ?EDM. Dr. Mehran Vahdani Moghaddam led the project of Conference paper 3 and the main study was conducted by himself. Although I did not have a major contribution in this study, but I did conduct some early experiments of filed-emission of nanotubes with a setup different from the one presented in the paper. Finally, although I did not have a significant role in the works presented in Journal paper 7 and Conference paper 3, they are presented in Appendices B and C to show the continuity of the work.  Carbon nanotube forest samples used for early experiments (presented in section 2.1) were grown by the chemical vapour deposition (CVD) system designed by Dr. Parham Yaghoobi in Prof. A. Nojeh lab. Later, the author and Anas Bsoul developed a replicate local heater and reaction tube of that system which was used to grow CNT samples for the work presented in section 2.2, and some experiments of Chapter 3. The rest of the CNT samples used in Chapter 3 viii  and all the CNT samples presented in Chapter 4 and 5 are grown by an improved CVD system developed by Dr. Mehran Vahdani Moghaddam in Prof. A. Nojeh lab.  ix  Table of Contents  Abstract .......................................................................................................................................... ii Preface ........................................................................................................................................... iv Table of Contents ......................................................................................................................... ix List of Tables ............................................................................................................................... xii List of Figures ............................................................................................................................. xiii Acknowledgements .................................................................................................................... xxi Dedication .................................................................................................................................. xxii Chapter 1: Introduction and Objective .......................................................................................1 1.1 Carbon Nanotubes ........................................................................................................... 1 1.2 Synthesis of Carbon Nanotubes ...................................................................................... 2 1.3 Carbon-Nanotube Forest and its Applications ................................................................ 3 1.4 Micro-Electro-Discharge Machining .............................................................................. 4 1.4.1 Introduction ................................................................................................................. 4 1.4.2 Process Description ..................................................................................................... 5 1.5 Micropatterning of Carbon Nanotubes ........................................................................... 8 1.6 Objectives of the Research .............................................................................................. 9 1.7 Potential Impact of the Research .................................................................................. 10 1.8 Research Methodology ................................................................................................. 11 1.9 Thesis Overview ........................................................................................................... 13 Chapter 2: Micro-Electro-Discharge Machining of Carbon Nanotube Forests; Feasibility and Characterization ...................................................................................................................14 x  2.1 ?EDM of CNT Forests; Feasibility .............................................................................. 14 2.1.1 Introduction ............................................................................................................... 14 2.1.2 Sample Preparation and Experimental Set-up .......................................................... 15 2.1.3 Results and Discussion ............................................................................................. 16 2.2 ?EDM of CNT Forests; Effect of Oxygen .................................................................... 27 2.2.1 Introduction ............................................................................................................... 27 2.2.2 Sample Preparation and Experimental Set-up .......................................................... 28 2.2.3 Results and Discussion ............................................................................................. 30 2.2.4 Conclusions ............................................................................................................... 36 Chapter 3: Toward High-Precision Micro-Electro-Discharge Machining of Carbon Nanotube Forests .........................................................................................................................37 3.1 Field-Emission-Assisted Approach .............................................................................. 37 3.1.1 Introduction ............................................................................................................... 38 3.1.2 Experimental Set-up.................................................................................................. 39 3.1.3 Results and Discussion ............................................................................................. 40 3.2 The Effect of Using Sulphur-Hexafluoride (SF6)  ........................................................ 52 3.2.1 Introduction ............................................................................................................... 52 3.2.2 Sample Preparation and Experimental Set-up .......................................................... 53 3.2.3 Results and Discussion ............................................................................................. 54 Chapter 4: Integrating Carbon Nanotube Forest in High-Power MEMS Switch .................67 4.1 Introduction and Background ....................................................................................... 68 4.2 Experimental ................................................................................................................. 71 4.3 Results and Discussion ................................................................................................. 75 xi  4.4 Conclusions ................................................................................................................... 83 Chapter 5: The Effects of Three-Dimensional Shaping of Vertically Aligned Carbon-Nanotube Contacts for Micro-Electro-Mechanical Switches ..................................................86 5.1 Introduction ................................................................................................................... 86 5.2 Principle and Basics ...................................................................................................... 88 5.3 Fabrication .................................................................................................................... 90 5.4 Results and Discussions ................................................................................................ 92 5.5 Conclusions ................................................................................................................... 98 Chapter 6: Conclusion .................................................................................................................99 6.1 Contributions................................................................................................................. 99 6.2 Future Work ................................................................................................................ 103 Bibliography ...............................................................................................................................105 Appendices ..................................................................................................................................119 Appendix A Carbon Nanotube Forest Growth System........................................................... 119 Appendix B Field-Emission from Carbon Nanotube Cones Fabricated by Micro-Electro-Discharge Machining .............................................................................................................. 121 Appendix C Cone-Shaped Forest of Aligned Carbon Nanotubes: An Alternative Probe for Scanning Microscopy.............................................................................................................. 126  xii  List of Tables  Table ?1-1: Properties of individual carbon nanotube ...................................................................... 2 Table ?1-2: Summary of micropatterning carbon nanotubes techniques ......................................... 9 Table C1: RRMS values of calibration dimple structures measured using fabricated CNT probes and commercial probe. ............................................................................................. 134   xiii  List of Figures  Figure ?1.1: Principle of one cycle of ?EDM process. .................................................................... 6 Figure ?1.2: RC relaxation type pulse generator of ?EDM .............................................................. 7 Figure ?1.3: 3D- and side- view of WEDG. ..................................................................................... 7 Figure ?2.1: ?EDM setup for machining CNT forest in air. .......................................................... 15 Figure ?2.2: Shrinkage effect in CNT forest after submerging in liquid; (a) before and after dipping in EDM oil, (b) SEM image of a dried CNT forest block after submerging in acetone, (c) top view of the CNT forest block shows the shrinkage volume, (d) close-up SEM image showing CNT forest roots are detached from the substrate due to the shrinkage force. .......................................................................................................... 17 Figure ?2.3: (a) Hole structures created in a CNT forest using EDM with 80 V and 30 V. (b) Hole structures made by mechanically drilling the forest using the same electrode with and without electrode rotation at 3000 rpm. ..................................................................... 18 Figure ?2.4: SEM images with the same magnification showing ?EDM results for the same pattern created with (a) 80 V, (b) 30 V, and (c) 10 V. ............................................... 18 Figure ?2.5: (a) Optical images during ?EDM of a CNT forest. A rotated 300-?m-electrode is scanned along a 500-?m?500-?msquare orbit in the X?Y plane. The inset in each image shows the top view of the electrode, its orbit (broken line), and the square CNT pattern to be obtained. (b) A 200-?m cube machined in the forest with this process. ....................................................................................................................... 20 Figure ?2.6: A sample waveform of discharge pulse observed during CNT machining with 30 V and 10 pF. ................................................................................................................... 21 xiv  Figure ?2.7: The electrode-forest gap clearance vs. machining voltage characterized by measuring diameters of holes drilled in a forest using a 100-?m-diameter electrode rotated at 3000 rpm for all data points. ...................................................................................... 22 Figure ?2.8: Multi-level microchannel structures patterned in a CNT forest. (a) The structures after nitrogen cleaning. (b) As-machined structures with debris before cleaning. .... 24 Figure ?2.9: Sidewalls of a channel created in a CNT forest, showing that the overall vertical orientation of the nanotubes is unaffected by ?EDM. ............................................... 25 Figure ?2.10: 3-D ?EDM of CNT forests using electrodes with cone-shaped tips performed at 35 V and 10 pF to form (a) a pyramid structure and (b) letters. Note the difference in the depth of the three letters U, B and C. ......................................................................... 25 Figure ?2.11: SEM and EDX results. (a) SEM image of the bottom surface of a hole machined with 30 V in a forest. (b) SEM image of the bottom surface of another hole machined with 80 V in the same forest. (c) EDX analysis of the surface processed at 30 V. (d) EDX analysis of the surface processed at 80 V. (e) EDX analysis of one of the particles observed on the surface processed at 80 V. ................................................. 26 Figure ?2.12: Experimental set-up for dry ?EDM of CNT forests with controlled oxygen concentrations in O2/N2 ambient. ............................................................................. 29 Figure ?2.13: SEM images of ?EDMed CNT forests in N2 gas mixed with (a) 0% O2 (oxygen free), (b) 10% O2, (c) 21% O2, (d) 50% O2. Drilling results in (e) pure N2 and (f) air. .................................................................................................................................... 30 Figure ?2.14: Progressions of electrode feeding in ?EDM of a CNT forest with different O2 concentrations in N2 at 30 V and 10 pF. .................................................................... 31 xv  Figure ?2.15: Measured average peak current of discharge pulses as a function of the O2 concentration at 30 V and 10 pF (the inset shows a typical pulse observed in 21% O2 at 30 V and 10 pF)...................................................................................................... 32 Figure ?2.16: SEM images of (a) original CNT forest before ?EDM, and ?EDMed CNT forest (b) in air, and with O2 concentrations of (c) 0%, and (d) 50% in N2 (scale bar size in each image is 200 nm). ....................................................................................................... 34 Figure ?2.17: Raman spectra of the CNT forest before and after ?EDM with different O2 concentrations in N2. .................................................................................................. 35 Figure ?3.1: The experimental set-up used for characterization of reverse polarity ?EDM of CNT forests. ........................................................................................................................ 39 Figure ?3.2: (a) Average peak current of discharge pulses measured at different voltages (with constant capacitance of 10 pF), (b) average pulse frequency at the same voltages and capacitance, and typical single pulse of discharge current generated using 20V and 10 pF with (c) normal polarity and (d) reverse polarity. A tungsten electrode with a 93-lm diameter was used for all the measurements. .................................................. 41 Figure ?3.3: Scanning electron microscope (SEM) images of micro-patterns machined in a CNT forest using 60V and 10 pF with (a) normal polarity and (b) reverse polarity. Each pattern was created by scanning a rotating electrode along a square shape (200 ?m?200 ?m) in the X-Y plane with continuous feeding of the electrode in the Z direction. .................................................................................................................... 43 Figure ?3.4: The upper two SEM images show shallow cavities machined in a CNT forest using 20V and 10 pF with (a) normal polarity and (b) reverse polarity. The lower two SEM images show micro-patterns machined in a CNT forest using 10V and 10 pF with (c) xvi  normal polarity and (d) reverse polarity, for a depth of 40 ?m with 1-?m-step electrode feeding in the Z direction. The images in (c) and (d) also show close-up views of the microstructures created in the cavities. ................................................. 45 Figure ?3.5: Electrode positions on the Z axis tracked in real time during machining with normal and reverse polarities, both using 10V and 10 pF. In both cases, the electrode was scanned along a square shape (100 ?m?100 ?m) in the X-Y plane with 1-?m-step feeding in the Z direction until reaching a depth of 40 ?m. ...................................... 47 Figure ?3.6: SEM images of patterned high aspect- ratio microstructures: (a) a cone shaped with normal polarity at 60V and 10 pF by scanning a tapered electrode along a circular orbit with 90-?m-diameter while feeding the electrode in the Z direction with 1-?m steps; (b) a cone shaped with reverse polarity at 10V and 10 pF under the same scanning/feeding conditions as in (a). The height of both cones is 120 ?m. ............. 49 Figure ?3.7: EDX analysis results for the CNT-forest surfaces machined with (a) normal polarity and (b) reverse polarity. ............................................................................................. 50 Figure ?3.8: Raman ID/IG ratios for a forest sample machined at 30V with different capacitor values. ........................................................................................................................ 51 Figure ?3.9: Experimental setup for dry ?EDM of pure CNT forest in different gas media. ........ 53 Figure ?3.10: Scanning electron microscope (SEM) images of the microstructures machined in a CNT forest in 50% SF6 and 50% O2 at 25 V with (a) the normal polarity and (b) the reverse polarity. Optical images of the tungsten electrode after machining with (c) the normal polarity and (d) the reverse polarity. ........................................................ 55 xvii  Figure ?3.11: Typical measured patterns of discharge current generated at 25 V with the reverse polarity in (a) 100% N2 and (b) 100% SF6, indicating much longer pulse duration (~70 ns) in the N2 ambient than in SF6 (~10 ns). ....................................................... 57 Figure ?3.12: SEM images of the microstructures machined in a CNT forest at 25 V with the reverse polarity in (a) 100% N2 and (b) 100% SF6. A close-up SEM image is also shown in each case. .................................................................................................... 58 Figure ?3.13: SEM images of the microstructures machined in a CNT forest: (a) 10% O2 in SF6 at 25 V; (b) 20% O2 in SF6 at 25 V; (c) 50% O2 in SF6 at 25 V; (d) 20% O2 in SF6 at 10 V; (e) 20% O2 in N2 at 25 V and (f) 20% O2 in N2 at 10 V. ...................................... 60 Figure ?3.14: (a) Average peak discharge current generated at 25 V and resultant discharge gap with different O2 concentrations in SF6. (b) Measured discharge gaps resulted from the SF6 and N2 environments (with 20% O2) and two different discharge voltages. . 61 Figure ?3.15: Electrode?s position along the Z axis with machining time for different gas media and EDM conditions measured during patterning shown in Figure  3.13. The retracting distance upon a short-circuit detection was set to 5 ?m for all the cases except for the conditions 20% O2 in N2 and SF6 at 10 V, in which the length was set to 1 ?m. ...................................................................................................................... 63 Figure ?3.16: EDX analysis results for the CNT-forest surfaces machined in SF6 with (a) 20% O2 and (b) 10% O2. .......................................................................................................... 66 Figure ?4.1: Schematic illustration of the top view of the contact switch device (top), along with the details of the SMA cantilever component with dimensions (bottom left) and a cross-sectional view of the device in the OFF state (bottom right). .......................... 72 xviii  Figure ?4.2: a) Overall optical image of the dry-processed switch device and close-up scanning-electron-microscope (SEM) images showing the SMA structure and top surfaces of the CNT forest integrated into the device. b) SEM images of the CNT forest in the device fabricated though the wet process, showing the densified structures of the CNTs that have lost their vertical alignment. ............................................................ 74 Figure ?4.3: The dependence of contact resistance on the heater driving current for (a) wet-processed and (b) dry-processed devices. c) The dependence of contact resistance on the SMA?s displacement (from the point at the cold state with full upward bending) for the dry-processed device; the inset shows an infrared image of the device under operation..................................................................................................................... 76 Figure ?4.4: Isig vs. heater driving current for the dry-processed device operated with a constant Vsig of 0.2 V, showing a dependence of Isig on the actuation level of the SMA. ........ 78 Figure ?4.5: Signal I-V relationships with approximate maximum Isig of (a) 100 mA, (b) 200 mA, (c) 300 mA, and (d) >400 mA; the last case shows a drop of the current indicating a failure of the contact. e) Collective data of contact resistance calculated from the results in (a)-(d), showing a non-linear dependence of the resistance on Isig along with a fitted curved of a power function. ........................................................................... 81 Figure ?4.6: Temporal response in contact resistance of the device operated with feedback control and a voltage waveform used to drive the heater (small steps in the waveform were attributed to non-ideal characteristics of the set-up used). ......................................... 82 Figure ?4.7: a) Trend of the ON-state resistance for over 106 cycles. b) SEM image and close-ups of two regions of the top surface of the CNT forest corresponding to the contact area (bottom) and outside of the area (top) after the 106 cycle test, showing different xix  surface textures between them and laterally oriented nanotube tips for the former case. ............................................................................................................................ 84 Figure ?5.1: (a) Cross sectional view of the switch device that integrates CNT forest contact and SMA cantilever actuator that has its contact angle of ?; and three examples in the form of the CNT-forest contact; (b) as-grown CNT forest showing the possible real contact region with the angled cantilever only at a top corner as highlighted; (c) patterned forest having inclined contact surface with the contact angle less than ? leading to partial planar contact; (d) patterned forest having inclined contact surface with the contact angle equal to ? allowing full planar contact with the cantilever. ... 89 Figure ?5.2: (a) (left) Scanning electron microscope (SEM) image of patterned CNT-forest contact with 26?-angled surface and (right) the surface profile captured with laser scanning confocal microscope (Olympus FV1000, Japan); (b) (left) overall optical image of the developed switch device and (right) close-up SEM image of the integrated SMA cantilever and sloped CNT-forest contact. ...................................... 93 Figure ?5.3: Contact resistances of the switches using patterned CNT forests with three different slope angles measured at Vsig = 2.2 V. ....................................................................... 94 Figure ?5.4: (a) Dependence of contact resistance on the displacement (measured with Isig of 10 mA). Comparisons between the switch with optimally angled CNT-forest contact and the one with bare forest contact for (b) contact resistance vs. Isig ,(c) Isig-Vsig, and (d) dissipated power vs. Isig. ............................................................................................. 95 Figure ?5.5: (a) Temporal response of the fabricated switch device showing its contact resistance and cantilever?s free-end displacement; (b) long-term trend of contact resistance for xx  ~1.4 million switching cycles demonstrating stable switching operation with an overall resistance range of 40-60 ?. .......................................................................... 97  Figure A1: Atmospheric Pressure CVD growth system of carbon nanotube. ........................... 120  Figure A2: Assembly of the silicon heater of the CVD growth system. ................................... 120  Figure B1: Schematic view of the experimental setup. ............................................................. 122  Figure B2: Field-emission from (a) ?EDMed pillar and (b) original pillar. .............................. 122  Figure B3: Machined and non-machined CNT forest pillars..................................................... 123  Figure B4: Image of electron emission from two machined CNT forest pillars with different heights: 120 ?m (dark circle) and 150 ?m (bright circle) at  (a) E=5.5?105 (V/m) and   (b) E=7?105 (V/m).. .................................................................................................. 124  Figure B5: SEM image of cone made by (a) ?EDM, and (b) photograph of field electron emission from that at E=12?105 (V/m). ................................................................... 125  Figure C1: Catalyst deposition on the Si cantilever................................................................... 128  Figure C2: (a) Tapered tungsten electrode used for ?EDM. Scale bar is 50 ?m. Illustration of the cone-shaping process at (b) the final stage of the process in which the forest is shaped to a sharp-tip cone. ....................................................................................... 129 Figure C3: SEM images of CNT probes; (a) single-step (b), (c) multi-step machining ............. 131 Figure C4: AFM images of calibration microstructures measured using (a) probe 2, (c) probe 3, and (e) commercial Si probe. Corresponding cross-sectional pro-files of multiple dimples are shown in (b), (d), and (f), respectively ................................................. 133      xxi  Acknowledgements In the Name of God, the Most Compassionate, the Most Merciful. I would like to express my deepest gratitude to Professor Kenichi Takahata, my honorable supervisor, and Professor Alireza Nojeh, my honorable co-supervisor. This work would have not been possible without the continuous and tremendous support of them.   I would like to thank the staff of the Nanofabrication Facility, the BioImaging Facility, Electron Microscopy Lab in materials engineering department, and Interfacial Analysis and Reactivity Lab (IARL) at the University of British Columbia for their help in fabricating and imaging my devices. I would like to thank Professor Edmond Cretu, Professor Lukas Chrostowski, and Professor Michael Chen for providing access to their equipment. I would like to thank Dr. Mohamed Sultan Mohamed Ali, Dr. Tanveer Saleh, Dr. Parham Yaghoobi, Dr. Mehran Vahdani Moghaddam, Alina Kupla, and Mario Beaudoin for their great comments and support. I would also like to thank my friends: Dr. Abdolreza Rashidi Mohammadi, Dr. Zhiming Xiao, Dr. Xing Chen, Babak Assadsangabi, Anas Bsoul, Min Hian Tee, Dan Brox, Mirza Saquib Sarwar, WangNing Yuan, and Muntakim Anwar for their useful comments and support. At the end, I would like to thank all those who helped me during this work whose names are not mentioned.  xxii  Dedication I dedicate this work to the ones whom I love. 1  Chapter 1: Introduction and Objective 1.1 Carbon Nanotubes  Carbon nanotubes are made by rolling an atom-thick sheet of hexagonal-oriented carbon atoms, called graphene [1]. Carbon nanotubes can be synthesized in two forms; single walled (SWNTs) or multiwalled (MWNTs) carbon nanotubes. SWNTs are made by rolling a single sheet of graphene while MWNTs consist of several concentric tubes. The diameter of the tubes ranges from few angstroms (SWNTs) [2], [3] to tens of nanometers (MWNTs). Nanotubes with lengths of up to several centimeters are reported [4], [5]. CNTs, with measured tensile strength of 63 GPa and Young?s modulus of about 4 TPa [6], [7], are considered the strongest materials yet discovered. The reported tensile strength of a single MWNT equates to that of a cable of 1 mm2 in diameter which can bear tension caused by a load of 6422 kg [8]. They are 100 times stronger than steel at one-sixth the weight [9], owing to their low density of 1.33 g/cm3 [10]. Depending on the chirality and diameter, SWNTs can be metallic or semiconducting, while MWNTs are always metallic [11], [12]. CNTs have been reported to have a high current capacity of more than 109 A cm-2 [13], [14]. Multi-walled CNTs exhibit a resistivity as low as 10-4 ? cm2 [15]. The measured thermal conductivity of 3000 W m-1 K-1 [16] for MWNTs and simulation value of 6600 W m-1 K-1 for SWNTs [17] shows the exceptional thermal properties of nanotubes. Table  1-1 summarizes the properties of nanotubes and providing a comparison with other materials [10].     2   Table ?1-1: Properties of individual carbon nanotube  Property Nanotube Comparison Tensile Strength 63 GPa  High-strength steel alloys break at about 2 GPa  Young?s Modulus 1-4 TPa  Steel: 200 MPa  Density 1.40 g/cm3  Aluminum has a density of 2.7 g/cm3  Current Carrying Capacity 109 A/cm2  Copper wires burn out at about 106 A/cm2  Thermal conductivity 6600 W/m.K  Nearly pure diamond transmits 3320 W/m?K  Temperature Stability Stable up to  2800 ?C in vacuum,  750 ?C in air  Metal wires in microchips melt at 600 to 1000 ?C  1.2 Synthesis of Carbon Nanotubes The three main synthesizing methods for fabricating carbon nanotubes are laser ablation [18], arc-discharge [1], and chemical vapor deposition (CVD) [19], [20]. In CVD, carbon nanotubes are grown from a catalyst deposited on a substrate. The direct attachment of nanotubes to substrate during the growth process reduces the extra steps of adhering them to a substrate in other growth methods. As compared to laser ablation and arc-discharge, CVD synthesizes carbon nanotubes at low temperatures (< 800 ?C) [21] and ambient pressure at high throughput. Therefore, CVD is the preferred method for growing CNTs. The working principle in the process is thermal decomposition of hydrocarbons over hot catalyst (commonly metals such as Co, Ni, or Fe). Catalyst-coated substrate is placed inside a tube and is heated to high temperatures. The hydrocarbon vapor is passed through the reaction tube. At sufficiently high temperatures carbon 3  nanotubes grow on the catalyst [19]. Hart et al. reported an affordable desktop CVD design to grow carbon nanotubes [22]. Details of the process for the growth system developed during this research are given in  Appendix A  .   1.3 Carbon-Nanotube Forest and its Applications As mentioned before, carbon nanotubes have exceptional properties. However, from the practical application point of view, employing full benefits of them in realistic systems requires a scalable approach in integration of nanotubes with MEMS devices [23]. Carbon-nanotube forest is a vertically aligned, densely packed array of CNTs with structural porosity of more than 90% [24], [25] with heights ranging from a few micrometers to several millimeters [26]?[28]. They could be either SWNT or MWNT. Throughout this thesis, all the forests used are metallic MWNT. Interesting properties such as large surface area [29] and promising electrical, mechanical, and thermal properties [7], [30]?[32] makes them a good candidate for MEMS applications. For example, high thermal conductivity of nanotubes makes them high performance thermal interface material [33]. Patterned microchannel cooling fins are used as chip-cooling heat sinks [34] due to the thermal properties and large interface surface of nanotubes. Outstanding mechanical and surface to volume ratio of nanotube forests makes them favorable in energy storage [35]. Patterned carbon nanotube towers with uniform length and diameter are used as electrochemical actuators with measured strains of up to 0.15% [36]. CNT forests are used as supercapacitors due to chemical stability, low resistivity and large surface area with a reported specific capacitance of 180 F/g [37], [38]. Other broad range of application opportunities includes thin-film electronic material [23], field-emitters [39], chip-coolers [34], biomimetic dry adhesives [40], and 3D micro-electro-mechanical devices [41]. 4   1.4 Micro-Electro-Discharge Machining 1.4.1 Introduction Conventional micromachining techniques used in MEMS are mostly based on semiconductor manufacturing processes. While surface micromachining techniques are limited to produce thin film 2D microstructures, bulk micromachining techniques, such as anisotropic wet etching and deep reactive ion etching (DRIE) are used to create 3D MEMS structures. However, these techniques are limited by material options. Successful integration of MEMS devices and materials that are not compatible with conventional MEMS micromachining techniques has been challenging. Therefore, there is a need to develop high precision, and high reproducible micromachining techniques capable of creating complex 3D structures [42]. Micro-electro-discharge machining (?EDMing) is a non-conventional, powerful machining technique, capable of machining any electrical conductive material and semiconductors. ?EDM is a non-contact process and mechanical forces are negligible; therefore, it is suitable for machining fragile materials. ?EDM is an electro-thermal process which utilizes controlled sparks generated between a microscopic electrode and a workpiece [42]. The miniaturized arc discharge locally melts and evaporates the material at the arc spot, and micromachining is performed by repeating the unit removal by a single pulse at high frequencies while controlling the relative position between the electrode and the workpiece. Typical ?EDM process for bulk materials is conducted in a dielectric liquid (such as oil or de-ionized water). Details of the process are presented in section  1.4.2.  5  1.4.2 Process Description Figure  1.1 shows the principle of ?EDM. As mentioned earlier, material removal is based on electro-thermal removal. High voltage is applied between the electrode and the workpiece (Figure  1.1 (1)). The two electrodes are electrically insulated by a dielectric medium between them. The dielectric is usually oil, but other mediums such as DI water [43] and air [44] are also reported. Having applied the voltage, electrode feeds toward the workpiece. This increases the electric field present between the electrode and the workpiece. At a certain gap distance, the electric field overcomes the breaking voltage of the medium and an electrically conductive channel (plasma) forms at the closest points between the electrodes (Figure  1.1 (2)). It is reported that the measured temperature of the plasma ranges between 8000 -10000 K [45]. High energy electrons are emitted from the cathode (tool) and hit the anode (workpiece). On the other hand, positively charged ions travel towards the cathode. Bombardment of the workpiece with electrons melts the workpiece locally and creates a crater-like cavity (Figure  1.1 (3)). As it is shown, the removal happens not only on the workpiece, but also on the tool side as well (tool wear). That is because, as the electrons and negatively charged particles hit the anode, positively charged particles hit the cathode. However, since electrons are lighter than positively charged particles, during the short time of one spark (several tens of ns to few ?s), number of positively charged particles hitting the cathode is less than the number of electrons that hit the anode. Therefore, material removal rate of the workpiece is higher than the tool wear. It should be noted that as the duration of a single discharge increases (formation of arc instead of spark), positively charged ions have enough time to melt the cathode and tool wear increases, which is not desired for the ?EDM process. Therefore, switching off the power after a single spark is critical to prevent formation of arc. As soon as the power shuts down the conducting channel starts to 6  disappear and the ionized dielectric medium starts to recover its insulating properties and the remaining debris are washed away by the liquid (Figure  1.1 (4)). It is necessary to have enough rest time, so that the medium can recover its insulating properties and the debris are washed away from the machining area, making it ready for the next spark (Figure  1.1 (5)). However, too long rest time decreases the discharge frequency and increases the machining time. The ?EDM machine has a feedback control system such that when a short circuit between the electrode and the sample (which prevents the discharge generation, i.e., material removal) is detected, the system retracts the electrode up while checking the status of the short circuit and resumes machining by feeding the electrode as soon as the circuit is opened. The pulse generator circuit is mainly RC-relaxation type (Figure  1.2), since it can deliver large amount of currents with short duration [46]. The amount of stored energy during the charge cycle depends on the values of V and (C+Cp). Cp models the parasitic (stray) capacitance of the circuit and exists between the wirings, electrode and the workpiece, electrode holder and the workpiece stage. However, the user can define the values of V and C. During the discharge cycle, most of the stored energy is delivered to the workpiece.  Figure ?1.1: Principle of one cycle of ?EDM process. 7   Figure ?1.2: RC relaxation type pulse generator of ?EDM  The standard tool used in ?EDM is cylindrical rods. This tool can be shaped into complex forms using wire electro-discharge grinding (WEDG). Machining principle is essentially the same as ?EDM, but with reverse polarity. A wire is fed continuously inside a groove and along a wire guide while the ?EDM tool is fed from the above and is machined (Figure  1.3).   Figure ?1.3: 3D- and side- view of WEDG.  8  1.5 Micropatterning of Carbon Nanotubes As mentioned earlier, a key in facilitating MEMS applications of a material is the ability to pattern the material in a batch mode with high precision and high reproducibility. It was also mentioned that CNT forests, due to their exceptional properties, are good candidates to be integrated with MEMS devices. However, to design and fabricate devices based on CNT forests, there is a fundamental need to define the dimensions of the forests precisely. CVD growth of patterned CNT forests has been implemented using pre-patterned catalyst layers defined by photolithography [47], [48], electron beam lithography [49], [50], soft mask [51], [52], and laser etching [24]. Patterning CNT forests during or after the growth using shadow mask [53], template growth (mold) [54], laser etching through mask [55], and densification [56], [57] has been presented. The mentioned techniques are, however, primarily for the formation of two-dimensional types of patterns (with uniform heights). Laser micromachining has been reported to shape CNT forests for different applications [34], [58]?[60] while exhibiting inherent limitations including tapered sidewalls, lack of high-precision depth control, and thermal damage [61]. Zhu et al. used scanning localized arc discharge lithography to locally truncate carbon nanotube forests [62]. The authors did not mention their machining resolution, but reported truncation and unraveling of nanotubes due to the passage of electrical current. Generally, arc machining is not very precise due to overheating the machining area, while spark machining (?EDM) is a precise machining by protecting the machining area from excessive heating [46]. Table  1-2 summarizes the machining techniques to pattern carbon nanotubes.     9  Table ?1-2: Summary of micropatterning carbon nanotubes techniques Technique Geometry Complexity Limitations Patterned catalyst before the growth [24], [47]?[53] 2D structures with uniform height  No complex 3D structures.  Template growth [54] 2D structures with uniform height  No complex 3D structures.  Laser etching through mask [55] 2D structures with uniform height  Structure is limited by the shape of mask. No complex 3D structures. Expensive.  Controlled densification [56], [57] 3D structures with pre-defined 2D base  Densification damages the morphology of nanotube arrays. No complex 3D structures.   E-beam (in SEM, STM, AFM) [63]?[65] Cutting individual nanotubes. Applicable only to individual nanotubes. Laser micromachining [34], [58]?[60] 3D structures  Lack of high-precision depth control. Thermal damage. Expensive.  Scanning localized arc discharge lithography [62] 3D structures  Not very precise and thermal damages due to the arc currents. Machining resolution is not mentioned.    1.6 Objectives of the Research As discussed earlier, techniques to micropattern carbon nanotubes are limited by their capability to produce complex 3D structures, the amount of thermal damage introduced to the sample, and machining resolution. To fill this gap and facilitate the application of CNTs to MEMS and other disciplines, it is essential to establish techniques to create 3D free-form microstructures from pre-grown forests. This research focused on developing a micromachining 10  technique to answer the mentioned challenges. The main objective of this work was to investigate the feasibility and characteristics of micro electro-discharge machining, as a powerful, high precision, and high reproducible machining technique, of carbon nanotube forests and its applications. For the feasibility and characterization sections, the focus was on developing the optimum machining conditions, such as machining energy, speed, and environment. Producing high-aspect-ratio, complex free-form 3D structures are demonstrated. Afterwards, methods to increase the precision of the machining process were investigated. Different approaches were taken to pursue this. Concerning the applications, developing high-power MEMS switches which integrate micropatterned carbon nanotube forests as electrical contact and shape memory alloy as actuation part was mainly considered. Moreover, micropatterned CNT forests as field emitters and AFM probe tips were developed.  1.7 Potential Impact of the Research Carbon nanotubes with exceptional mechanical, electrical, chemical, optical and other properties are excellent candidates for MEMS and other applications. However, a major obstacle in integrating this material with MEMS and other devices is the limitation in post-growth patterning of nanotubes. Micro electro-discharge machining is an affordable, compatible to batch mode, and clean fabrication technique, developed to produce complex structures which are difficult and sometimes impossible to produce with other micro machining techniques. The micropatterning technique developed in this thesis provides a strong tool to shape CNT forests to complex free-form 3D microstructures for MEMS and other applications. The precision ?EDM provides is not achievable with other fabrication methods. Potential applications include gas 11  sensors, electrical vias and contact, optical and mechanical switches, microfluidics, field emitters and AFM probes.  1.8 Research Methodology Three milestones were defined to achieve the goals of this research: a) demonstrating the feasibility of ?EDM of CNT forests; b) developing techniques to increase the precision of the process; c) demonstrating the application of the process in a MEMS switch device. In order to achieve these milestones, the following approaches were taken:  A. Feasibility In the initial stage of the research, the focus was on feasibility of micro-electro-discharge machining of CNT forests. Dry and wet ?EDM of CNT forests with different conditions were carried out. Optimum conditions for the dry process were then demonstrated. Limitations of the machining tolerance in the process were also described. Furthermore, micropatterned 3D structures were produced to show the effectiveness of the process. Finally, elemental analyses were carried out to describe the level of possible cross-contaminations of the process.   B. Mechanism of Dry ?EDM of CNT Forests Having shown the feasibility of the process, the working principle of dry micro-electro-discharge machining of CNT forests was investigated. The experiments were to investigate the effect of oxygen in the process. Machining was carried out in a controlled environment with pre-defined ratios of oxygen/nitrogen. A different mechanism was suggested to be valid for 12  ?EDMing of CNT forests rather than normal mechanism known for typical EDM process. Elemental and molecular analyses were also carried out to investigate the level of possible cross-contaminations in the process.  C. Field-Emission-Assisted ?EDM of CNT Forests This step modified the conventional configuration of electro-discharge machining and investigated dry ?EDM of CNT forests assisted by field emission properties of carbon nanotubes. Discharge currents were measured for both conventional and proposed configurations (reverse configuration). The reverse machining made it possible to machine at lower discharge currents; therefore, the machining tolerance was improved. Finally, elemental and molecular analyses were carried out to investigate the cross-contaminations involved in the process.   D. The Effect of Using Sulphur-Hexafuoride (SF6) in Dry ?EDM of CNT Forests This step, similar to step C, was to find a method to improve the machining tolerance of dry ?EDM of CNT forests.  Use of a high dielectric gas such as SF6 was suggested to achieve this goal. Moreover, discharge current waveforms were measured for different SF6/O2 mixture ratios. Lower Machining voltage was also made it possible for using SF6 in reverse machining, leading to finder micropatterning. Finally, elemental analyses were carried out to investigate the level of possible cross-contamination during the process.  E. Integrating CNTs into a High-Power MEMS Switch This step involved applying carbon nanotube forests in MEMS devices. To do so, a high power MEMS switch device was developed. The switch is thermally actuated by using a shape 13  memory alloy. CNT forests were then used as contact material of the switch. The results revealed that combination of CNT forests and shape memory alloy actuators could be a promising path to realize reliable MEMS contact switches for high-power applications. It is also worth mentioning that bare CNT forests were used in this step. Micropatterned CNT forests with ?EDM will be pursued in the next step.  F. Effects of Using 3D Micropatterned CNT Forests in MEMS Switches In this phase, the focus was on 3D micropatterning CNT forests integrated with MEMS switches. CNT forests with different 3D profiles were patterned and compared. Furthermore, improvement of switch parameters reported in step E was shown.  1.9 Thesis Overview This thesis is divided into 6 chapters. The Introduction, Objectives and Research Methodology of this study are discussed in chapter 1. Chapter 2 mainly focuses on the feasibility of ?EDM of CNT forests. Characterization and working principle of dry micro-electro-discharge machining of CNT forests are also discussed in the same chapter (A and B in section  1.8). Chapter 3 will be a discussion on improving the precision of the process. Field-emission-assisted ?EDM and the effect of using SF6 are also going to be discussed in the same section (C and D in section  1.8). In chapter 4, a MEMS switch integrated with bare CNT forest and shape memory alloy is presented (E in section  1.8). The final chapter, Chapter 5 is going to focus on 3D micropatterned CNT forests to be used in high power MEMS switches (F in section  1.8).   14  Chapter 2: Micro-Electro-Discharge Machining of Carbon Nanotube Forests; Feasibility and Characterization In this chapter, first, the feasibility of micro machining carbon nanotube forests using micro-electro-discharge machining will be reported. 3D free-form micro-structures with high-aspect-ratios will be demonstrated next. Characterizations of the process for different machining conditions will be also discussed. Furthermore, machining tolerance is going to be reported for different machining conditions by investigating the discharge gap. Also, cross-contaminations during the process will be investigated by analyzing the elemental analyses. Later on in the chapter, there will be a report on the characterization of the dry machining process with respect to the machining gas ambient. Controlled environment with pre-defined ratios of N2/O2 will be explained provided for different machining conditions. Finally, a different machining mechanism is going to be suggested for ?EDMing of CNT forests rather than the typical melting and removal principle in the ?EDM process.    2.1 ?EDM of CNT Forests; Feasibility1 2.1.1 Introduction Micro-electro-discharge machining of carbon nanofibers [14, 15] as well as DC arc discharge machining of CNT forests [16] have been reported for surface patterning of the materials with                                                  1 A portion of this section has been published in a peer-reviewed journal (Reused with permission from ?W. Khalid, M. Sultan Mohamed Ali, M. Dahmardeh, Y. Choi, P. Yaghoobi, A. Nojeh, and K. Takahata, ?High-aspect-ratio, free-form patterning of carbon nanotube forests using micro-electro-discharge machining?, Diamond and Related Materials, 19(11), 1405, 2010.?, Copyright (2010), with permission from Elsevier) [66]. Parts of this section were also presented in a proceeding (Reused with permission from M. Dahmardeh, W. Khalid, M.S. Mohamed Ali, Y. Choi, P. Yaghoobi, A. Nojeh, and K. Takahata, High-aspect-ratio, ?3-D micromachining of carbon-nanotube forests by micro-electro-discharge machining in air?, Proceeding IEEE MEMS, Mexico, Jan 23-27, 2011, pp. 272 - 275?, Copyright ? 2011 IEEE) [67]. 15  low aspect ratios. In this section pulsed ?EDM process for 3-D, high-aspect-ratio micromachining of pure CNT forests is presented [66], [67]; this technique is shown to be highly effective in forming arbitrary micro-scale geometries in the forests.  2.1.2 Sample Preparation and Experimental Set-up CNT forest samples used in ?EDM experiments were prepared as follows: First, a 10-nm-thick layer of aluminum was evaporated on a highly-doped silicon wafer (<100> n-type, resistivity 0.008?0.015 ? cm). Subsequently, a 2-nm-thick layer of iron was deposited. CNT growth was performed in an atmospheric-pressure CVD system. In a typical growth process, after loading the sample, the temperature was ramped up from room temperature to 750 ?C in 20 min while maintaining a flow of 500 sccm of hydrogen and 200 sccm of argon in the reaction tube. The sample was then annealed for 3 min under these flow conditions at 750 ?C. Subsequently, flow rates of 50 sccm of ethylene, 40 sccm of hydrogen, and 75 sccm of argon were used for 90 min at this temperature for CNT growth, before cooling the samples down to room temperature again. Forests of vertically aligned multi-walled were obtained with lengths of up to several hundreds of micrometers. ?EDM setup for machining CNT forests is shown in Figure  2.1.   Figure ?2.1: ?EDM setup for machining CNT forest in air. 16  2.1.3 Results and Discussion Typical ?EDM process is conducted in a dielectric liquid (such as oil or de-ionized water). However, due to the shrinkage/densification of CNT forests by submerging them in liquid caused by liquid capillary force [68], [69], there is a possibility of damaging the CNT forests. To better illustrate this effect Figure  2.2 a shows cylinders of nanotube forests with 300 ?m diameter before and after submerging in EDM oil. The inset shows a close-up of the tip of a shrunk forest. Figure  2.2 (b)-(d) show a block of CNT forest submerged in acetone and is then left to dry at room temperature. It can be seen that the forests shrank from the original volume. The observed cracks seem to be a purely physical phenomenon related to this effect induced by the capillary force.  In order to avoid the above shrinkage effect in bare CNT forests, experiments were also performed in air. It was observed, however, that processing with the same parameters (80 V, 120 pF) that enabled good patterning and shapes in oil (before drying) caused the local destruction of CNTs with uncontrolled large sparks in air, resulting in non-uniform surfaces and poor sharpness in the structures. The voltage and capacitance were then substantially lowered to 30?35 V and 10 pF, respectively, which produced highly promising results. Figure  2.3 (a) compares holes drilled at 80 V and 30 V (both with 10 pF) in air, indicating sharper edges and smoother surfaces obtained at 30 V compared with those at 80 V. Note that the bottom of both holes are defined by the tips of CNTs shortened by the machining process. As a control experiment, holes were also created with 0 V, i.e., mechanically drilling the CNT forest, both with and without rotation of the electrode (Figure  2.3 (b)); it can be seen that the drilled portion is simply dislocated downward and remains within the hole. The results in Figure  2.3 thus clearly show the effect of removal by an EDM mechanism. 17   Figure ?2.2: Shrinkage effect in CNT forest after submerging in liquid; (a) before and after dipping in EDM oil, (b) SEM image of a dried CNT forest block after submerging in acetone, (c) top view of the CNT forest block shows the shrinkage volume, (d) close-up SEM image showing CNT forest roots are detached from the substrate due to the shrinkage force.  In order to systematically investigate the effect of voltage on the patterning of CNT forests, experiments were performed using 80, 30, 20 and 10 V with 10 pF as process parameters. Figure  2.4 (a), (b), and (c) show the contrast when the same pattern (500-?m?350-?m X?Y scanning for a depth of 100 ?m) was machined with 80, 30 and 10 V, respectively, using a 150-?m-diameter electrode.  18   Figure ?2.3: (a) Hole structures created in a CNT forest using EDM with 80 V and 30 V. (b) Hole structures made by mechanically drilling the forest using the same electrode with and without electrode rotation at 3000 rpm.   Figure ?2.4: SEM images with the same magnification showing ?EDM results for the same pattern created with (a) 80 V, (b) 30 V, and (c) 10 V.   19  The forest sample was continuously moved along the rectangle pattern using the X?Y stage while feeding the electrode until it reached the target depth. It can be seen in Figure  2.4 (a) that machining of CNT forests in air at 80 V led to a distorted structure due to large discharge sparks similar to the result in Figure  2.3 (a). In contrast, processes with 30 V produced very fine and stable discharge pulses, resulting in well-controlled CNT removal as can be seen in Figure  2.4 (b). The results with 20 V were similar to those with 30 V. At 10 V, machining exhibited signs of mechanical grinding (Figure  2.4 (c)). Figure  2.5 (a) shows a machining process using a 300-?m-diameter cylindrical electrode at 35 V and 10 pF for forming a square pattern in a CNT forest, showing light emission from discharge pulses at the interface between the electrode bottom and the CNT surface. The cubic structure (approximately 200 ?m on all sides) in Figure  2.5 (b) was obtained under these conditions. Figure  2.6 shows a typical waveform of a discharge pulse generated at 30 V and 10 pF measured using a current probe (CT-1, Tektronix, Inc., USA) inserted in the discharge circuit as shown in Figure  2.1. The measurement result indicates that a pulse with the peak current and pulse duration of approximately 60 mA and 32 ns flows through the CNT forest during the discharge. The peak current is about 1?2 orders of magnitude smaller than those seen in conventional ?EDM. This is related to the discharge pulse energy defined by the machining condition, which is expressed as CV2/2, where C is the capacitance of the R-C circuit and V is the machining voltage, if parasitic capacitances are neglected [70]. With this, a theoretical energy value of 4.5 nJ is calculated with 30 V and 10 pF for machining CNT forests, which is approximately 85 times smaller than that with 80 V and 120 pF, a typical combination used in ?EDM of conventional materials.  20   Figure ?2.5: (a) Optical images during ?EDM of a CNT forest. A rotated 300-?m-electrode is scanned along a 500-?m?500-?msquare orbit in the X?Y plane. The inset in each image shows the top view of the electrode, its orbit (broken line), and the square CNT pattern to be obtained. (b) A 200-?m cube machined in the forest with this process.  An important consideration is the gap created between the EDM electrode and the machined structure due to the discharge process, since this gap affects the final dimensions of the structure. 21   Figure ?2.6: A sample waveform of discharge pulse observed during CNT machining with 30 V and 10 pF.   In general, smaller gaps are preferred for achieving higher precision and tighter tolerances. The dependence of this gap on the voltage was characterized by measuring the diameter of holes, all of which were drilled using a 100-?m-diameter electrode rotated at 3000 rpm. Figure  2.7 plots the average gap distance, G, calculated using the measured diameter of the hole, DH, and that of the electrode, DE as G = (DH ? DE)/2, at various voltages. No measurable change in DE due to the electrode wear was observed (DE ? 100 ?m). The result shows the nonlinear increase of the gap with voltage, and also shows that the gap is around 10 ?m at the optimal voltage of 30 V. Note that the effective discharge gap can be smaller than this value as any wobbling of the rotating electrode due to non-idealities in WEDG shaping will make the effective diameter of the electrode during the process larger than DE. Nevertheless, it can be seen that the measured gaps are smaller by factors of 3?5 than the results reported in [71] if the gaps at the same voltages (60?110 V) are compared. This may be related to a difference in the 22  capacitance used (10 pF plus parasitic capacitance in the present characterization, as opposed to parasitic capacitance only in the above report), and/or structural differences between the nanofibers involved in the above report and the nanotubes targeted in the present work.   Figure ?2.7: The electrode-forest gap clearance vs. machining voltage characterized by measuring diameters of holes drilled in a forest using a 100-?m-diameter electrode rotated at 3000 rpm for all data points.  To demonstrate the ability of this technique in 3-D patterning, multi-level, complex micro channels were created in CNT forests. A 50-?m-diameter electrode with 35 V and 10 pF was used to shape the channel structures with depths of 50, 100 and 150 ?m (Figure  2.8). The 100-?m-deep channel structures achieved a minimum feature size of ~5 ?m (Figure  2.8 (a)), corresponding to an aspect ratio of 20. Even after machining of narrow channel structures, the orientation of aligned nanotubes was intact as can be seen in Figure  2.9. The debris on the 23  patterned structures observed after the process (Figure  2.8 (b)) were easily removed by gently blowing nitrogen onto the sample. The structures in Figure  2.8 (a) were cleaned by this method; no damage in the structures, including the high-aspect-ratio 5-?m feature, was observed. Another important issue in 3-D patterning is the creation of angled surfaces. This was demonstrated using the electrodes whose shapes were customized by WEDG. Figure  2.10 (a) shows a pyramid structure machined using an electrode with a cone-shaped tip. The tip of the pyramid has an approximately 38?38-?m2 area and a 130-?m height. The cone shaped electrode was also used to pattern letters of the alphabet on a CNT forest (Figure  2.10 (b)). The depths of the letters U, B and C are 120, 150 and 50 ?m, respectively. A zoomed view of the middle section of the ?B? structure seen in Figure  2.10 (b) shows sharp edges and smooth, angled surfaces formed in the CNT forest. Surface analysis of machined forest structures was performed using a scanning electron microscope (SEM) with an energy-dispersive X-ray (EDX) spectroscopic analyzer (Hitachi S-3000 N). The bottom surfaces of 50-?m-deep holes drilled at 30 V and 80 V (both with 10 pF) in the same forest with a height of several hundreds of microns were characterized for this purpose. The EDX analyses (Figure  2.11 (c)?(e)) show a high level of silicon in addition to carbon; this is most likely due to the presence of the substrate below the forest. Low levels of catalytic materials (iron was detected but not visible in the plots) as well as oxygen were also detected. At 30 V (Figure  2.11 (c)), tungsten, the electrode material, was not detected on the surface, suggesting almost zero consumption of the electrode. At 80 V (Figure  2.11 (b)), the machined surfaces were observed to have submicron-/nanoscale particles. Another EDX focused on the particles revealed that these particles are a compound of tungsten and carbon (Figure  2.11 (e)). This indicates that at high voltages, the discharge causes some consumption of the electrode at the tip (mainly on the bottom surface, as no measurable diameter change was observed), and 24  tungsten melted from the electrode and carbon removed from the forest are fused to form these particles. It can be seen in Figure  2.11 that the surface processed at 30 V (Figure  2.11 (a)) is smoother and denser than the surface at 80 V (Figure  2.11 (b)), which exhibits a texture closer to that of the original forest. This dense surface with 30 V is likely because the carbon removed from the forests tends to remain inside the hole and be spread over the bottom by the rotating electrode when a lower voltage (i.e., lower discharge energy) is used. This hypothesis is supported by the results seen in Figure  2.3 (a), which shows carbon debris around the hole machined at 80 V but much less particles when 30 V is used. Moreover, the relatively weaker EDX signal of the silicon substrate in the 30-V case (Figure  2.11 (c)) compared to that in the 80-V case (Figure  2.11 (d)) can be attributed to the denser surface of the former, as it can attenuate the substrate signal more.    Figure ?2.8: Multi-level microchannel structures patterned in a CNT forest. (a) The structures after nitrogen cleaning. (b) As-machined structures with debris before cleaning.  25   Figure ?2.9: Sidewalls of a channel created in a CNT forest, showing that the overall vertical orientation of the nanotubes is unaffected by ?EDM.   Figure ?2.10: 3-D ?EDM of CNT forests using electrodes with cone-shaped tips performed at 35 V and 10 pF to form (a) a pyramid structure and (b) letters. Note the difference in the depth of the three letters U, B and C. 26   Figure ?2.11: SEM and EDX results. (a) SEM image of the bottom surface of a hole machined with 30 V in a forest. (b) SEM image of the bottom surface of another hole machined with 80 V in the same forest. (c) EDX analysis of the surface processed at 30 V. (d) EDX analysis of the surface processed at 80 V. (e) EDX analysis of one of the particles observed on the surface processed at 80 V.  The difference in the degree of debris ejection from a machining gap can be related to the magnitude of pressure waves that are caused by thermal expansion of air at the gap induced by discharge pulses, i.e., the smaller the voltage or discharge energy, the lower the ejection pressure hence more debris tend to stay in the hole. Thus, for fine machining with lower energies, debris removal during the process is anticipated to be a key factor for deep or high-aspect-ratio drilling, 27  which will need further investigation. It is worth noting that the situation should be different when the electrode is scanned horizontally, because there are more paths for removed carbon atoms to be ejected from the machining gap compared to the hole-drilling case where the electrode tip is fully enclosed by machined sidewalls of the forest. In fact, the scanned results obtained at 30?35 V in Figure  2.4 (b) and Figure  2.8 (b) show ejected debris that have been left on the machined structures. This suggests that machining that creates some open space around the electrode may lead to more debris removal from the machining area, which may aid in achieving deeper machining compared to the hole-drilling case.  2.2 ?EDM of CNT Forests; Effect of Oxygen 2.2.1 Introduction This section2 investigates the working principle of dry micro-electro-discharge machining of vertically aligned carbon-nanotube forests by evaluating the effect of oxygen on the process. The machining experiments with controlled oxygen/nitrogen ratios indicate a correlation between the peak current of discharge pulses and the oxygen concentration, suggesting not only a vital role for oxygen in the process, but also a removal mechanism fundamentally different from that in typical electro-discharge machining based on direct melting and evaporation of the sample material. The highest surface quality and uniformity in the machined forest microstructures as well as smooth machining without short circuiting are achieved at an approximate oxygen concentration of 20% under the discharge condition of 30 V and 10 pF, revealing that air is an                                                  2 A version of this chapter has been published in a peer-reviewed journal (Reused with permission from ?M. Dahmardeh, A. Nojeh, and K. Takahata, ?Possible mechanism in dry micro-electro-discharge machining of carbon-nanotube forests: A study of the effect of oxygen?, Journal of Applied Physics, 109(9), 093308, 2011?, Copyright ? 2011, AIP Publishing LLC) [82]. 28  optimal medium for the removal process. Elemental and molecular analyses show no evidence of significant crystalline deterioration or contamination in the nanotubes processed with the technique. 2.2.2 Sample Preparation and Experimental Set-up The CNT forest samples used in ?EDM experiments were prepared as follows: First, a 10-nm-thick layer of aluminum was evaporated on a highly-doped silicon wafer (<100> n-type, resistivity 0.008-0.015 ? cm). Subsequently, a 2-nm-thick layer of iron was deposited. CNT growth was performed in an atmospheric-pressure CVD system. In a typical growth process, after loading the sample, the temperature was ramped up from room temperature to 750?C while maintaining a flow of 500 sccm of hydrogen and 200 sccm of argon in the reaction tube. The sample was then annealed for 3 min under these flow conditions at 750 ?C. Subsequently, flow rates of 50 sccm of ethylene, 40 sccm of hydrogen, and 75 sccm of argon were used for 90 min at this temperature for CNT growth, before cooling the samples down to room temperature again. Forests of vertically aligned multi-walled CNTs (as characterized by scanning and transmission electron microscopy) were obtained with lengths of up to several hundreds of micrometers. Various samples with lateral dimensions as large as a few centimeters were grown [67]. ?EDM experiments were carried out with a 3-axis ?EDM machine (EM203, SmalTec International, USA) that employed relaxation-type resistor-capacitor (R-C) circuitry for pulse generation/timing [72]. As opposed to the DC arc technique [62], this ?EDM process that uses nanosecond pulses of arc discharge for machining potentially enables precise control of discharge energy delivered to a CNT forest, while protecting the sample from overheating [46]. The experimental set-up used for dry ?EDM characterization is shown in Figure  2.12. Oxygen is first mixed with nitrogen, an inert dilute gas, inside a buffer chamber, and the mixed gas is 29  introduced to the machining chamber, where the O2 concentration is measured using an oxygen sensor (VN202, Vandagraph Co., UK).  The flow rates of O2 and N2 are adjusted so that the O2 concentration reaches the target value and is stabilized in the machining chamber for at least 10 minutes prior to machining. In this study, the O2 concentrations of 0% (oxygen free), 6%, 10%, 21% (approximately equal to the ratio in air), and 50% were tested. A tungsten electrode (32-100 ?m diameter) and the CNT forest are connected as the cathode and the anode, respectively, with the R-C circuit as shown in Figure  2.12. The electrode feed rate in the vertical (Z) direction was set to be 0.5 ?m/s.  The machining experiments were conducted with an optimal condition (machining voltage 30 V; capacitance 10 pF; electrode rotation speed 3000 rpm; X-Y feed rates 1 mm/min) that we have previously developed for ?EDM of CNT forests in air [66].    Figure ?2.12: Experimental set-up for dry ?EDM of CNT forests with controlled oxygen concentrations in O2/N2 ambient.  30  2.2.3 Results and Discussion Figure  2.13 (a) - (d) show the microstructures machined with different O2 concentrations and the machining conditions noted above, by scanning a 100-?m-diameter electrode along a rectangular pattern (300 ?m ? 400 ?m) in the X-Y directions while feeding the tool in the Z direction to a depth of 40 ?m. The results indicate that the process at 0% O2 [Figure  2.13 (a)] resulted in the lowest removal quality, and that the uniformity and the surface smoothness of the machined structures were consistently improved as the O2 concentration was increased to 21% [Figure  2.13 (c)]. As can be seen in the drilling results in N2 [Figure  2.13 (e)] and in air [Figure  2.13 (f)] performed under the same EDM conditions, the CNT removal in air is as effective as the 21% O2 case.   Figure ?2.13: SEM images of ?EDMed CNT forests in N2 gas mixed with (a) 0% O2 (oxygen free), (b) 10% O2, (c) 21% O2, (d) 50% O2. Drilling results in (e) pure N2 and (f) air.  Machining in 50% O2 [Figure  2.13 (d)], however, led to rougher surfaces compared to those at 21% O2 as apparent from the SEM images. To visualize the progression of electrode feeding 31  and the impact of oxygen on the feeding, the Z position of the electrode was tracked during drilling of a forest to a depth of 50 ?m with different O2 concentrations [Figure  2.14]. The results with the 21% and 50% O2 concentrations showed smooth feeding with no short-circuit detection, reaching the target depth within the ideal machining time of 100 seconds (i.e. (50 ?m)/(0.5 ?m/s)). As the concentration was lowered, zigzag patterns appeared, due to the occurrence of short circuits and the resultant controlled retraction of the electrode. It is clearly seen that lowering the O2 concentration below 21% deteriorates the machining efficiency. For the oxygen-free case, machining beyond 10 ?m of depth was not achieved.   Figure ?2.14: Progressions of electrode feeding in ?EDM of a CNT forest with different O2 concentrations in N2 at 30 V and 10 pF.  To probe the EDM dependence on oxygen, the discharge current was measured under identical EDM conditions (30 V, 10 pF) with varying O2 concentrations using a current probe (CT-1, Tektronix, USA) inserted in the discharge circuit as shown in Figure  2.12. Figure  2.15 plots the average peak current of discharge pulses (n=600) measured as a function of the O2 32  concentration. It can be seen that the average peak current drops as the O2 concentration increases and saturates (~13 mA) at around 21% O2. This saturation is likely related to the results in Figure  2.14 that shows similar straight feeding paths for the O2 concentrations of 21% and 50%. The discharge current is the highest (~23 mA) at 0% O2 while proper machining at this condition is barely feasible. This condition does not follow the typical relationship between the discharge current and the material removal rate in regular EDM, in which larger discharge currents lead to higher removal rates in general.  It has been suggested that the lower current carrying capacity of MWNTs in the presence of oxygen is mainly because of the loss of individual carbon shells due to thermal oxidation [73], [74].    Figure ?2.15: Measured average peak current of discharge pulses as a function of the O2 concentration at 30 V and 10 pF (the inset shows a typical pulse observed in 21% O2 at 30 V and 10 pF).  33  This characteristic is consistent with the measured result in Figure  2.15 that shows the highest current in the absence of oxygen and little removal of CNTs. Based on the results observed, the removal of CNTs in the ?EDM process may be related to the thermally enhanced oxidation, rather than direct melting/evaporation due to heat provided by the discharge pulses as the typical removal mechanism in EDM ? in other words, this CNT ?EDM may essentially be a pulsed process of local oxygen plasma etching of the nanotubes.  To evaluate the structures of the processed nanotubes, high-resolution scanning electron microscopy (SEM) was performed for the surfaces of the machined structures shown in Figure  2.13 and for an unprocessed area near the structures [Figure  2.16 (a)-(d)]. For low concentrations of O2 (up to 21%), some of the CNTs developed sharp tips (indicated by arrows in Figure  2.16 (c)), resembling the needle-shaped bundles of CNTs after plasma etching reported in [75], while other CNTs exhibited different morphologies with rougher surfaces compared to those of the original CNTs with smooth surfaces (similar results were reported in [76]). For 50% O2, thickening of CNTs is evident; this could be related to the formation of thicker bundles of individual CNTs and/or the adsorption of the carbon debris produced in the machining process [66].  The analysis of the machined samples with energy-dispersive X-ray spectroscopy (EDX) showed no noticeable difference between the results with different O2 concentrations. Tungsten was not observed in the EDX results, suggesting negligible electrode consumption during the ?EDM process under the employed machining conditions. In order to evaluate the impact of ?EDM on the crystalline properties of the CNTs, Raman spectra were collected from the ?EDMed regions in the structures shown in Figure  2.13 (a), (c), and (d) machined in the forest 34  with the O2 concentrations of 0%, 21%, and 50%, respectively, as well as from the original CNTs in the same forest for comparison [Figure  2.17].  Figure ?2.16: SEM images of (a) original CNT forest before ?EDM, and ?EDMed CNT forest (b) in air, and with O2 concentrations of (c) 0%, and (d) 50% in N2 (scale bar size in each image is 200 nm).  The G mode that arises from the sp2 C crystalline structures and the D mode related to crystalline defects [77] are seen in the collected data. As shown in the graph, the ID/IG ratios in the original and ?EDMed forest surfaces are very close (0.76-0.79), suggesting that the impact of ?EDM on the CNT?s crystalline properties under the used conditions is minimal. The DC arc discharge machining of CNT forests has been reported to lower the ID/IG ratio [62]. It is also known that high-density oxygen plasma treatment of CNTs causes defects in them [78]. The 35  Raman analysis in the present study provided no evidence of significant ID/IG ratio reduction or increased defects in the CNTs processed with the pulsed ?EDM used.    Figure ?2.17: Raman spectra of the CNT forest before and after ?EDM with different O2 concentrations in N2.  A closer look at Figure  2.17 reveals that for the oxygen-free (0%) condition, the G and D peaks are shifted by ~22 and ~12 cm-1 from the peaks (at 1570 and 1340 cm-1, respectively) for the non-zero O2 concentrations to the higher wavenumber side. The shifts were observable in Raman data from some locations on the surfaces machined under the oxygen-free condition but were not consistent over the entire area of the surfaces. It has been suggested that the compressive thermal strain causes shifts of the G-mode peak [79]?[81]. In the machining 36  process, uncontrolled spontaneous large sparks were occasionally observed at low concentrations (0-10%) of oxygen ? the observed Raman shifts in the oxygen-free case could be related to high current stressing of the carbon nanotubes caused by the large sparks.  2.2.4 Conclusions It was the main concern of this stage to investigate the effect of oxygen in dry ?EDM of CNT forests. The experimental results revealed that oxygen plays a critical role in the removal process, suggesting that localized oxygen plasma etching induced by pulsed arc discharges, unlike the direct thermal removal in regular ?EDM, may be the main removal mechanism of the process. It was found that the use of oxygen-free, 100%-nitrogen ambient prevents proper CNT removal. The process at the O2 concentration of ~21% in nitrogen achieved not only the highest machining quality among all O2 concentrations tested but also efficient CNT removal without suffering from short circuiting during the process. This suggests that air may be a suitable, and in fact optimal, medium for dry ?EDM of the forests. Furthermore, thickening of the tips of the processed CNTs was observed, which could be related to the adsorption of the removed carbon atoms and bundling of the CNTs. The Raman and EDX analyses, however, suggest that the ?EDM process may not cause significant crystalline deterioration in the CNTs and contamination with the electrode elements. 37  Chapter 3: Toward High-Precision Micro-Electro-Discharge Machining of Carbon Nanotube Forests Having demonstrated the feasibility of producing complex 3D free-form CNT forest structures using dry ?EDM, and characterization of the process in the earlier chapter, this chapter aims to improve the precision of the process by studying the discharge gap as the main parameter. This will be pursued by taking two approaches; employing enhanced field emission properties of carbon nanotubes; and using them as cathode and source of electron for generating discharges during the micro-electro-discharge machining. By doing so, it was possible to carry out the machining at low energies and consequently, tighter tolerances. Later on in this chapter, SF6 as the machining ambient will be employed. Due to the high dielectric strength and arc extinguishing properties of SF6, it could be a good candidate for the dry ?EDM process of CNT forests to achieve tighter discharge gaps and increased precision.  3.1 Field-Emission-Assisted Approach This section3 is to explain dry micro-electro-discharge machining of vertically aligned carbon nanotube forests that are used as cathodes in the process, as opposed to conventional EDM where the material to be machined forms the anode, toward achieving higher precision in the patterned microstructures. The new configuration with the reversed polarity is observed to                                                  3 A portion of this section has been published in a peer-reviewed journal (Reused with permission from ?T. Saleh, M. Dahmardeh, A. Bsoul, A. Nojeh, and K. Takahata, ?Field-emission-assisted approach to dry micro-electro-discharge machining of carbon-nanotube forests?, Journal of Applied Physics, 110, 103305, 2011?, Copyright ? 2011, AIP Publishing LLC) [91]. Parts of this section were also presented in a proceeding (Reused with permission from T. Saleh, M. Dahmardeh, A. Bsoul, A. Nojeh, and K. Takahata, ?High-precision dry micro-electro-discharge machining of carbon-nanotube forests with ultralow discharge energy?, Proceeding IEEE MEMS, Paris, Jan 29-Feb2, 2012, pp. 259-262?, Copyright ? 2012 IEEE) [98]. 38  generate higher discharge currents in the process, presumably due to effective field-emission from CNTs. This effect allows the process to be performed at very low discharge energies, approximately 80? smaller than in the conventional normal-polarity case, with the machining voltage and tolerance down to 10V and 2.5 ?m, respectively, enabling high-precision high-aspect-ratio micro-patterning in the forests. The new approach is also demonstrated to make the process faster, cleaner, and more stable than conventional processing. Spectroscopic analyses of the forests processed by reverse ?EDM show no evidence of significant crystalline deterioration or contamination in the CNTs.  3.1.1 Introduction In typical EDM (including ?EDM), the workpiece and the electrode are generally arranged to be the anode and the cathode, respectively, as this polarity usually results in efficient material removal with small electrode wear. To the best of our knowledge, all previous studies on carbon-nanofibers or CNT-forest ?EDM have used this conventional polarity, i.e., the carbon material has served as the anode while the tungsten electrode has been used as the cathode [66], [82]. However, the effect of the polarity of the CNT forest on the EDM removal of the material has not been studied. Because of their extremely small tips with nanometer radii and high aspect ratios, CNTs are known to significantly enhance an applied electric field and thus have excellent electron emission properties. In fact, CNT cathodes were reported to reduce gas breakdown voltage while increasing the discharge current compared to tungsten cathodes [83]; although the set-up and the ranges of the breakdown voltage and current reported are different (one to two orders of magnitude) from those involved in ?EDM, the above suggests that the use of reverse polarity in ?EDM of CNT forests could be effective in lowering the discharge voltage and gap 39  clearance (for higher precision) as well as increasing the discharge current (for effective CNT removal) in the process. To explore this hypothesis, the present work investigates reverse-polarity ?EDM of pure CNT forests in dry air experimentally. The characteristics of reverse ?EDM for CNT forests and the patterned structures are studied to reveal various advantages over the conventional process with the normal polarity, achieving higher precision in the forest patterning process including that for high-aspect-ratio geometries.  3.1.2 Experimental Set-up The experimental set-up arranged for the machining tests and characterization is illustrated in Figure  3.1. As shown, a current probe (CT-l, Tektronix, USA) was used to monitor pulses of the discharge current in real time. The discharge current data were captured from the oscilloscope using a GPIB interface and stored in a computer for subsequent analysis.   Figure ?3.1: The experimental set-up used for characterization of reverse polarity ?EDM of CNT forests. 40  A series of ?EDM experiments at both normal and reverse polarities were performed for various energy levels that were controlled by varying the discharge voltage in a range of 10- 60V with a fixed capacitance of 10 pF in the R-C circuit. The electrode (tungsten, diameter of 40-93 ?m) was rotated at 3000 rpm during all the machining experiments. The X-Y scanning rate and the electrode feed rate in the Z direction were set to 1 mm/min and 10 ?m/min, respectively.  3.1.3 Results and Discussion The discharge pulses generated with both reverse and normal polarities were first characterized to observe the differences between the two conditions. Figure  3.2 (a) and (b) show the comparison of the average peak current and frequency of the pulses (n = 400) measured at different voltage levels. Typical discharge pulses for the normal and reverse polarities measured with the current probe are also shown in Figure  3.2 (c) and (d). It can be understood from the comparison in Figure  3.2 (a) and (b) that both the peak current and the frequency with the reverse polarity are higher than those with the normal polarity, and that the difference in the peak current at lower voltages is significant. It was also observed that short circuiting between the electrode and the CNT forest was a predominant factor in the normal polarity case when lower voltages were used (e.g., for 10V and 10 pF, the probability of short circuits with the normal polarity was ~5? higher than that with the reverse polarity). More occurrences of short circuits may result in mechanical grinding of the CNT forest as observed in Ref. [66], which can distort the orientation of the CNTs. A possible reason behind the higher discharge current in the reverse-polarity case that uses the forest as the cathode could be enhanced field-emission (FE) from CNTs [53], [84]?[87]; the forest cathode can more easily emit electrons from the nanoscale tips of the CNTs compared to a conventional metallic cathode (such as the tungsten tip) at a given 41  voltage. The increased discharge current with the reverse polarity is also consistent with the results reported in Ref. [83]. As will be discussed later, the reverse-polarity condition at 10V was found to produce an approximate discharge gap clearance of 2.5 ?m; this means that the applied electric field established at the gap is ~4 V/?m under the condition.     Figure ?3.2: (a) Average peak current of discharge pulses measured at different voltages (with constant capacitance of 10 pF), (b) average pulse frequency at the same voltages and capacitance, and typical single pulse of discharge current generated using 20V and 10 pF with (c) normal polarity and (d) reverse polarity. A tungsten electrode with a 93-lm diameter was used for all the measurements.  42  Past studies show FE current densities of ~0.5 mA/cm2 for tungsten cathodes [88] and ~1 mA/cm2 for CNT-forest cathodes [53] for this level of field, suggesting an approximately 2? increase in the FE current with the CNT forest compared to tungsten. Another study shows that discharge currents (in a corona phase) with CNT-forest cathodes are 6-7? larger than those with tungsten cathodes [83]. The discharge current densities calculated from Figure  3.2 (a) at 10V for the normal and reverse polarities (assuming that the current is uniformly distributed over the bottom surface of the 93-?m-diameter electrode) are 10.3 A/cm2 and 29.5 A/cm2, respectively, giving a 2.86? increase in the discharge current when the reverse polarity or the CNT-forest cathode is used. It is notable that although the discharge conditions and properties involved in the above cases (FE, corona, and arc) are substantially different [89], the levels of current enhancement observed with CNT-forest cathodes are somewhat similar under the experimental conditions used. It should also be noted that at high discharge voltages (>50 V), the pulse frequency was not necessarily higher for the reverse polarity, contrary to what was observed for low voltages as shown in Figure  3.2 (b). At 60V and 10 pF, for example, the pulse frequency with the reverse polarity was measured to be ~90 KHz, whereas that with the normal polarity was ~1360 KHz. This is likely related to the fact that the operation at high voltages with the reverse polarity often produced uncontrolled large sparks, which resulted in an abnormally large removal of the forest material and a very large gap clearance between the electrode and the forest, preventing discharge generation until the gap was reduced by the electrode feeding motion. This problematic phenomenon caused not only the decrease of the pulse frequency but also damages to the forest structure. In contrast, normal-polarity machining at high voltages was observed to produce stable discharge pulses with nearly no short circuiting, leading to higher frequency. This 43  process dependence on the voltage polarity at high voltages can be clearly seen in the results shown in Figure  3.3 that compares square patterns produced under identical voltage and capacitance (60V and 10 pF) with normal and reverse polarities. The significant distortion of the pattern for the reverse-polarity case is evident. This result is most likely due to the occurrence of the uncontrolled large sparks under the high-voltage condition mentioned earlier; at voltages greater than 50 V, more effective and efficient removal was possible with the normal polarity condition.  Figure ?3.3: Scanning electron microscope (SEM) images of micro-patterns machined in a CNT forest using 60V and 10 pF with (a) normal polarity and (b) reverse polarity. Each pattern was created by scanning a rotating electrode along a square shape (200 ?m?200 ?m) in the X-Y plane with continuous feeding of the electrode in the Z direction.  It was found that for lower voltages, however, the reverse-polarity ?EDM exhibited a different behavior, with advantageous effects towards further miniaturization in the CNT removal process with higher precision. The previous studies reported that the lowest value of the optimal voltage with the normal polarity was around 30 V, and that further lowering of the 44  voltage led to mechanical grinding of the CNTs [66]. To evaluate the effect of the reverse polarity in ?EDM of the forest for comparison, first shallow cavities were machined on a CNT forest with both the polarities at low-voltage levels. As can be seen in the sample results at 20V shown in Figure  3.4 (a) and (b), reverse-polarity machining produced smoother surfaces compared to normal-polarity processing. One possible cause of this result may be the difference in the discharge current between the two cases?the reverse-polarity process produces higher current than the normal-polarity for the same voltage as demonstrated in Figure  3.2 (a). This may allow the former to remove CNTs effectively, whereas the latter suffers from insufficient removal because of smaller discharge currents, possibly causing mechanical abrasion of the forest. The above effect was further verified with deeper patterning with both polarities, in a similar manner as that described in Figure  3.3 but at 10V (with 10 pF) in this case. The results in Figure  3.4 (c) and (d) indicate that the reverse-polarity process produced sharper, smoother, and cleaner microstructures compared to the normal-polarity case. Moreover, the patterns created with the reverse polarity exhibited a narrower width in the machined grooves compared to those with the normal polarity even though identical electrode and machining conditions were used. From the dimensions shown in Figure  3.4, as well as the diameter (93 ?m) of the electrode used, the discharge gap clearance is calculated to be 7.5 ?m for the normal polarity, whereas for the reverse polarity it is 2.5 ?m, which is 3? smaller (and shows a ~4? improvement over the previous result reported in Ref. [66]). This means that reverse ?EDM enables much tighter tolerances and higher precision in CNT forest patterning. 45   Figure ?3.4: The upper two SEM images show shallow cavities machined in a CNT forest using 20V and 10 pF with (a) normal polarity and (b) reverse polarity. The lower two SEM images show micro-patterns machined in a CNT forest using 10V and 10 pF with (c) normal polarity and (d) reverse polarity, for a depth of 40 ?m with 1-?m-step electrode feeding in the Z direction. The images in (c) and (d) also show close-up views of the microstructures created in the cavities.  46  Machining stability is another important aspect of ?EDM processing of CNT forests. In the process, electrode feeding is feedback controlled so that when a short circuit is detected, the Z stage retracts the electrode upward until the circuit is opened and then moves the electrode downward to resume its feeding and the machining process. The process becomes unstable and slow if many short circuits occur during the process because of frequent up/down motion of the Z axis. It was observed that reverse-polarity machining at low voltages was consistently more stable than normal-polarity machining for CNT forests. This tendency can be seen in Figure  3.5 that plots the electrode position on the Z axis during each of the machining processes conducted under the same conditions except for the voltage polarity. It is clear from the graph that the normal-polarity case produced frequent short circuits that led to ripples in the electrode motion, whereas the reverse-polarity case resulted in much more stable and faster electrode feeding or removal of the CNTs. For this particular machining condition, the total machining time with the reverse polarity was approximately 60% shorter than the time with the normal polarity.  Another interesting observation is that reverse-polarity machining (at low voltages) produced very small or almost no debris, leading to highly clean surfaces after the machining as can be seen in Figure  3.4 (d). In contrast, as reported in Ref. [66], normal-polarity machining produced a substantial amount of debris that was left on the machined surfaces [Figure  3.4 (c)]. It was also observed that the debris accumulated on and stuck to the electrode surfaces, increasing the effective diameter of the electrode in a random and non-uniform manner. In reverse ?EDM, the forest is the cathode and thus not subject to electron bombardment (leading to the conventional thermal removal) in principle; therefore, CNT removal is expected to be almost entirely due to oxygen plasma etching that can decompose CNTs into volatile products, forming virtually no debris. With the normal-polarity condition, in contrast, the forest is the anode that is 47  bombarded by electrons during the process and may be subject to some level of thermal removal where parts of the CNTs are melted and blown by pressure waves induced by the intense heat that the pulsed discharge arc produces, leaving resolidified carbon debris on the work zone and the electrode. A single MWNT was reported to be heated up to 2000K due to emitting a FE current of 1l A [90]; this self-heating effect of CNTs may need to be considered with respect to the removal process. However, in the present case, the current passed through a single CNT by a discharge pulse is estimated to be significantly lower, e.g., ~300 pA for the 10V condition that involves an average peak discharge current of 2 mA [Figure  3.2 (a)] considering the typical CNT density of ~1011/cm2 observed in the forest samples and the electrode diameter of 93 ?m used in the experiment. Moreover, the ?EDM process uses pulsed currents as opposed to DC current involved in Ref. [90]. Therefore, the impact of the self-heating effect on the removal process is likely minimal under the relevant machining condition.  Figure ?3.5: Electrode positions on the Z axis tracked in real time during machining with normal and reverse polarities, both using 10V and 10 pF. In both cases, the electrode was scanned along a square shape (100 ?m?100 ?m) in the X-Y plane with 1-?m-step feeding in the Z direction until reaching a depth of 40 ?m. 48  A key factor for high-aspect-ratio patterning in CNT forest is debris removal [66]. It is also essential to maintain the electrode surfaces free from debris accumulation that degrades the machining precision and can destroy the high aspect-ratio microstructures produced during the process. It was observed in the present study that the use of relatively high discharge energy (e.g., 60V and 10 pF) was effective in reducing debris generation under the normal polarity condition (in Ref. [67], needle-like microstructures were created with this method), which could be due to the enhancement of the oxygen-plasma etching phenomenon; however, the use of higher voltages tends to cause lower tolerance and more roughness in the machined surfaces. All these issues may be effectively addressed through reverse ?EDM because of its cleanness and the possibility of usage of low voltages as discussed above. The effectiveness of reverse ?EDM in high aspect-ratio micromachining of CNT forests was evaluated by patterning conical micro-structures with the conventional normal-polarity condition at 60V as well as with the reverse-polarity condition at 10V (both with 10 pF) using a tapered cylindrical electrode. As can be seen from the results in Figure  3.6, reverse ?EDM achieved finer structures with higher aspect ratios and smoother surfaces, demonstrating its effectiveness with low discharge energies for high-aspect-ratio patterning in CNT forests. Since the discharge energy is equal to CV2/2, where C is the capacitance of the R-C circuit (ignoring parasitic) and V is the voltage, the above electrical conditions and results suggest that the discharge energy involved in the reverse-polarity case is 0.5 nJ, 36? smaller than the energy for the normal-polarity case (and ~80? smaller than the energy used for the high-aspect-ratio machining reported in Ref. [67]), and that reverse-polarity ?EDM enables proper CNT removal using such low discharge energies. 49   Figure ?3.6: SEM images of patterned high aspect- ratio microstructures: (a) a cone shaped with normal polarity at 60V and 10 pF by scanning a tapered electrode along a circular orbit with 90-?m-diameter while feeding the electrode in the Z direction with 1-?m steps; (b) a cone shaped with reverse polarity at 10V and 10 pF under the same scanning/feeding conditions as in (a). The height of both cones is 120 ?m.  Energy-dispersive X-ray spectroscopy (EDX) of the CNT forest surfaces machined at 10V and 10 pF [Figure  3.7] indicated no detectable signals relevant to the electrode material (tungsten) with both the reverse and normal polarities, suggesting that the use of the reverse polarity involves almost zero consumption of the electrode and thus does not cause contamination of the processed forest surfaces with the electrode material. A high level of silicon detected in addition to carbon is most likely due to the presence of the substrate below the forest. These observations are also consistent with the previous results [66]. (Although the data shows a larger silicon peak for the normal-polarity case as seen in Figure  3.7, we have not observed a similar behavior in other samples studied. Therefore, we believe this could be related to the non 50  ?uniformity in the initial thickness of the forest that has led to a difference in the forest thickness left after the removal, rather than an effect related to polarity.) Raman spectroscopy provided no evidence of significant ID/IG ratio reduction, i.e., increased defects in the CNTs processed with either polarity under different conditions [Figure  3.8].  Figure ?3.7: EDX analysis results for the CNT-forest surfaces machined with (a) normal polarity and (b) reverse polarity.  The effect of using reverse polarity in ?EDM was investigated for micro-patterning of pure CNT forests. It was found that the process with the reverse-polarity condition increased the discharge current, possibly because of the field-emission properties of CNTs that serve as the cathode in reverse ?EDM. 51   Figure ?3.8: Raman ID/IG ratios for a forest sample machined at 30V with different capacitor values.  This concept was utilized to achieve ?EDM of CNT forests at lower machining voltages or discharge energies compared to the previously reported conditions. The structures machined with reverse ?EDM at low voltages were found to have higher precision and smoother and cleaner surfaces with almost no debris compared with those produced using conventional, normal-polarity processing. Reverse ?EDM was also found to enable more stable and faster machining of the forests. Raman and EDX analyses revealed that reverse ?EDM did not cause significant crystalline defects in the processed CNTs and contamination of the forest surfaces with the electrode element, respectively.   52  3.2 The Effect of Using Sulphur-Hexafluoride (SF6) 4 In this section, the effect of using sulphur hexafluoride (SF6), a high-dielectric-strength gas, for dry microelectro-discharge machining of carbon-nanotube forests is investigated. It was found that SF6 enables ?EDM of CNTs without O2, which is known to be essential for CNT machining in N2. The process in the SF6 ambient at a discharge voltage of 25 V was found to lead to a smaller discharge gap, i.e., tighter tolerance as well as higher machining quality compared with the N2 case at the same voltage. The N2 environment produced smaller discharge gap when 10 V was used; however, both the quality and rate of machining were somewhat lower in this case. The mixture with 20% O2 in SF6 is revealed to be an optimum condition for machining tolerance and quality. Also, CNT forests were used as the cathode in the process, as opposed to conventional ?EDM where the workpiece formed the anode. This configuration in the SF6?O2 mixture was observed to generate higher discharge currents at low voltages, presumably due to effective field-emission by the CNTs, leading to finer and cleaner machining. Furthermore, energy-dispersive X-ray analysis revealed that the optimal conditions result in less contamination by the electrode element on the processed forest surfaces.  3.2.1 Introduction In section  3.1, it was reported that reverse-polarity ?EDM of CNT forests that were defined as the cathode in air ambient enhanced the patterning tolerances and quality with decreased discharge energies [91]. In addition, all these previous studies have used the mixture of N2 and O2 (in the form of air in most cases) as the machining medium, and the effect of other                                                  4 A portion of this section has been published in a peer-reviewed journal (Reused with permission from ?T. Saleh, M. Dahmardeh, A. Nojeh, and K. Takahata, ?Dry micro-electro-discharge machining of carbon-nanotube forests using sulphur-hexafluoride?, Carbon, 52, 288, 2013?, Copyright (2013) with permission from Elsevier) [159]. 53  gases on the ?EDM performance has not been investigated so far. Sulphur hexafluoride (SF6) has a dielectric strength much higher than that of N2 (by a factor of ~3) [92]. This suggests that in SF6, the machining electrode and the workpiece (CNT forest) have to come closer in order to cause a gas breakdown for a given electric field strength. Hence, the use of SF6 may further reduce the discharge gap in the ?EDM process compared to the case with N2 under the same discharge voltage.  3.2.2 Sample Preparation and Experimental Set-up The CNT samples are prepared according to the procedure given in section  2.2.2. The experimental set-up arranged for the machining tests and characterization is illustrated in Figure  3.9.   Figure ?3.9: Experimental setup for dry ?EDM of pure CNT forest in different gas media.  As shown, a current probe (CT-l, Tektronix, OR, USA) was used to monitor pulses of the discharge current in real time. The discharge current data were captured from the oscilloscope using a GPIB interface and stored in a computer for subsequent analysis. Further, O2 was first 54  mixed with either SF6 or N2 inside a buffer chamber, and the mixed gas was introduced to the machining chamber, where the O2 concentration was measured using an oxygen sensor (VN202, Vandagraph Co., UK). The flow rates of O2 and SF6/N2 were adjusted so that the O2 concentration reached the target value and was stabilized in the machining chamber for at least 5 min prior to machining.  3.2.3 Results and Discussion As mentioned earlier, in air, ?EDM of CNT forests with the reverse polarity using the forests as the cathode was demonstrated to result in higher performance than the normal polarity case [91]. In this configuration, effective removal at very low voltages (~10 V) was achieved due to higher currents of the discharge pulses apparently because of the field-emission properties of the CNT cathode. To evaluate the effect of the SF6 environment for reverse ?EDM of CNT forests, machining tests with both the normal and reverse polarities were first conducted in a gas mixture of 50% SF6 and 50% O2. A 64-?m-diameter electrode was scanned along a square path of 200 ?m by 200 ?m in a CNT forest while machining it at 25 V to a depth of 25 ?m. Figure  3.10 shows the structures machined at both polarities, as well as the electrode used for each case imaged after the process without cleaning. It is clear from Figure  3.10 a and b that the reverse-polarity process resulted in finer and sharper structures compared with the normal-polarity case, the latter exhibiting distorted shapes and rough surfaces in the machined structures. In this EDM condition at 25 V, the reverse-polarity process was observed to generate relatively high discharge current (~16.3 mA) that is evidently sufficient to induce desirable material removal governed by electrical discharges. In the case of the normal polarity, however, the discharge current was much lower (~5mA), leading to insufficient removal by electrical 55  discharges, therefore causing mechanical abrasion and distorted structures/surfaces. These tendencies in terms of the discharge current and machining quality are consistent with the results obtained in air [91].  Figure ?3.10: Scanning electron microscope (SEM) images of the microstructures machined in a CNT forest in 50% SF6 and 50% O2 at 25 V with (a) the normal polarity and (b) the reverse polarity. Optical images of the tungsten electrode after machining with (c) the normal polarity and (d) the reverse polarity.  As also can be seen in Figure  3.10, reverse-polarity ?EDM resulted in much cleaner (less debris) structures, whereas the normal-polarity case produced more debris left on the structures and the debris accumulated on and stuck to the electrode. The debris accumulation on the electrode increases its effective diameter in a random and non-uniform manner, leading to an undesired larger gap between the walls of the resultant structures (as shown in Figure  3.10 a and b), thus lowering the precision in the machining process. A possible explanation for the above result may be similar to the case with reverse ?EDM in air [91] ? with reverse polarity, the CNT 56  forest is the cathode and thus not subject to electron bombardment (which induces conventional thermal removal as noted earlier) in principle; therefore, CNT removal is expected to be almost entirely due to oxygen and/or fluorine plasma etching [82], [93], [94] that decomposes CNTs into volatile products, forming minimal debris. With the normal-polarity condition, in contrast, the forest is the anode that is bombarded by electrons during the process and may be subject to some level of thermal removal where parts of the CNTs are melted and blown by pressure waves induced by the intense heat that the pulsed discharge produces, leaving resolidified carbon debris on the work zone and on the electrode. Based on the comparison above, the subsequent experimentations in the present study were carried out in the reverse-polarity mode, which provides more desirable results in machining precision and quality. In order to evaluate the effect of SF6, in comparison with the conventional N2, used as the dielectric gas on reverse ?EDM of CNT forests, machining tests were performed with 100% SF6 ambient as well as with 100% N2 ambient. Two basic discharge phenomena in EDM should be noted before the results of the above experiment are discussed. Gas discharges can be classified into several categories based on the corresponding voltage and current values. In EDM, there are two types of discharges that most commonly occur, namely spark and arc. Spark is characterized with higher discharge voltages compare to arc. Furthermore, spark is a transient process which may ultimately lead to a continuous arc if certain electrical conditions are fulfilled. In EDM, including ?EDM, it is desired to have spark discharge rather than arcing, which is detrimental as it results in excessive heating that causes severe damage to the sample surfaces [95]. Figure  3.11 a and b show typical discharge currents measured with 100% N2 and 100% SF6, respectively.  57   Figure ?3.11: Typical measured patterns of discharge current generated at 25 V with the reverse polarity in (a) 100% N2 and (b) 100% SF6, indicating much longer pulse duration (~70 ns) in the N2 ambient than in SF6 (~10 ns).  As represented in Figure  3.11 b, the 100% SF6 case produced short pulses (pulse duration ~10 ns), indicating a spark-mode discharge. These pulses usually had a peak current of around 10 mA or less but also showed spontaneous very large peak currents (30?40 mA) occasionally. For the 100% N2 case (Figure  3.11 a), the process tended to produce longer pulse durations that led to frequent abnormal arcing. (These two different discharge modes; spark and arc, were also distinguishable visually through the on-machine microscope; the former case was typically 58  observed to emit white light whereas the latter mode was with yellow?orange light emissions.) Moreover, arcing resulted in frequent short-circuit detections, preventing proper machining. Figure  3.12 a and b compare the results obtained in 100% N2 and 100% SF6, respectively, to produce the same square patterns as those in Figure  3.10 a and b.    Figure ?3.12: SEM images of the microstructures machined in a CNT forest at 25 V with the reverse polarity in (a) 100% N2 and (b) 100% SF6. A close-up SEM image is also shown in each case.  Figure  3.12 a exhibits damage on the forest surface with almost no material removal. In contrast, Figure  3.12 b shows the result of much more stable discharges in the form of sparks and stable removal. It has been reported in previous studies that SF6 plasma is suitable for etching of CNTs [93] and that carbon reacts with fluorine (fluorination of carbon) and forms various gaseous compounds during a plasma treatment of CNTs in an SF6 environment [94]. Although 59  these reports did not use spark discharge, similar chemical etching phenomena may occur with plasmas in the form of spark discharge, which could be the case shown in Figure  3.12 b. The possible reason of frequent arcing in 100% N2 can be understood from the fact that N2 is less electrically resistive than SF6 and thus permits the discharge gap to sustain a continuous arc between the electrode and the forest surface [92], [96], [97]. As noted earlier, the presence of O2 in the ?EDM process was reported to be essential for proper removal of CNTs in N2 ambient, in which the optimal concentration of O2 was ~20% [67], [82]. To study the role of O2 in the SF6 case, the ?EDM process was characterized with O2 concentrations of 10%, 20%, and 50%. The results machined at 25 V shown in Figure  3.13 suggest that the structural and surface quality improved with increasing O2 concentration up to 20% (Figure  3.13 a and b), and that the structures became distorted (e.g., the top surface of the center post as seen in Figure  3.13 c, possibly due to sparks propagating and etching portions of it) again when the concentration was further increased to 50%. The structure machined at 10 V and 20% O2 in SF6 shown in Figure  3.13 d indicates deteriorated structural quality compared to Figure  3.13 b, the 25-V case under the same ambient. The results obtained at the same voltage levels, 25 V and 10 V, with 20% O2 in N2 ambient are shown in Figure  3.13 e and f, respectively. These suggest that, in contrast to the SF6?O2 ambient cases, the 10-V condition led to higher machining quality than the 25-V condition in the N2?O2 ambient; this result is consistent with the previous findings reported in [91]. However, a comparison between Figure  3.13 b and f, representing the conditions that provided the highest machining quality for the SF6 and N2 environments, respectively, suggests that SF6 results in sharper corners (without extended portions at the corner of the center post) than N2, and the sidewall of the resultant structure is smoother (free from extended layers) for SF6 ambient. Possible sources of the different optimal 60  voltage levels for the SF6 and N2 environments (25 and 10 V, respectively) found above will be discussed later.    Figure ?3.13: SEM images of the microstructures machined in a CNT forest: (a) 10% O2 in SF6 at 25 V; (b) 20% O2 in SF6 at 25 V; (c) 50% O2 in SF6 at 25 V; (d) 20% O2 in SF6 at 10 V; (e) 20% O2 in N2 at 25 V and (f) 20% O2 in N2 at 10 V.  Figure  3.14 a shows the measured values of the average peak discharge current (calculated from 300 individual discharge pulses) and of the discharge gap as a function of O2 61  concentration in SF6. The discharge gap was calculated as the half of the dimensional difference between the measured width of a groove machined in a forest and the diameter of the electrode used. Figure  3.14 b compares the discharge gaps measured in the structures obtained with the SF6 and N2 environments (both at 20% O2) at the two discharge voltages of 25 and 10 V.   Figure ?3.14: (a) Average peak discharge current generated at 25 V and resultant discharge gap with different O2 concentrations in SF6. (b) Measured discharge gaps resulted from the SF6 and N2 environments (with 20% O2) and two different discharge voltages.  Figure  3.15 shows the measured Z position of the electrode captured during machining processes with different gas compositions and discharge voltages. The ripples seen in Figure  3.15 were caused by the retraction motion of the Z stage due to the short-circuit events 62  occurred in the processes as noted earlier. As shown in Figure  3.14 a, the discharge current was observed to have an increasing trend with O2 concentration in SF6 at 25 V; however, the discharge gap exhibited the minimal value (of 4.2 ?m) at 20% O2. These results may be explained as follows: The O2 concentration at 10% caused frequent short circuit detections as shown in Figure  3.15, which provided more energy for plasma etching noted earlier, resulting in a larger gap. For the 100% SF6 (O2 free) case, although the machining was observed to be smooth (with little short-circuit detection as shown in Figure  3.15) with a low average peak current (~5mA), the very large pulses (with peaks of 30?40 mA) occasionally observed in this 100% SF6 condition discussed earlier may have caused larger removal and discharge gap. For an O2 concentration above 20%, the process generated short pulses with higher peak currents in a consistent manner, which also led to a larger discharge gap. As discussed above and shown in Figure  3.14 b, in SF6 (at 20% O2), processing at 25 V resulted in the minimum discharge gap and the highest machining quality; however, this is not the case for N2 (at the same O2 concentration), in which processing at 10 V led to the minimum gap and the highest machining quality. In the SF6 environment, 25 V was a suitable voltage level to produce spark discharge pulses and perform smooth machining (Figure  3.14 b); however, 10 V may have made the discharge gap too small because of the high-dielectric ambient and thus caused physical touching and mechanical rubbing between the rotating electrode and the forest surface (which may have caused the circular marks on the bottom of the structure shown in Figure  3.14 d) due to non-ideal mechanical/positioning instability in the ?EDM system used, deteriorating the processed structure/surfaces. This undesired mechanical contact may have also occurred on the sidewalls of the patterned structures and caused slight bending or displacement of the CNTs on the walls, which may be the probable cause of the larger gap compared with the N2 environment case at 10 63  V (Figure  3.14 b) and of the very frequent short circuits or long machining time (Figure  3.15). In the N2 environment, on the contrary, the tendency of uncontrolled large spark and/or arcing (similar to the 100% N2 case) was evident when 25 V was used. This is believed to be a major source of the structural distortion observed (Figure  3.13 e) as well as the enlarged gap (Figure  3.14 b).    Figure ?3.15: Electrode?s position along the Z axis with machining time for different gas media and EDM conditions measured during patterning shown in Figure  3.13. The retracting distance upon a short-circuit detection was set to 5 ?m for all the cases except for the conditions 20% O2 in N2 and SF6 at 10 V, in which the length was set to 1 ?m.  Lowering the voltage to 10 V improved the removal quality (Figure  3.13 f) while decreasing the discharge gap. However, these favourable features at 10 V in the N2 case come with the price of machining stability and efficiency ? a relatively high rate of short circuiting, 64  presumably due to the low discharge energy causing insufficient removal, was observed to slow the process at this 10-V condition (e.g., Figure  3.15 shows that the removal at 10 V in N2 was ~1.7? slower than the case at 25 V in the SF6 environment). This high rate of short circuiting may have also introduced the undulations of the sidewalls of the resultant structure as shown in Figure  3.13 f. The elements on the surfaces of the forest microstructures (the bottom of the trenches) reverse-?EDMed in the SF6?O2 ambient were characterized using energy-dispersive X-ray spectroscopy (EDX) at 20-keV beam voltage (the beam spot size was ~2 ?m, almost 40? smaller than the width of the trenches, which ensured that the EDX data were obtained from the bottom of the trenches). The results from the surfaces processed with 20% and 10% O2 (Figure  3.16) indicate insignificant detectable sulphur and fluorine in both cases, suggesting that the use of SF6 does not cause any considerable contamination caused by the ambient gas itself. A low level of silicon detected is most likely due to the presence of the silicon substrate below the forest. The results also show that the level of tungsten, the contaminant due to wear of the electrode, is much lower at 20% O2 than the 10% O2 case, a favourable characteristic that the process at the optimal O2 concentration provides. Interestingly, it can be seen that the level of silicon is substantially reduced (by a factor of ~9) with the lower O2 concentration of 10%. A potential reason behind this could be as follows; at 10% O2, the tungsten level is almost 3? higher (6.46% as indicated), which may form a thin layer on the machined surface and reduce the electron beam penetration, as well as the escape of the generated X-rays from underneath, leading to less detection of silicon. Another fact that is worth noting is that iron was not detected in the machined surfaces using both O2 concentrations. This result could be related to the following two possibilities. One is that the CNTs may be predominantly root grown, thus iron remains on the substrate surface 65  that is too far from the probed forest surfaces to be detected, given its small amount (as can be seen, even silicon shows up with very small signals, although the substrate is bulk silicon). The other is that iron may be originally present on the forest surface (due to potential tip growth of CNTs) but removed by the ?EDM process. As regards the tungsten level in the N2?O2 environment, a previous report shows 1.25% of tungsten contamination under the condition corresponding to the optimal N2 case [98]. The tungsten level observed in the current study for SF6 (2.2%, Figure  3.16 a) is somewhat larger than the above level; it should be noted, however, that the electrode?s (tungsten?s) consumption condition can be affected by not only the gas medium but also the electrical contact (contact resistance) to the forest, which can vary from sample to sample, and may lead to variations in the level of tungsten. 66   Figure ?3.16: EDX analysis results for the CNT-forest surfaces machined in SF6 with (a) 20% O2 and (b) 10% O2.  67  Chapter 4: Integrating Carbon Nanotube Forest in High-Power MEMS Switch Having demonstrated the feasibility of ?EDMing of CNT forests, characterizations, suggesting the machining principle, and achieving machining with higher-precisions, the first application of the developed machining technique in a high power MEMS switch was presented. It was demonstrated that employing ?EDMed CNT forests as contact material would improve the performance of high-power MEMS switches. To do this, in the beginning of this chapter5 there is a discussion on the integration of vertically aligned carbon nanotube forests as an electrical contact material with a high-power, normally-open switch based on MEMS technology. Later in  Chapter 5: the integration of micropatterned CNT forest with the high-power MEMS switch as the first application of the ?EDMed CNT forests is shown. It is further explained that the high-power MEMS switch integrates a shape-memory-alloy (SMA) cantilever that is thermally actuated to enable switching between the movable CNT forest and the copper electrode formed on the SMA. The out-of-plane SMA actuator provided high forces to enable distributed contacts with the CNT forest, achieving low contact resistances and high ON/OFF resistance ratios. The ON state of the switch showed contact resistances as low as 35 ? with a dependence on the operating current. The device operation was then performed with over 5-W input powers. Long-term operation with more than 1?106 switching cycles was demonstrated next. The results indicated that a combination of the CNT-based contact and the SMA actuator                                                  5 Part of this chapter has been published in a peer-reviewed journal (Reused with permission from ?M. Dahmardeh, M. S. Mohamed Ali, T. Saleh, T. M. Hian, M. Vahdani Moghaddam, A. Nojeh, and K. Takahata, ?High-power MEMS switch enabled by carbon-nanotube contact and shape-memory-alloy actuator?, physica status solidi (a), 210: 631?638, 2013?, Copyright ? 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [148]. 68  may be a promising path to realizing reliable MEMS contact switches for high-power applications.   4.1 Introduction and Background Micro-electro-mechanical switches have benefits over solid-state switches such as low OFF-state leakage current, high ON/OFF resistance ratio, high breakdown voltage, and low distortion [99]?[101]. Due to the physical limitations of power transistors, high-power switches based on MEMS technology have been attracting more attention [102], [103]; however, there are still essential issues in this application area [103]. Reliability and lifetime of the contact are among the challenges of MEMS contact switches, especially for the above area. The contact resistance depends on a variety of factors, including the contact force, contact roughness and the material properties [104]. Typical contact materials used for MEMS contact switches are pure metals such as gold [100], [105], nickel [106], and aluminum [107]. The actual contact area of these metallic contacts tends to be limited by the presence of discrete contact points and thus much smaller than the designed area [29], [108]. Furthermore, these point contacts cause welding, shorting, and stiction failures [109]. For instance, metallic MEMS switches were reported to fail after cycles in the orders of 103 [105] and 104 [106] due to damages at localized point contacts. A variety of contact designs have been reported to improve device performance toward high power applications, including a ball-grid-array contact [110], a liquid-metal wetted contact [111], a gold contact with a meshed drain electrode [112], and a ruthenium contact switch with a corrugated diaphragm [104]. The use of an arc-suppression circuit was also reported to enable arcless operation of micro relays for power applications [113]. Actuation-69  assisted release mechanisms [103], [114], [115] have also been actively studied to address stiction related failures in MEMS contact switches.  Carbon-nanotube forest, because of its large surface area [29] and promising electrical, mechanical, and thermal properties [30], is a good candidate for a more reliable contact material for MEMS switches. For example, CNTs have been reported to have a high current capacity of more than 109 A cm-2 [13], [14]. Multi-walled CNTs exhibit a resistivity as low as 10-4 ? cm2 [15]. They also exhibit very high thermal conductivity (1400 W m-1 K-1) and operational temperatures (up to 2000 K) [116]. The junctions of CNTs and metallic electrodes have been reported to show resistance levels of 104-109 ? [117]. High-density CNT forests for interconnect applications [118] as well as a wafer-scale growth of patterned forests [119] were reported. There have been efforts to integrate CNTs with MEMS [41], [120]?[127]. A normally-closed MEMS switch that uses a contact of two CNT forests has been reported [128]. The design of this switch is based on electrostatic actuation and requires high voltages to detach the two forests off and achieve the OFF state. The condition in the ON state that no force is actively applied to the contact may limit the ability to minimize the contact resistance. In addition, the device layout based on in-plane actuation can occupy more chip space.  MEMS switches have also been reported with other actuation methods, including electrothermal [129], [130], magnetic [131], and piezoelectric principles [132]. Electrothermal actuation generally offers large forces, large displacements, and ease of on-chip integration. The former two features are advantageous in making stable, low-resistance contacts at the ON state and high isolation at the OFF state, respectively [129].  Shape memory alloys are smart materials that undergo crystal transformations upon heating or cooling to their austenitic and martensitic phase transformation temperatures, 70  respectively. While in the low temperature (martensitic state), SMA can be easily deformed by external stress, at high temperatures (austenitic state), it returns to the initial shape [133]. The transformation phase temperature of the SMA depends on the material composition. Among various types of SMA, NiTi alloy, known as Nitinol is mostly used. Applications of SMA include microgrippers [134], [135], cantilever actuators [136], and micropumps [137] and microvalves [138]. As one type of thermal (often electrothermal) actuators, they offer similar characteristics together with high fatigue resistance to cyclic operations, which is also advantageous for switching applications [139], [140]. Out-of-plane actuations can be easily achieved with simple SMA structures as opposed to typical electrothermal actuators that have in-plane actuation schemes, potentially enabling a small device footprint. In general, SMA actuation is slow compared to other principles; however, this may not be a major issue for high-power switching applications [102], [141]. Moreover, given the attractive features outlined above, SMA actuators may be a suitable choice for DC/low-frequency contact switches in the above application field.  Here, we present a normally-open, SMA-based MEMS switch that is integrated with a CNT forest used as contact material. The characteristics of the electrical contact between the CNT forest and a metallic electrode are investigated along with their dependences on control parameters for the device operation. Results from high-current operation and long-term switching tests are reported as well. The outcome of these experiments suggests that the integration of SMAs and CNT forests is a promising path toward realizing a reliable MEMS contact switch for high-power applications.   71  4.2 Experimental Figure  4.1 shows the layout of the developed switch device and the SMA actuator component used in the device. The SMA has a cantilever structure as shown, being fabricated to bend upward in its martensitic (cold) state at room temperature and vertically actuated downward when it is heated and enters its austenite (hot) state [136]. Thus, the signal terminal 1 (CNT forest) and terminal 2 (bonding pad for SMA) indicated in Figure  4.1 are disconnected at the cold state. As also shown in the figure, the cantilever is designed to have the bonding region with a cavity and perforations used for the bonding process. When the temperature of the SMA is elevated to exceed its threshold (austenite-phase) temperature, the cantilever is actuated and returns to its memorized flat shape. This thermal actuation is performed using resistive heaters integrated on the substrate (Si with a 2-?m-thick SiO2 insulation layer) by passing a driving current between terminals 3 and 4. As the cantilever is actuated, it makes contact with the top surface of the CNT forest arranged underneath the cantilever, at which point the switch is closed (i.e., terminals 1 and 2 are shorted). SMAs generally have relatively high resistivity levels. To minimize resistance at the contact and through the switch circuit, the bottom side of the SMA cantilever is coated with copper (with 200-nm thickness). The cantilever structure is bulk-micromachined in 300-?m-thick nickel-titanium SMA (so called Nitinol) sheets with a threshold temperature of 65 ?C (Alloy M, Memory Metalle GmbH, Germany).  Prior to copper coating, the bottom side of the structure is deposited with 4-?m-thick SiO2 that works as a compressive reset layer for the SMA actuator, so that the cantilever bends towards the uncoated (top) side at room temperature due to the stress applied by the layer. When the SMA is heated beyond the threshold temperature, its phase-transition force overcomes the bending moment caused by the SiO2 layer and makes the cantilever flat to push the CNTs 72  downward, achieving good electrical contacts with the individual nanotubes. As heat is removed, the cantilever restores its cold-state shape by bending up, making the switch open.    Figure ?4.1: Schematic illustration of the top view of the contact switch device (top), along with the details of the SMA cantilever component with dimensions (bottom left) and a cross-sectional view of the device in the OFF state (bottom right).  A forest of multi-walled CNTs with a thickness range of 100-200 ?m is directly grown on the heater substrate. To mechanically and thermally couple the SMA component with the substrate on which a CNT forest is present, we have investigated two integration approaches, wet and dry methods. In the wet method, the bonding region of the SMA is coupled with the bonding pad on the substrate using photolithography-assisted copper electroplating [134]. This process requires the CNTs to become wet and then dried. In the other approach, the dry method, the SMA is directly fixed onto the bonding pad of the substrate using liquid polyimide (PI) by filling 73  it in the cavity of the region and curing it to fix the cantilever. In this case, the CNT forest remains dry throughout the fabrication of the device. Images of the devices developed through the above dry and wet processes are shown in Figure  4.2 (a) and (b), respectively. As can be seen in Figure  4.2 (b), the forest that went through the wet process was densified, due to capillary effects [66], [69], forming random structures of bundled nanotubes. In contrast, as shown in Figure  4.2 (a), the device fabricated through the dry process maintains the original forest structures and surfaces. As in typical CNT forests, the forests grown in this study had some level of thickness variation within each forest.  For the fabrication of the heater circuit and signal/bonding pads on the substrate, Cr (15 nm) and Cu (200 nm) layers were first e-beam evaporated on a lightly doped Si wafer with a 2-?m-thick SiO2 layer. Photoresist (SPR 220-7, Rohm and Hass Co., PA, USA) was spin coated and patterned to define the layout of the heater and pads. Cu electroplating was performed for 50 ?m of thickness over the exposed Cu layer within the photo-defined region, followed by striping the photoresist and etching away the Cr-Cu layers underneath. Al2O3 (10 nm) and Fe (1.5 nm) were then e-beam evaporated on the corresponding Cu pad through a shadow mask as the catalyst layer for the CNT growth. A multi-walled CNT forest was synthesized for a height of up to 200 ?m through ethylene-based atmospheric chemical vapor deposition.  The SMA cantilever (Figure  4.1) was shaped with chemical etching of the Nitinol sheet for thinning and then shaped with micro-electro-discharge machining (?EDM) using a commercial ?EDM system (EM203, SmalTec International Inc., IL, USA) [136]. (This ?EDM technique may also be used to planarize the top surface of the CNT forest and adjust its height [66], [82] to address the thickness variation of the forest discussed earlier.) The SiO2 reset layer was deposited on the bottom side of the SMA cantilever with plasma-enhanced CVD at 350 ?C 74  while keeping it flat as the memorized state, followed by e-beam evaporation of the Cu layer on top of the reset layer. For bonding the SMA to the substrate, in the wet method, the component was fixed onto the substrate using an electroplating bonding technique [134]. After fixing the bonding region of the SMA using the SPR photoresist spin-coated on the substrate as a temporary adhesive (covering the CNT forest as well) followed by soft baking at 45 ?C for 1 hour, the photoresist on the region was exposed to ultraviolet light (the other regions were masked) and developed so that the SPR on top and in the perforations of the region was removed.   Figure ?4.2: a) Overall optical image of the dry-processed switch device and close-up scanning-electron-microscope (SEM) images showing the SMA structure and top surfaces of the CNT forest integrated into the device. b) SEM images of the CNT forest in the device fabricated though the wet process, showing the densified structures of the CNTs that have lost their vertical alignment.  75  This region was then electroplated with Cu for 120-?m thickness to bond the SMA to the substrate. The device was completed by dissolving the SPR in acetone, rinsing the device with isopropyl alcohol and deionized water, and drying it in air. In the dry method, the bonding region of the SMA was aligned to the bonding pad, and then liquid PI (HD-4010, HD Microsystems, DE, USA) was directly applied to the cavity and perforations and cured to complete the bonding. The experiments performed for the temporal control and long-term operations of the SMA actuator as well as the electrical measurements in these tests (to be discussed in the subsequent section) used a set-up centered on an electronic workstation board (NI ELVIS, National Instrument Co., TX, USA) coupled with LabVIEW programs. An amplifier was used to drive the heater of the devices using the signals generated by the board. The feedback loop used for the temporal control of the switch was established as follows: The heater driving current was increased while monitoring the contact resistance. As soon as the contact resistance dropped below a threshold level (set to be 10 K? in the experiments), the heater current was terminated to cool the device and open the contact, after which the current was applied again to repeat the ON-OFF cycle. The displacement measurement of the SMA cantilever was performed using a laser displacement sensor (LK-G32, Keyence, ON, Canada) with a resolution of 10?nm and a spot size of 30 ?m.  4.3 Results and Discussion  We first evaluated the performance of the two types of devices with the densified and original CNT forests. Heating the device substrates to a temperature of ~100 ?C or greater with a driving current of 0.5-0.9 A was observed to enable full deflections of the cantilevers. Figure  4.3 (a) and (b) show the dependence of the contact resistance (probed between terminals 1 and 2) on 76  the driving current for the wet- and dry-processed devices (both operated in air), respectively. The open resistances of these two devices were 20 M? or more (not shown in the figures). The wet-processed device exhibited significantly higher contact resistance than the dry-processed one; the dry one showed a resistance of ~500 ? in repeated switching consistently, while the wet-bonded device showed a resistance of a few K? at the first contact and then even higher resistances (in the order of 10 K?) after that as depicted in Figure  4.3 (a). The more resistive contact with the wet-processed device may be associated with the non-uniform (and probably more rigid) structures of the densified forest, potentially leading to a smaller actual contact area. The mismatch between the OFF-to-ON and ON-to-OFF paths in each result is presumably due to a hysteresis in thermomechanical response of SMA under a heating-cooling cycle [134].    Figure ?4.3: The dependence of contact resistance on the heater driving current for (a) wet-processed and (b) dry-processed devices. c) The dependence of contact resistance on the SMA?s 77  displacement (from the point at the cold state with full upward bending) for the dry-processed device; the inset shows an infrared image of the device under operation. Another possible cause of the mismatch could be related to the geometry of the individual CNTs that have somewhat wavy shapes; their tips can adhere to the cantilever surface due to the van der Waals force [142], potentially elongating the nanotube structures and maintaining the contacts during an early stage of the upward displacement of the cantilever, and then eventually detached from the surface. This type of the SMA cantilever used can exert very high actuation forces (~840 mN [136]) that are larger than those available with typical MEMS switches based on electrostatic designs by three to four orders of magnitude [143]. This experiment verifies that the high SMA force contributes to achieving a high ON/OFF resistance ratio and that the restoring force generated by the SiO2 reset layer is sufficient to overcome the surface force induced at the contact interface, releasing the cantilever from the forest to fully open the switch. Based on the above result, the rest of the experiments were conducted with the dry-processed device in air. Figure  4.3 (c) shows the dependence of the contact resistance on the vertical displacement of the cantilever tracked using the laser displacement sensor. For this measurement, the heater current was increased until observing a minimal ON-state resistance and then decreased while recording the displacement and the resistance. The laser spot of the sensor was directed to a location nearly at the edge of the free end of the cantilever. At a displacement of 65 ?m, a readable resistance (~500 K?) started to appear, at which point presumably initial contact between parts of the forest (that had thickness variation as noted earlier) and the copper was made. As the displacement was increased, the resistance continued to drop until reaching several 10?s of K?, where the resistance was relatively stable for displacements of up to around 82 ?m. The displacement beyond this level further decreased the contact resistance to 500 ? as 78  can be seen in the close-up plot in Figure  4.3 (c). The small decrease of the displacement near the maximum point while the resistance still decreased may be related to the following condition: As the cantilever actuates downward, due to its curvature and angular motion, it may first touch the inner edge (the one closer to the SMA bonding region) of the CNT forest. This contact edge of the forest could act as a pivot axis for the cantilever to make a seesaw-like behavior, leading to an upward motion of the cantilever?s tip that can decrease the displacement reading. The small increase of the displacement observed while decreasing the driving current can also be explained with the same hypothetical effect. Figure  4.4 shows variations of the signal current (Isig) generated by applying a constant signal voltage (Vsig = 0.2 V) between terminals 1 and 2 observed while displacing the cantilever with the driving current. This result clearly indicates that the ON-state Isig depends on the displacement, i.e., the contact force applied by the cantilever.    Figure ?4.4: Isig vs. heater driving current for the dry-processed device operated with a constant Vsig of 0.2 V, showing a dependence of Isig on the actuation level of the SMA. 79  This dependence can be understood considering the fact that CNTs are physically flexible, and thus increasing the force may result in more new contacts established with CNTs (while maintaining the existing contacts with other CNTs that are deformed by the force), lowering the contact resistance [144] and increasing Isig. The result in Figure  4.4 also shows a transistor-like nonlinear response; Isig rapidly increases at a threshold level of the input driving current (or that of the resultant displacement or force) that the contact is established, and its dependence diminishes when the displacement/force reaches a certain level (corresponding to a driving current of ~0.7 A in this test).   To assess the device ability and limitation in high-power operations, we conducted a destructive test, in which Vsig and Isig fed to the ON-state switch were increased until the contact failed. Figure  4.5 shows the results of the tests for four different current levels (approximately 100 mA, 200 mA, 300 mA, and >400 mA) in which Isig was increased and then decreased by adjusting Vsig. As shown, the cases with the maximum Isig of up to 300 mA (Figure  4.5 (a), (b) and (c)) showed consistent I-V behaviors. The result in Figure  4.5 (d) indicates that Isig similarly increased to 400 mA and more, corresponding to an input power of  more than 5 W, but started to drop at 470 mA while still increasing Vsig, and that the decreasing I-V path did not follow the increasing path unlike the other three cases. An optical observation verified that the copper layer on the bottom side of the cantilever had partly burnt and portions of the CNT forest were stuck to the cantilever side. This failure level of Isig is significantly higher than those reported for gold contacts in MEMS (e.g., 10 mA [105]). The contact resistance was calculated from the above results (Vsig divided by the resultant Isig) and plotted as a function of Isig in Figure  4.5 (e). This graph clearly shows that the contact resistance has a dependence on Isig (or Vsig), more on the lower Isig side, exhibiting a decreasing trend with which a power function fits well as displayed 80  on the figure. This trend is advantageous in terms of the targeted application area. Figure  4.5 (e) also shows that the resistance at the peak Isig in Figure  4.5 (c), the case tested with the highest peak Isig (300 mA) showing repeatable response, is 34.9 ?; this level is significantly lower than the minimum value (286 ?) reported for the device that used a CNT-forest contact [128]. It also shows that the contact failed when it reached 30.6 ? corresponding to the peak Isig in Figure  4.5 (d). The nonlinear dependence of the resistance shown in Figure  4.5 (e) and its reversibility imply a possibility for using the contact of a CNT forest and copper as a varistor-like component, in which its base resistance level is adjustable with the contact force generated by the actuator (Figure  4.3 (c) and Figure  4.4). A similar dependence of the resistance on the signal current was reported for metallic contacts of a MEMS switch operated in air [145]. Although the mechanism of the dependence observed in our device is not clear, since its operating ambient was air as well, it might be associated with breakdown of an insulating film (formed on the copper surface) as suggested in the report; further study is necessary to analyze this characteristic of the contact. The dynamic response of the switch was characterized using a feedback-looped set-up described in Experimental section. A typical temporal behavior of the switch resistance is shown in Figure  4.6, along with the driving-voltage waveform applied to the heater for the SMA actuation, showing an ON-OFF cycle time of 2.3 s in this particular example. The shortest cycle time observed in this experiment was 1.7 s. These values are significantly smaller than the times observed in other SMA actuators with similar dimensions (e.g., ~8.2? faster than the case previously reported [134]) in the same environment (air without forced cooling). 81   Figure ?4.5: Signal I-V relationships with approximate maximum Isig of (a) 100 mA, (b) 200 mA, (c) 300 mA, and (d) >400 mA; the last case shows a drop of the current indicating a failure of the contact. e) Collective data of contact resistance calculated from the results in (a)-(d), showing a non-linear dependence of the resistance on Isig along with a fitted curved of a power function. 82   Figure ?4.6: Temporal response in contact resistance of the device operated with feedback control and a voltage waveform used to drive the heater (small steps in the waveform were attributed to non-ideal characteristics of the set-up used).  The cycle time is mainly defined by the speed of heat transfer to/from the SMA as well as the threshold temperature and response hysteresis of the material. Reducing the size of the SMA cantilever and selecting SMAs with lower threshold temperatures are expected to shorten the necessary heating time and improve the temporal response further. The integration of CNTs with SMA may be another possibility to improve the actuation frequency of SMA [146]. Nevertheless, the demonstrated speed may be sufficient for the targeted application area discussed earlier. Using the same control set-up, long-term switching tests were performed. Figure  4.7 (a) displays the trend of the ON-state resistance for over 1?106 cycles, showing relatively stable values at around 650-700 ? for the entire test period. This experiment revealed that both the contact and the SMA actuator were still functional after the above cycles without 83  showing noticeable degradations. Given the small strains (estimated to be <10-3) that the SMA is subject to during the actuation, the above result on the SMA seems to match the characteristics reported for macro-scale samples (millions of cycles for small strains) [147]. Figure  4.7 (b) shows the top surface of the forest after the 106 cycle, at an area where one of the side edges of the cantilever was present. On the contact region in the image, the tips of the CNTs seem to have been more flattened and laterally oriented (along the direction of the arrow shown in the image) than the portion outside of the contact area that appears to maintain the original surface morphology without directionality. This may be evidence that the physical flexibility of CNTs enabled vastly distributed contact points at the ON state of the switch, as opposed to the case with metallic MEMS switches that involves limited point contacts leading to premature failures as described earlier. A combination of this effect and other favorable features of CNT as well as those of SMA may have contributed to the performance demonstrated in this test and others discussed previously.  4.4 Conclusions We reported an SMA-based MEMS switch that utilized a contact combination of a CNT forest and copper. The device fabricated through dry processing showed much lower and consistent contact resistance compared to the case that used the wet process which resulted in densification of the forest structure. The dry-processed device exhibited contact resistances from ~500 ? down to 35 ? or less for higher contact currents, providing high ON/OFF resistance ratios. This outcome was brought about by the use of the bulk-micromachined SMA actuator that enabled firm contacts with the CNT forest to achieve low ON-state resistances as well as releasing from the CNT forest while overcoming the surface force induced by the forest. 84   Figure ?4.7: a) Trend of the ON-state resistance for over 106 cycles. b) SEM image and close-ups of two regions of the top surface of the CNT forest corresponding to the contact area (bottom) and outside of the area (top) after the 106 cycle test, showing different surface textures between them and laterally oriented nanotube tips for the former case. 85  The SMA actuator was feedback controlled to demonstrate an ON-OFF cycle time of 1.7 s, much faster than similarly sized SMA actuators reported in the past. The device was operated with input powers of >5 W and exhibited a failure at a current of 470 mA, both of which are higher than those involved in typical MEMS contact switches by one order of magnitude or more [7, 52]. A visual analysis revealed that the failure was associated with the copper layer on the SMA cantilever rather than the CNTs, suggesting that even higher powers could be fed to the contact safely with optimized copper thickness. Switching cycles of >1?106 were demonstrated, without observing a sign of fatigue in the SMA actuation. This long-term experiment suggests that the CNT-copper contact tested may be a more reliable option than metal contacts widely used in MEMS. This is presumed to be enabled mainly by the excellent electrical, thermal, and mechanical properties of CNTs as well as the large effective contact area provided by the aligned CNT forest. The combination of this contact material and SMA actuator forms a robust contact switch potentially suitable for high-power applications. The CNT-copper contact was also observed to exhibit a varistor-like characteristic, implying another potential application of the contact material. 86  Chapter 5: The Effects of Three-Dimensional Shaping of Vertically Aligned Carbon-Nanotube Contacts for Micro-Electro-Mechanical Switches Having discussed the integration of a bare CNT forest as contact electrode in high-power MEMS switch, this chapter6 is going to deal with the improvement of the switch by integrating micropatterned CNT forests machined with ?EDM as the contact electrode. During the study, an out-of-plane shape-memory-alloy cantilever was thermally actuated to enable switching between the carbon nanotube forest and the copper layer deposited on the SMA cantilever. While, the switch presented in the previous chapter [148] suffers from large contact resistance, the present work employs micro-electro-discharge machining to shape the CNT forest according to the SMA cantilever in order to improve the performance of the switch. Providing contact resistances in the 10 ? range with an enhanced current capacity was achieved by improved contact areas. An SMA actuator was then integrated to demonstrate stable switching for ~1.4 million ON-OFF cycles with no sign of damage. The results proved that post-growth micropatterning of CNTs is a promising path to improved and reliable micro contact switches enabled by arrayed CNT contacts for high-power applications.    5.1 Introduction As mentioned in the introduction section, the ability to grow CNT forests, through CVD opened up opportunities to develop different types of novel devices enabled by the material [41], [120]. Owing to the unique and superior characteristics of the CNT forest in terms of mechanical                                                  6 A version of this chapter has been accepted for publication in a peer-reviewed journal (Reused with permission from ?M. Dahmardeh, M. Vahdani Moghaddam, T. M. Hian, A. Nojeh, and K. Takahata, ?The effects of three-dimensional shaping of vertically aligned carbon-nanotube contacts for micro-electro-mechanical switches?, Appl. Phys. Lett., 103, 171603, 2013?, Copyright ? 2013, AIP Publishing LLC) [160]. 87  [149], electrical [53], thermal [150], chemical [151], and optical [152], [153] properties, the integration of the material into MEMS is a very attractive approach to advancing the functionality and performance of the devices. A key in facilitating MEMS applications of the material is the ability to pattern the material in a batch mode with high precision and high reproducibility. As mentioned in section  1.5, the CVD growth of patterned CNT forests has been implemented using pre-patterned catalyst layers defined by photolithography, electron beam lithography, soft mask, and laser etching. Patterning CNT forests during or after the growth using shadow mask, mold, laser etching through mask, and densification has been presented. The mentioned techniques are, however, primarily for the formation of two-dimensional types of patterns (with uniform heights). Laser micromachining has been reported to shape CNT forests for different applications while exhibiting inherent limitations including tapered sidewalls, lack of high-precision depth control, and thermal damage. The developed micropatterning method in the previous chapters provides free-form, three-dimensional (3D) patterning of CNT forests. The 3D-pattened CNT forests are being utilized to realize novel and high-performance devices such as field-emitters [53], [154] and AFM scanning probes [155], extending their application opportunities to other areas, including microfluidics [156], gas sensors [157], [158], energy absorbing coatings [149], and heat sinks [33], [34]. This chapter utilizes the mentioned micropatterning technique to micropattern the CNT forest to improve the characteristics of the switch. CNT forests could make a significant contribution in electrical contact switches due to the promising characteristics. CNTs, with high current capacity of more than 109 A/cm2 [13], [14], low resistivity of 10-4 ?cm2 [15], and large surface area in the form of forests [29], are good candidates for interconnect [118] and switching [128] applications. The developed switch in the 88  previous chapter operated using a micromachined SMA actuator, was demonstrated to show stable switching for over 1?106 cycles. However, it exhibited relatively high ON-state resistances (~500 ?). The main cause of the high resistance is associated with the real contact area between the metallic electrode and the CNT forest contact as will be described later. This chapter reports 3D micropatterning of the CNT-forests contact enabled by the dry ?EDM process as a very effective route to addressing the issues related to the contact area, hence improving the conductance of the switch. Lowering the contact resistance leads to improvements in signal current (Isig) capacity and power loss.    5.2 Principle and Basics The micro contact switch developed is normally open and integrated with a micro-patterned CNT forest that serves as the electrical contact material. The switching operation is controlled with a SMA cantilever actuator that is thermally operated using a resistive heater integrated on the device. As shown in Figure  5.1 (a), the SMA cantilever structure is coated with a stress layer that bends the structure upward in its martensite (cold) state at room temperature and is vertically actuated downward when heated to enter the austenite (hot) state of the SMA. The signal terminals (terminals 1 and 2) are disconnected in the OFF state in which the cantilever is in the bent condition. Once the heater is activated (by applying a current through it) to reach the austenite threshold temperature of the SMA, the cantilever is actuated toward its memorized flat shape, making a contact with the CNT forests and closing the switch, entering the ON state. This contact occurs at a top edge of the forest that generally has a rectangular cross section, rather than making a planar contact on the top surface of the forest, before the cantilever reaches its flat shape (i.e., the cantilever is still in a bent condition) as illustrated in Figure  5.1 (b). This contact 89  condition (as in the case of the device reported in [148]) significantly limits the real contact area, thus increasing the ON-state resistance of the switch. This study aims to investigate the effect of 3D shaping of the CNT-forest contacts toward improving the contact area and resistance. In particular, we utilize dry ?EDM to achieve controlled, tapered surfaces with different angles in the CNT forests to evaluate the effect and maximize the contact area and hence the ON-state conductance of the switch.   Figure ?5.1: (a) Cross sectional view of the switch device that integrates CNT forest contact and SMA cantilever actuator that has its contact angle of ?; and three examples in the form of the CNT-forest contact; (b) as-grown CNT forest showing the possible real contact region with the angled cantilever only at a top corner as highlighted; (c) patterned forest having inclined contact surface with the contact angle less than ? leading to partial planar contact; (d) patterned forest having inclined contact surface with the contact angle equal to ? allowing full planar contact with the cantilever.  90  Figure  5.1 (c) displays the case where the angle of the sloped surface of the forest is smaller than the tip angle of the SMA cantilever at the touch-down condition, leading to a partial contact of the cantilever with the surface, whereas the case where the forest?s slope angle matches the angle of the cantilever tip results in a full contact as illustrated in Figure  5.1 (d), thus in principle leading to a higher contact conductance.    5.3 Fabrication The switch device is fabricated as follows. The heater and the pads are lithographically patterned in a 50-?m-thick Cu layer created on a lightly doped silicon wafer with a SiO2 layer on top. A catalyst layer (Al2O3/Fe) is deposited on the corresponding Cu pad (terminal 2, Figure  5.1 (a)), on which multi-walled CNT forests are grown with heights of up to 200 ?m using an ethylene-based atmospheric-pressure CVD process. The SMA film with 100-?m of thickness (adjusted using wet chemical etching) is patterned to form the cantilever structure with the bonding pad using a standard wet ?EDM process [42]. The dimensions of the cantilever are defined to be the same as those of the device reported in [148]. A 4-?m-thick SiO2 film, the stress layer for the SMA, is then deposited using plasma-enhanced CVD on the SMA cantilever, followed by evaporation of Cu (300 nm), the other electrode material that makes a direct contact with the CNTs when the switch is turned on. Further details of the fabrication described above can be found in [148]. The CNT forests grown on the heater substrates are then processed by dry ?EDM to create inclined surfaces with different angles. These inclined surfaces are established by creating fine staircase profiles with varying depths and heights of the stair (this process will be described later). The SMA cantilever component is then aligned and bonded on the Cu pad (terminal 1, Figure  5.1 (a)) created on the substrate. This bonding is performed using a 91  conductive dry-film adhesive (WaferGrip, Dynatex, Santa Rosa, CA, USA) that is inserted between the Cu pad on the substrate and the bonding pad created in the SMA cantilever. This assembly is annealed on a hotplate for 3-5 minutes at 120 ?C while applying a force (of 10 N) to the bonding site of the SMA from its top surface; cooling the assembly down to room temperature completes the bonding process. In order to enhance the electrical connection of the bonded SMA cantilever to the Cu pad on the substrate, the bonding site is coated with a 200-nm-thick layer of Cu through a shadow mask.  Dry ?EDM used for CNT-forest patterning was performed in air using a commercial 3-axis system with a positioning resolution of 100 nm (EM203, Samltec International, IL, USA). Details of the dry ?EDM process are reported in  Chapter 2: The inclined surfaces of the forests with different angles were patterned using 300-?m-diameter cylindrical tungsten electrodes with flattened bottoms. The device substrate on which a CNT forest was formed was placed on the X-Y stage of the system to control its lateral position relative to the electrode tip, whereas the vertical position of the electrode was controlled with the Z stage of the system. The formation of these surfaces was achieved by creating staircase profiles, patterned by scanning the electrode over a CNT forest grown on the substrate along the X axis of the system, while making a common 5-?m step in the Z direction and a varying step in the Y direction in each scan, to define a certain angle of the staircase slope. Fabricated samples of the SMA cantilever in its cold state were measured to have an approximate angle of 26? between the free-end region of the cantilever and the substrate plane. This cantilever was integrated on the substrate so that its bottom surface of the free-end region was in close proximity to the CNT forest and made contact with a minimal displacement to minimize the switching time. Thus, the contact angle of the cantilever was presumed to be close to the initial angle (26?). Following this estimation, we patterned the sloped 92  surfaces with an angle of 26?, as well as with other angles, 45? and 14?, to assess the dependence of the contact resistance on the angle of the forest?s contact surface. These three angles were determined by setting the Y-axis step in the scanning ?EDM process to be 10 ?m, 20 ?m, and 5 ?m, respectively. Figure  5.2 (a) shows a patterned CNT forest with a 26?-angled surface that is observed to exhibit a reasonably good flatness and uniformity, suggesting the effectiveness of the 3D dry ?EDM process for the fabrication of CNT-forest contacts with customized 3D shapes. The overall images of the integrated device are shown in Figure  5.2 (b).   5.4 Results and Discussions The electrical and thermomechanical characteristics of the fabricated devices as well as their dynamic behaviors were investigated using the set-up cantered around an electronic workstation board (NI ELVIS II, National Instrument Co., TX, USA) coupled with LabVIEW programs that were used for data acquisition. The NI board was also used to generate drive signals for the integrated heater to control the switch. The generated drive current was amplified to supply sufficient levels of current for the heater-switch operation. A thermal couple (OMEGA HH802U) was used to record temperature of the heater circuit. A laser displacement sensor (LK-G32, Keyence, ON, Canada) with a sensing resolution of 10 nm and a spot size of 30 ?m was used to capture the displacement of the SMA cantilever at its free end. The dependence of the ON-state contact resistance on the forest angle was first characterized using the devices fabricated to have 14?-, 26?-, and 45?-angled forest surfaces as described earlier; Figure  5.3 shows a measurement result of the resistance recorded while applying a periodic signal voltage (Vsig) of 2.2 V (between terminals 1 and 2) for 10 seconds followed by an off time of 5 seconds.  93   Figure ?5.2: (a) (left) Scanning electron microscope (SEM) image of patterned CNT-forest contact with 26?-angled surface and (right) the surface profile captured with laser scanning confocal microscope (Olympus FV1000, Japan); (b) (left) overall optical image of the developed switch device and (right) close-up SEM image of the integrated SMA cantilever and sloped CNT-forest contact.  All the three devices were observed to exhibit OFF-state contact resistances on the order of 10 M? and ON-state contact resistances lower than those reported with the device with as-grown forest contacts (~500 ?) [148]. Among them, as can be seen in Figure  5.3, the device with the 26? contact angle showed the lowest contact resistance of ~40 ? (leading to Isig of 55 mA), approximately 3-6? lower than the other two (14? and 45?) cases. This result verifies that the 94  inclined CNT-forest contact with the angle tailored to that of the cantilever at its free end provides the minimal resistance, presumably due to an enhanced real contact area achieved by the particular configuration, whereas the devices with smaller or larger angles lead to smaller contact areas and hence higher resistances. Based on these observations, the rest of the experiments were carried out using the devices with the optimal, 26?-angled CNT-forest contacts.   Figure ?5.3: Contact resistances of the switches using patterned CNT forests with three different slope angles measured at Vsig = 2.2 V.  The ON-state contact resistance of the defined device was observed to depend on the post-touchdown displacement of the SMA cantilever. As can be seen in the measurement result shown in Figure  5.4 (a), the resistance dropped abruptly at about 75-?m travel distance of the cantilever from the rest state, at which the cantilever made the first contact with the CNTs, and 95  the resistance continued to drop as the cantilever was displaced further, most likely because the bottom electrode of the cantilever established more contact points with the tips of CNT arrays in the forest as displaced more. The higher contact resistance (~200 ?) compared with the result shown in Figure  5.3 is due to dependence of the resistance on Isig, as will be shown in Figure  5.4 (b), and the smaller Isig (10 mA) used in this measurement. The non-linear dependence of the resistance shown in Figure  5.4 (b) is consistent with the results of our previous work [148].    Figure ?5.4: (a) Dependence of contact resistance on the displacement (measured with Isig of 10 mA). Comparisons between the switch with optimally angled CNT-forest contact and the one with bare forest contact for (b) contact resistance vs. Isig ,(c) Isig-Vsig, and (d) dissipated power vs. Isig.  96  Figure  5.4 (a) also indicates a reverse behavior of the displacement near its maximum point, which is potentially associated with a pivoting effect that lifted the tip of the cantilever caused by making a hard contact with the forest. These observations (displacement dependence of the resistance and the reverse behavior of the cantilever tip) are consistent with the results previously reported [148]. The temporal response of the developed device was characterized through automated sequential control of the SMA cantilever actuator. As the drive current was fed to the heater circuit, the cantilever actuator traveled down toward the tapered surface of the CNT forest and closed the switch by making contact with the surface. The LabVIEW program was arranged to maintain the drive current until the detected resistance reached a value as low as 200 ?, after which the current was turned off to cool the cantilever, which was forced to go back to the original position caused by the reset layer, and open the switch for the resistance to reach 1 M?. This ON-OFF cycle was repeated while recording the switch?s resistance and the displacement of the cantilever tip (Figure  5.5 (a)). The initial displacement was ~80 ?m, which decreased to 35-40 ?m for the following cycles due to the threshold levels of the resistance set in the control sequence. One cycle was observed to complete in ~3 seconds with the set-up used. A similar program (with a modified threshold level of the ON-state contact resistance of 60 ?) was used to perform long-term operation of the developed device. Figure  5.5 (b) shows the ON-state contact resistance tracked during 1.4 million cycles of switching operated with Isig of 60 mA. As can be seen, the average resistance was measured to be almost within the range of 40-60 ? for the entire test. As a comparison, the device with a bare forest contact [148] exhibited a resistance range of 600-700 ?  in long-term operation (~1 million switching cycles with several-fold smaller Isig). The improved contact area of the developed device, enabled by micropatterning of CNT-forest 97  contacts, was demonstrated to achieve an order of magnitude lower contact resistance with a higher current carrying capacity, stably operating for a larger number of cycles with no sign of negative impact on the contact materials or the overall device.   Figure ?5.5: (a) Temporal response of the fabricated switch device showing its contact resistance and cantilever?s free-end displacement; (b) long-term trend of contact resistance for ~1.4 million switching cycles demonstrating stable switching operation with an overall resistance range of 40-60 ?.  98  5.5 Conclusions In this Chapter, the effect of 3D patterning of CNT forests toward advancing high-power micro-electro-mechanical switches enabled by integrated CNT-forest contacts was studied. The experiments performed using fabricated devices verified that the approach improved the ON-state conductance and the current carrying capacity of the switch, presumably because the ?EDM-patterned forest surfaces with controlled angles would maximize the real contact area, i.e., the number of contact points between the individual CNTs in the forest and the cantilever. The results observed with differently angled forest contacts supported this hypothesis. The electromechancial behaviors of the fabricated switch device with an optimal contact angle were characterized to reveal ON-state contact resistances as low as 13.2 ? with Isig of over 500 mA and suppressed power dissipations in the device. The stable operation of the device was further demonstrated for switching cycles of ~1.4?106, with 10-17? lower ON-state contact resistances than the device with a CNT-forest contact with a flat top surface.         99  Chapter 6: Conclusion 6.1 Contributions The research contributions are presented in the following chapter sequence:  I. Chapter 2 In this chapter, feasibility of micropatterning carbon nanotube forests using micro-electro-discharge machining was demonstrated. Due to the wet nature of typical ?EDM process, effect of submerging CNTs in liquid was shown. For the dry process, micro patterned structures at different machining conditions were characterized and the corresponding measured discharge current waveforms were reported. The discharge gap between the machining electrode and the CNT forest sample was also measured at different machining conditions to characterize the machining tolerance. Various 3D and high aspect ratio micropatterns were machined. Furthermore, EDX analyses were reported to show the level of possible cross-contamination of the process.  Later on in this chapter, the effect of ambient gas and specifically oxygen on dry ?EDM of CNT forests was investigated in a rational way to investigate the principle of the process. Controlled concentrations of nitrogen and oxygen were then introduced to the machining area in a controlled environment. The necessary role of oxygen during the machining process was also shown. Furthermore, a different machining mechanism for ?EDM of CNT forests was reported compared to the typical ?EDM process which was based on melting and removal of the workpiece. Next, different levels of oxygen were investigated and corresponding discharge current waveforms were measured. EDX analyses were then carried out to measure the level of possible cross-contamination of the process. Raman spectra were collected from the ?EDMed 100  regions to investigate the effect of the process on the crystalline properties of the carbon nanotubes.  II. Chapter 3 In the previous chapter there was a discussion on feasibility, effectiveness, and characterization of ?EDM process on CNT forests. The goal of this chapter is to investigate methods that would increase the precision of the process. Two approaches were taken to pursue this goal; first, use of the enhanced field emission properties of carbon nanotubes forests as an electron source for the electro-discharges generated during the ?EDM process. In this approach, CNT forests were used as cathodes, as opposed to conventional ?EDM process where the material to be machined forms the anode. It is reported in this chapter that by doing so, machining at low discharge energies, and consequently tighter machining tolerance would be feasible, possibly due to the enhanced field emission properties of carbon nanotubes. The discharge current and pulse frequency were also measured for both conventional and reverse machining configurations at various machining conditions. EDX analysis and Raman spectra were then collected to investigate the possible cross-contamination or damage to carbon nanotubes during the process. The second taken approach in this chapter was the effect of using SF6 gas in the machining ambient. This gas because of the high dielectric strength was selected to achieve tighter discharge gaps. Machining with the presence of different ratios of N2/O2/SF6 was then illustrated. Also, discharge currents were measured at different machining conditions. EDX analyses were presented too to show the level of cross-contamination during the machining process.  101  III. Chapter 4 In order to demonstrate an application of ?EDM of CNT forests, firstly there was a demonstration about the integration of carbon nanotube forest as an electrical contact material with a high-power MEMS switch as a preliminary effort toward MEMS application of the developed micropatterning method. Carbon nanotubes because of large surface area, excellent mechanical and electrical properties were selected for conducting high power signals in a MEMS switch. The switch principle was based on a thermally actuated shape memory alloy cantilever for DC and low-frequency applications. Shape memory alloy was selected to achieve high forces in the ON state, to ensure reliable contact with the CNT forests. The SMA was bonded to the device using two different techniques; wet and dry ones. These techniques and the effects on the CNT forest electrode were also described in details. The electrical performance of the CNT forest, thermomechanical characterization of the SMA cantilever, high power handling and temporal response of the switch were then investigated. The switch was actuated for over one million cycles, without any sign of damage. Finally, it was suggested that integration of CNT forests and SMAs are a promising path towards high-power MEMS switches.  IV. Chapter 5 Having demonstrated the feasibility of integration of CNT forests into high-power MEMS applications in Chapter 4, this chapter was a report on the improvement of the switch by 3D micropatterning of CNT forest as an electrical contact ?EDM. The design, except for the machined CNT forest and the bonding technique was similar to the design presented in the previous chapter. Here, CNT forests with different 3D profiles were patterned and tested as electrical contact for the MEMS switch. It was shown that ?EDM of CNT forests to produce 3D 102  micropatterns provides a reliable and improved contact for MEMS switches. The low contact resistance of the optimally micropatterned CNT forest provided an electrical path for the switch with low dissipated powers. The electrical performance of the micropatterned CNT forest, thermomechanical characterization of the SMA cantilever, high power handling and temporal response of the switch were detailed. The device was then actuated for more than 1.4 million cycles with no sign of damage.   V. Appendices Appendix A reports the developed atmospheric pressure chemical vapor deposition system to grow carbon nanotube forests.  Appendix B demonstrates field emission application of the micropatterened CNT forests with ?EDM. Uniform field emission and at lower electric fields of ?EDMed CNT forests compared to bare ones was observed. Confined field emission from the tips of micropatterened CNT forests was also demonstrated, which is not achievable by non-patterned CNT forests.  Appendix C details the application of ?EDMed CNT forests as AFM probes. Forests were grown on commercial Si AFM probes with no tip and are patterned to produce cone structures of carbon nanotubes. Remarkably higher mechanical stability and robustness, as well as potential batch manufacturing compatibility with higher precision at low costs were reported and discussed. Comparison with commercial probes shows very good agreement in terms of the performance and suggests potential application of the CNT forests with further improvements.   The contribution of this research to the field can be summarized as developing a powerful micro-machining technique to pattern carbon nanotube forests with high precision and high 103  reproducibility. The developed machining process is high-precision, compared to other CNT machining techniques, applicable to batch machining processes, and clean. The machining technique is used to shape CNT forest into 3D structure as a reliable and improved electrode of MEMS switches for high-power applications. Micropatterened CNT forests produced by ?EDMed are used as field emitters and AFM probes to demonstrate few applications of the developed process. In terms of the fabrication method, in this research, devices were fabricated by combining non-traditional micromachining methods such as ?EDM with standard MEMS fabrication methods such as lithography, E-beam deposition, sputter disposition, PECVD, and electro-chemical deposition. There was also an integration of bulk materials such as the SMA sheet into the standard MEMS fabrication process. Carbon nanotube growth using chemical vapour deposition technique between the fabrication steps was further demonstrated. The developed fabrication processes to realize the devices prove that unique and high performance MEMS devices can be built by combining non-traditional micro-machining process and materials with standard MEMS fabrication approach.  6.2 Future Work Future work may be divided into two categories: increasing the precision of ?EDM patterning of CNT forests, and improving the application area. The former may be pursued by decreasing the size of the machining sparks. Possible methods to achieve this, besides the ones presented in this research, can be studied, such as optimizing the discharge path to maximize the discharge energy delivered to the discharge gap. Producing high resolution electrodes machined by methods other than WEDG, such as electrochemical etching to narrow the electrode, could potentially increase the precision of the process. For the application area, integration of the 104  micropatterned CNT forests in other MEMS devices, such as gas sensors, field emitters, and electron sources can be studied. Moreover, the performance of the developed devices, e.g., switching time, and bonding method for the MEMS switch device integrated with CNT forests, could be optimized. 105  Bibliography [1] S. Iijima, ?Helical microtubules of graphitic carbon,? Nature, vol. 354, no. 6348, pp. 56?58, Nov. 1991. [2] K. Hata, D. N. Futaba, K. Mizuno, T. Namai, M. Yumura, and S. Iijima, ?Water-assisted highly efficient synthesis of impurity-free single-walled carbon nanotubes.,? Science, vol. 306, no. 5700, pp. 1362?4, Nov. 2004. [3] L. Guan, K. Suenaga, and S. Iijima, ?Smallest carbon nanotube assigned with atomic resolution accuracy.,? Nano Lett., vol. 8, no. 2, pp. 459?62, Mar. 2008. [4] X. Wang, Q. Li, J. Xie, Z. Jin, J. Wang, Y. Li, K. Jiang, and S. Fan, ?Fabrication of ultralong and electrically uniform single-walled carbon nanotubes on clean substrates.,? Nano Lett., vol. 9, no. 9, pp. 3137?41, Oct. 2009. [5] R. Zhang, Y. Zhang, Q. Zhang, H. Xie, W. Qian, and F. Wei, ?Growth of half-meter long carbon nanotubes based on Schulz-Flory distribution.,? ACS Nano, vol. 7, no. 7, pp. 6156?61, Jul. 2013. [6] M. Yu, ?Strength and Breaking Mechanism of Multiwalled Carbon Nanotubes Under Tensile Load,? Science (80-. )., vol. 287, no. 5453, pp. 637?640, Jan. 2000. [7] M. M. J. Treacy, T. W. Ebbesen, and J. M. Gibson, ?Exceptionally high Young?s modulus observed for individual carbon nanotubes,? Nature, vol. 381, no. 6584, pp. 678?680, Jun. 1996. [8] J. P. Metters and C. E. Banks, ?Electrochemical utilisation of chemical vapour deposition grown carbon nanotubes as sensors,? Vacuum, vol. 86, no. 5, pp. 507?519, Jan. 2012. [9] M. Paradise and T. Goswami, ?Carbon nanotubes ? Production and industrial applications,? Mater. Des., vol. 28, no. 5, pp. 1477?1489, Jan. 2007. [10] P. G. Collins and P. Avouris, ?Nanotubes for electronics.,? Sci. Am., vol. 283, no. 6, pp. 62?9, Dec. 2000. [11] H. Dai, ?Carbon nanotubes: opportunities and challenges,? Surf. Sci., vol. 500, no. 1?3, pp. 218?241, Mar. 2002. [12] H. Li, W. Yin, K. Banerjee, and J.-F. Mao, ?Circuit Modeling and Performance Analysis of Multi-Walled Carbon Nanotube Interconnects,? IEEE Trans. Electron Devices, vol. 55, no. 6, pp. 1328?1337, Jun. 2008. 106  [13] Z. Yao, C. Kane, and C. Dekker, ?High-Field Electrical Transport in Single-Wall Carbon Nanotubes,? Phys. Rev. Lett., vol. 84, no. 13, pp. 2941?2944, Mar. 2000. [14] B. Q. Wei, R. Vajtai, and P. M. Ajayan, ?Reliability and current carrying capacity of carbon nanotubes,? Appl. Phys. Lett., vol. 79, no. 8, p. 1172, Aug. 2001. [15] Y.-H. Li, J. Wei, X. Zhang, C. Xu, D. Wu, L. Lu, and B. Wei, ?Mechanical and electrical properties of carbon nanotube ribbons,? Chem. Phys. Lett., vol. 365, no. 1?2, pp. 95?100, Oct. 2002. [16] P. Kim, L. Shi, a. Majumdar, and P. McEuen, ?Thermal Transport Measurements of Individual Multiwalled Nanotubes,? Phys. Rev. Lett., vol. 87, no. 21, p. 215502, Oct. 2001. [17] S. Berber, Y. Kwon, and D. Tomanek, ?Unusually high thermal conductivity of carbon nanotubes,? Phys. Rev. Lett., vol. 84, no. 20, pp. 4613?6, May 2000. [18] T. Guo, P. Nikolaev, A. Thess, D. T. Colbert, and R. E. Smalley, ?Catalytic growth of single-walled manotubes by laser vaporization,? Chem. Phys. Lett., vol. 243, no. 1?2, pp. 49?54, Sep. 1995. [19] M. Kumar and Y. Ando, ?Chemical Vapor Deposition of Carbon Nanotubes: A Review on Growth Mechanism and Mass Production,? J. Nanosci. Nanotechnol., vol. 10, no. 6, pp. 3739?3758, Jun. 2010. [20] A. J. Hart and A. H. Slocum, ?Rapid growth and flow-mediated nucleation of millimeter-scale aligned carbon nanotube structures from a thin-film catalyst.,? J. Phys. Chem. B, vol. 110, no. 16, pp. 8250?7, Apr. 2006. [21] J. Prasek, J. Drbohlavova, J. Chomoucka, J. Hubalek, O. Jasek, V. Adam, and R. Kizek, ?Methods for carbon nanotubes synthesis?review,? J. Mater. Chem., vol. 21, no. 40, p. 15872, 2011. [22] A. J. Hart, L. van Laake, and A. H. Slocum, ?Desktop growth of carbon-nanotube monoliths with in situ optical imaging.,? Small, vol. 3, no. 5, pp. 772?7, May 2007. [23] S. J. Kang, C. Kocabas, T. Ozel, M. Shim, N. Pimparkar, M. a Alam, S. V Rotkin, and J. a Rogers, ?High-performance electronics using dense, perfectly aligned arrays of single-walled carbon nanotubes.,? Nat. Nanotechnol., vol. 2, no. 4, pp. 230?6, May 2007. [24] M. Terrones, N. Grobert, and J. Olivares, ?Controlled production of aligned-nanotube bundles,? Nature, vol. 388, no. July, pp. 52?55, 1997. [25] O. Yaglioglu, A. Cao, a. J. Hart, R. Martens, and a. H. Slocum, ?Wide Range Control of Microstructure and Mechanical Properties of Carbon Nanotube Forests: A Comparison 107  Between Fixed and Floating Catalyst CVD Techniques,? Adv. Funct. Mater., vol. 22, no. 23, pp. 5028?5037, Dec. 2012. [26] H. Kim, J. Kang, Y. Kim, B. H. Hong, J. Choi, and S. Iijima, ?Synthesis of Ultra-Long Super-Aligned Double-Walled Carbon Nanotube Forests,? J. Nanosci. Nanotechnol., vol. 11, no. 1, p. 4, 2011. [27] S. Chakrabarti, K. Gong, and L. Dai, ?Structural Evaluation along the Nanotube Length for Super-long Vertically Aligned Double-Walled Carbon Nanotube Arrays,? J. Phys. Chem. C, vol. 112, no. 22, pp. 8136?8139, Jun. 2008. [28] S. Chakrabarti, T. Nagasaka, Y. Yoshikawa, L. Pan, and Y. Nakayama, ?Growth of Super Long Aligned Brush-Like Carbon Nanotubes,? Jpn. J. Appl. Phys., vol. 45, no. No. 28, pp. L720?L722, Jul. 2006. [29] M. Park, B. A. Cola, T. Siegmund, J. Xu, M. R. Maschmann, T. S. Fisher, and H. Kim, ?Effects of a carbon nanotube layer on electrical contact resistance between copper substrates,? Nanotechnology, vol. 17, no. 9, pp. 2294?2303, May 2006. [30] N. Grobert, ?Carbon nanotubes ? becoming clean,? Mater. Today, vol. 10, no. 1?2, pp. 28?35, Jan. 2007. [31] N. Hamada, S. Sawada, and A. Oshiyama, ?New one-dimensional conductors: Graphitic microtubules,? Phys. Rev. Lett., vol. 68, no. 10, pp. 1579?1581, Mar. 1992. [32] J. Lu, ?Elastic Properties of Carbon Nanotubes and Nanoropes,? Phys. Rev. Lett., vol. 79, no. 7, pp. 1297?1300, Aug. 1997. [33] Y. Fu, N. Nabiollahi, T. Wang, S. Wang, Z. Hu, B. Carlberg, Y. Zhang, X. Wang, and J. Liu, ?A complete carbon-nanotube-based on-chip cooling solution with very high heat dissipation capacity.,? Nanotechnology, vol. 23, no. 4, p. 045304, Feb. 2012. [34] K. Kord s, G. T th, P. Moilanen, M. Kumpum ki, J. V h kangas, a. Uusim ki, R. Vajtai, and P. M. Ajayan, ?Chip cooling with integrated carbon nanotube microfin architectures,? Appl. Phys. Lett., vol. 90, no. 12, p. 123105, 2007. [35] E. Frackowiak and F. B?guin, ?Electrochemical storage of energy in carbon nanotubes and nanostructured carbons,? Carbon N. Y., vol. 40, no. 10, pp. 1775?1787, Aug. 2002. [36] Y. Yun, V. Shanov, Y. Tu, M. J. Schulz, S. Yarmolenko, S. Neralla, J. Sankar, and S. Subramaniam, ?A multi-wall carbon nanotube tower electrochemical actuator.,? Nano Lett., vol. 6, no. 4, pp. 689?93, Apr. 2006. 108  [37] K. H. An, W. S. Kim, Y. S. Park, J.-M. Moon, D. J. Bae, S. C. Lim, Y. S. Lee, and Y. H. Lee, ?Electrochemical Properties of High-Power Supercapacitors Using Single-Walled Carbon Nanotube Electrodes,? Adv. Funct. Mater., vol. 11, no. 5, pp. 387?392, Oct. 2001. [38] C. Du and N. Pan, ?High power density supercapacitor electrodes of carbon nanotube films by electrophoretic deposition,? Nanotechnology, vol. 17, no. 21, pp. 5314?5318, Nov. 2006. [39] O. Gr ning, O. M. K ttel, C. Emmenegger, P. Gr ning, and L. Schlapbach, ?Field emission properties of carbon nanotubes,? J. Vac. Sci. Technol. B Microelectron. Nanom. Struct., vol. 18, no. 2, p. 665, 2000. [40] L. Ge, S. Sethi, L. Ci, P. M. Ajayan, and A. Dhinojwala, ?Carbon nanotube-based synthetic gecko tapes.,? Proc. Natl. Acad. Sci. U. S. A., vol. 104, no. 26, pp. 10792?5, Jun. 2007. [41] Y. Hayamizu, T. Yamada, K. Mizuno, R. C. Davis, D. N. Futaba, M. Yumura, and K. Hata, ?Integrated three-dimensional microelectromechanical devices from processable carbon nanotube wafers.,? Nat. Nanotechnol., vol. 3, no. 5, pp. 289?94, May 2008. [42] K. Takahata, ?Micro-Electro-Discharge Machining Technologies for MEMS,? in Micro Electronic and Mechanical Systems, no. December, InTech, 2009, p. 386. [43] G. Kibria, B. R. Sarkar, B. B. Pradhan, and B. Bhattacharyya, ?Comparative study of different dielectrics for micro-EDM performance during microhole machining of Ti-6Al-4V alloy,? Int. J. Adv. Manuf. Technol., vol. 48, no. 5?8, pp. 557?570, Sep. 2009. [44] M. Kunieda, M. Yoshida, and N. Taniguchi, ?Electrical Discharge Machining in Gas,? CIRP Ann. - Manuf. Technol., vol. 46, no. 1, pp. 143?146, Jan. 1997. [45] K. Albinski, K. Musiol, A. Miernikiewicz, S. Labuz, and M. Malota, ?The temperature of a plasma used in electrical discharge machining,? Plasma Sources Sci. Technol., vol. 5, no. 4, pp. 736?742, Nov. 1996. [46] M. Kunieda, B. Lauwers, K. P. Rajurkar, and B. M. Schumacher, ?Advancing EDM through Fundamental Insight into the Process,? CIRP Ann. - Manuf. Technol., vol. 54, no. 2, pp. 64?87, Jan. 2005. [47] J. I. Sohn, S. Lee, Y.-H. Song, S.-Y. Choi, K.-I. Cho, and K.-S. Nam, ?Patterned selective growth of carbon nanotubes and large field emission from vertically well-aligned carbon nanotube field emitter arrays,? Appl. Phys. Lett., vol. 78, no. 7, p. 901, 2001. [48] O. Yaglioglu, R. Martens, a. J. Hart, and a. H. Slocum, ?Conductive Carbon Nanotube Composite Microprobes,? Adv. Mater., vol. 20, no. 2, pp. 357?362, Jan. 2008. 109  [49] Z. F. Ren, Z. P. Huang, D. Z. Wang, J. G. Wen, J. W. Xu, J. H. Wang, L. E. Calvet, J. Chen, J. F. Klemic, and M. a. Reed, ?Growth of a single freestanding multiwall carbon nanotube on each nanonickel dot,? Appl. Phys. Lett., vol. 75, no. 8, p. 1086, 1999. [50] K. B. K. Teo, M. Chhowalla, G. a. J. Amaratunga, W. I. Milne, D. G. Hasko, G. Pirio, P. Legagneux, F. Wyczisk, and D. Pribat, ?Uniform patterned growth of carbon nanotubes without surface carbon,? Appl. Phys. Lett., vol. 79, no. 10, p. 1534, 2001. [51] H. Kind, J.-M. Bonard, C. Emmenegger, L.-O. Nilsson, K. Hernadi, E. Maillard-Schaller, L. Schlapbach, L. Forr , and K. Kern, ?Patterned Films of Nanotubes Using Microcontact Printing of Catalysts,? Adv. Mater., vol. 11, no. 15, pp. 1285?1289, Oct. 1999. [52] L. Nilsson, O. Groening, C. Emmenegger, O. Kuettel, E. Schaller, L. Schlapbach, H. Kind, J.-M. Bonard, and K. Kern, ?Scanning field emission from patterned carbon nanotube films,? Appl. Phys. Lett., vol. 76, no. 15, p. 2071, 2000. [53] S. Fan, ?Self-Oriented Regular Arrays of Carbon Nanotubes and Their Field Emission Properties,? Science (80-. )., vol. 283, no. 5401, pp. 512?514, Jan. 1999. [54] J. Li, C. Papadopoulos, J. M. Xu, and M. Moskovits, ?Highly-ordered carbon nanotube arrays for electronics applications,? Appl. Phys. Lett., vol. 75, no. 3, p. 367, 1999. [55] F. C. Cheong, K. Y. Lim, C. H. Sow, J. Lin, and C. K. Ong, ?Large area patterned arrays of aligned carbon nanotubes via laser trimming,? Nanotechnology, vol. 14, no. 4, pp. 433?437, Apr. 2003. [56] D. Jiang, T. Wang, S. Chen, L. Ye, and J. Liu, ?Paper-mediated controlled densification and low temperature transfer of carbon nanotube forests for electronic interconnect application,? Microelectron. Eng., vol. 103, pp. 177?180, Mar. 2013. [57] T. Wang, D. Jiang, S. Chen, K. Jeppson, L. Ye, and J. Liu, ?Formation of three-dimensional carbon nanotube structures by controllable vapor densification,? Mater. Lett., vol. 78, pp. 184?187, Jul. 2012. [58] K. Y. Lim, C. H. Sow, J. Lin, F. C. Cheong, Z. X. Shen, J. T. L. Thong, K. C. Chin, and A. T. S. Wee, ?Laser Pruning of Carbon Nanotubes as a Route to Static and Movable Structures,? Adv. Mater., vol. 15, no. 4, pp. 300?303, Feb. 2003. [59] Z. H. Lim and C.-H. Sow, ?Laser-Induced Rapid Carbon Nanotube Micro-Actuators,? Adv. Funct. Mater., vol. 20, no. 5, pp. 847?852, Mar. 2010. [60] W. H. Hung, R. Kumar, A. Bushmaker, S. B. Cronin, and M. J. Bronikowski, ?Rapid prototyping of three-dimensional microstructures from multiwalled carbon nanotubes,? Appl. Phys. Lett., vol. 91, no. 9, p. 093121, 2007. 110  [61] K. Jain and J. Chae, ?Ablation assistor enables low-fluence photoablation of carbon nanotubes,? SPIE Newsroom, pp. 2?3, 2009. [62] Y. W. Zhu, C.-H. Sow, M.-C. Sim, G. Sharma, and V. Kripesh, ?Scanning localized arc discharge lithography for the fabrication of microstructures made of carbon nanotubes,? Nanotechnology, vol. 18, no. 38, p. 385304, Sep. 2007. [63] T. D. Yuzvinsky, a. M. Fennimore, W. Mickelson, C. Esquivias, and a. Zettl, ?Precision cutting of nanotubes with a low-energy electron beam,? Appl. Phys. Lett., vol. 86, no. 5, p. 053109, 2005. [64] A. Rubio, S. P. Apell, L. C. Venema, and C. Dekker, ?A mechanism for cutting carbon nanotubes with a scanning tunneling microscope,? Eur. Phys. J. B, vol. 17, no. 2, pp. 301?308, Sep. 2000. [65] J.-Y. Park, Y. Yaish, M. Brink, S. Rosenblatt, and P. L. McEuen, ?Electrical cutting and nicking of carbon nanotubes using an atomic force microscope,? Appl. Phys. Lett., vol. 80, no. 23, p. 4446, 2002. [66] W. Khalid, M. S. M. Ali, M. Dahmardeh, Y. Choi, P. Yaghoobi, A. Nojeh, and K. Takahata, ?High-aspect-ratio, free-form patterning of carbon nanotube forests using micro-electro-discharge machining,? Diam. Relat. Mater., vol. 19, no. 11, pp. 1405?1410, Nov. 2010. [67] M. Dahmardeh, W. Khalid, M. S. Mohamed Ali, Y. Choi, P. Yaghoobi, a. Nojeh, and K. Takahata, ?High-aspect-ratio, 3-D micromachining of carbon-nanotube forests by micro-electro-discharge machining in air,? 2011 IEEE 24th Int. Conf. Micro Electro Mech. Syst., pp. 272?275, Jan. 2011. [68] N. Chakrapani, B. Wei, A. Carrillo, P. M. Ajayan, and R. S. Kane, ?Capillarity-driven assembly of two-dimensional cellular carbon nanotube foams.,? Proc. Natl. Acad. Sci. U. S. A., vol. 101, no. 12, pp. 4009?12, Mar. 2004. [69] D. N. Futaba, K. Hata, T. Yamada, T. Hiraoka, Y. Hayamizu, Y. Kakudate, O. Tanaike, H. Hatori, M. Yumura, and S. Iijima, ?Shape-engineerable and highly densely packed single-walled carbon nanotubes and their application as super-capacitor electrodes.,? Nat. Mater., vol. 5, no. 12, pp. 987?94, Dec. 2006. [70] T. Masaki, K. Kawata, and T. Masuzawa, ?Micro electro-discharge machining and its applications,? in IEEE Proceedings on Micro Electro Mechanical Systems, An Investigation of Micro Structures, Sensors, Actuators, Machines and Robots., 1990, pp. 21?26. 111  [71] B. H. Kim, J. G. Ok, Y. H. Kim, and C. N. Chu, ?Electrical Discharge Machining of Carbon Nanofiber for Uniform Field Emission,? CIRP Ann. - Manuf. Technol., vol. 56, no. 1, pp. 233?236, Jan. 2007. [72] Y. . Wong, M. Rahman, H. . Lim, H. Han, and N. Ravi, ?Investigation of micro-EDM material removal characteristics using single RC-pulse discharges,? J. Mater. Process. Technol., vol. 140, no. 1?3, pp. 303?307, Sep. 2003. [73] P. Collins, M. Hersam, M. Arnold, R. Martel, and P. Avouris, ?Current Saturation and Electrical Breakdown in Multiwalled Carbon Nanotubes,? Phys. Rev. Lett., vol. 86, no. 14, pp. 3128?3131, Apr. 2001. [74] P. M. Ajayan, T. W. Ebbesen, T. Ichihashi, S. Iijima, K. Tanigaki, and H. Hiura, ?Opening carbon nanotubes with oxygen and implications for filling,? Nature, vol. 362, no. 6420, pp. 522?525, Apr. 1993. [75] Y. Liu, L. Liu, P. Liu, L. Sheng, and S. Fan, ?Plasma etching carbon nanotube arrays and the field emission properties,? Diam. Relat. Mater., vol. 13, no. 9, pp. 1609?1613, Sep. 2004. [76] K. Yu, Z. Zhu, Y. Zhang, Q. Li, W. Wang, L. Luo, X. Yu, H. Ma, Z. Li, and T. Feng, ?Change of surface morphology and field emission property of carbon nanotube films treated using a hydrogen plasma,? Appl. Surf. Sci., vol. 225, no. 1?4, pp. 380?388, Mar. 2004. [77] M. S. Dresselhaus, G. Dresselhaus, R. Saito, and a. Jorio, ?Raman spectroscopy of carbon nanotubes,? Phys. Rep., vol. 409, no. 2, pp. 47?99, Mar. 2005. [78] C.-P. Juan, C.-C. Tsai, K.-H. Chen, L.-C. Chen, and H.-C. Cheng, ?Effects of High-Density Oxygen Plasma Posttreatment on Field Emission Properties of Carbon Nanotube Field-Emission Displays,? Jpn. J. Appl. Phys., vol. 44, no. 11, pp. 8231?8236, Nov. 2005. [79] Z. Li, P. Dharap, S. Nagarajaiah, E. V. Barrera, and J. D. Kim, ?Carbon Nanotube Film Sensors,? Adv. Mater., vol. 16, no. 7, pp. 640?643, Apr. 2004. [80] J. Sandler, M. Shaffer, a. Windle, M. Halsall, M. Montes-Mor?n, C. Cooper, and R. Young, ?Variations in the Raman peak shift as a function of hydrostatic pressure for various carbon nanostructures: A simple geometric effect,? Phys. Rev. B, vol. 67, no. 3, p. 035417, Jan. 2003. [81] S. L. Ruan, P. Gao, X. G. Yang, and T. X. Yu, ?Toughening high performance ultrahigh molecular weight polyethylene using multiwalled carbon nanotubes,? Polymer (Guildf)., vol. 44, no. 19, pp. 5643?5654, Sep. 2003. 112  [82] M. Dahmardeh, A. Nojeh, and K. Takahata, ?Possible mechanism in dry micro-electro-discharge machining of carbon-nanotube forests: A study of the effect of oxygen,? J. Appl. Phys., vol. 109, no. 9, p. 093308, May 2011. [83] B. Liang, A. Ogino, and M. Nagatsu, ?Discharge characteristics of a nano-sized electrode with aligned carbon nanotubes grown on a tungsten whisker tip under various gas conditions,? J. Phys. D. Appl. Phys., vol. 43, no. 27, p. 275202, Jul. 2010. [84] W. A. de Heer, A. Ch telain, and D. Ugarte, ?A Carbon Nanotube Field-Emission Electron Source,? Science (80-. )., vol. 270, no. 5239, pp. 1179?1180, Nov. 1995. [85] J. Bonard, M. Croci, C. Klinke, R. Kurt, O. Noury, and N. Weiss, ?Carbon nanotube films as electron field emitters,? Carbon N. Y., vol. 40, no. 10, pp. 1715?1728, Aug. 2002. [86] P. G. Collins and a. Zettl, ?A simple and robust electron beam source from carbon nanotubes,? Appl. Phys. Lett., vol. 69, no. 13, p. 1969, 1996. [87] Y. Saito, Carbon Nanotube and Related Field Emitters. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA, 2010. [88] K. Sun, J. Y. Lee, B. Li, W. Liu, C. Miao, Y.-H. Xie, X. Wei, and T. P. Russell, ?Fabrication and field emission study of atomically sharp high-density tungsten nanotip arrays,? J. Appl. Phys., vol. 108, no. 3, p. 036102, 2010. [89] H. C. Miller, ?Electrical discharges in vacuum 1877-1979,? IEEE Trans. Electr. Insul., vol. 25, no. 5, pp. 765?860, 1990. [90] S. Purcell, P. Vincent, C. Journet, and V. Binh, ?Hot Nanotubes: Stable Heating of Individual Multiwall Carbon Nanotubes to 2000 K Induced by the Field-Emission Current,? Phys. Rev. Lett., vol. 88, no. 10, p. 105502, Feb. 2002. [91] T. Saleh, M. Dahmardeh, A. Bsoul, A. Nojeh, and K. Takahata, ?Field-emission-assisted approach to dry micro-electro-discharge machining of carbon-nanotube forests,? J. Appl. Phys., vol. 110, no. 10, p. 103305, 2011. [92] T. Rokunohe, Y. Yagihashi, F. Endo, and T. Oomori, ?Fundamental insulation characteristics of air; N2, CO2, N2/O2, and SF6/N2 mixed gases,? Electr. Eng. Japan, vol. 155, no. 3, pp. 9?17, May 2006. [93] Z. Hou, B. Cai, H. Liu, and D. Xu, ?Ar, O2, CHF3, and SF6 plasma treatments of screen-printed carbon nanotube films for electrode applications,? Carbon N. Y., vol. 46, no. 3, pp. 405?413, Mar. 2008. 113  [94] A. Barlow, A. Birch, A. Deslandes, and J. Quinton, ?Plasma Fluorination of Highly Ordered Pyrolytic Graphite and Single Walled Carbon Nanotube Surfaces,? 2006 Int. Conf. Nanosci. Nanotechnol., pp. 103?106, 2006. [95] A. Descoeudres, ?Characterization of electrical discharge machining plasmas,? EPFL, 2006. [96] R. Geballe and M. Reeves, ?A Condition on Uniform Field Breakdown in Electron-Attaching Gases,? Phys. Rev., vol. 92, no. 4, pp. 867?868, Nov. 1953. [97] T. Nitta and Y. Shibuya, ?Electrical Breakdown of Long Gaps in Sulfur Hexafluoride,? IEEE Trans. Power Appar. Syst., vol. PAS-90, no. 3, pp. 1065?1071, May 1971. [98] T. Saleh, M. Dahmardeh, A. Bsoul, A. Nojeh, and K. Takahata, ?High-precision dry micro-electro-discharge machining of carbon-nanotube forests with ultralow discharge energy,? in 2012 IEEE 25th International Conference on Micro Electro Mechanical Systems (MEMS), 2012, no. February, pp. 259?262. [99] B. McCarthy, G. G. Adams, N. E. McGruer, and D. Potter, ?A dynamic model, including contact bounce, of an electrostatically actuated microswitch,? J. Microelectromechanical Syst., vol. 11, no. 3, pp. 276?283, Jun. 2002. [100] S. Zhou, X.-Q. Sun, and W. N. Carr, ?A monolithic variable inductor network using microrelays with combined thermal and electrostatic actuation,? J. Micromechanics Microengineering, vol. 9, no. 1, pp. 45?50, Mar. 1999. [101] G. M. Rebeiz and J. B. Muldavin, ?RF MEMS switches and switch circuits,? IEEE Microw. Mag., vol. 2, no. 4, pp. 59?71, 2001. [102] P. G. Steeneken and O. Wunnicke, ?Performance limits of MEMS switches for power electronics,? in 2012 24th International Symposium on Power Semiconductor Devices and ICs, 2012, pp. 417?420. [103] Y.-H. Song, C.-H. Han, M.-W. Kim, J. O. Lee, and J.-B. Yoon, ?An Electrostatically Actuated Stacked-Electrode MEMS Relay With a Levering and Torsional Spring for Power Applications,? J. Microelectromechanical Syst., vol. 21, no. 5, pp. 1209?1217, Oct. 2012. [104] F. Ke, J. Miao, and J. Oberhammer, ?A Ruthenium-Based Multimetal-Contact RF MEMS Switch With a Corrugated Diaphragm,? J. Microelectromechanical Syst., vol. 17, no. 6, pp. 1447?1459, Dec. 2008. [105] S. T. Patton and J. S. Zabinski, ?Fundamental studies of Au contacts in MEMS RF switches,? Tribol. Lett., vol. 18, no. 2, pp. 215?230, Feb. 2005. 114  [106] M.-A. Gr?tillat, F. Gr?tillat, and N. F. de Rooij, ?Micromechanical relay with electrostatic actuation and metallic contacts,? J. Micromechanics Microengineering, vol. 9, no. 4, pp. 324?331, Dec. 1999. [107] C. L. Goldsmith, Z. Yao, S. Eshelman, and D. Denniston, ?Performance of low-loss RF MEMS capacitive switches,? IEEE Microw. Guid. Wave Lett., vol. 8, no. 8, pp. 269?271, 1998. [108] E. J. J. Kruglick and K. S. J. Pister, ?Lateral MEMS microcontact considerations,? J. Microelectromechanical Syst., vol. 8, no. 3, pp. 264?271, 1999. [109] R. A. Coutu, P. E. Kladitis, K. D. Leedy, and R. L. Crane, ?Selecting metal alloy electric contact materials for MEMS switches,? J. Micromechanics Microengineering, vol. 14, no. 8, pp. 1157?1164, Aug. 2004. [110] L. L. W. Chow, J. L. Volakis, K. Saitou, and K. Kurabayashi, ?Lifetime Extension of RF MEMS Direct Contact Switches in Hot Switching Operations by Ball Grid Array Dimple Design,? IEEE Electron Device Lett., vol. 28, no. 6, pp. 479?481, Jun. 2007. [111] A. Cao, P. Yuen, and L. Lin, ?Microrelays With Bidirectional Electrothermal Electromagnetic Actuators and Liquid Metal Wetted Contacts,? J. Microelectromechanical Syst., vol. 16, no. 3, pp. 700?708, Jun. 2007. [112] Y.-H. Song, D.-H. Choi, H.-H. Yang, and J.-B. Yoon, ?An Extremely Low Contact-Resistance MEMS Relay Using Meshed Drain Structure and Soft Insulating Layer,? J. Microelectromechanical Syst., vol. 20, no. 1, pp. 204?212, Feb. 2011. [113] H.-S. Lee, C. H. Leung, J. Shi, S.-C. Chang, S. Lorincz, and I. Nedelescu, ?Integrated microrelays: concept and initial results,? J. Microelectromechanical Syst., vol. 11, no. 2, pp. 147?153, Apr. 2002. [114] J. Oberhammer and G. Stemme, ?Active Opening Force and Passive Contact Force Electrostatic Switches for Soft Metal Contact Materials,? J. Microelectromechanical Syst., vol. 15, no. 5, pp. 1235?1242, Oct. 2006. [115] W. Simon, B. Schauwecker, A. Lauer, A. Wien, and I. Wolff, ?EM design of broadband RF multiport toggle switches,? Int. J. RF Microw. Comput. Eng., vol. 14, no. 4, pp. 329?337, Jul. 2004. [116] C. Hierold, Carbon Nanotube Devices: Properties, Modeling, Integration and Applications. Wiley-VCH, Weinheim, Germany, 2008. [117] L. Dong, S. Youkey, J. Bush, J. Jiao, V. M. Dubin, and R. V. Chebiam, ?Effects of local Joule heating on the reduction of contact resistance between carbon nanotubes and metal electrodes,? J. Appl. Phys., vol. 101, no. 2, p. 024320, Jan. 2007. 115  [118] J. Robertson, G. Zhong, C. S. Esconjauregui, B. C. Bayer, C. Zhang, M. Fouquet, and S. Hofmann, ?Applications of Carbon Nanotubes Grown by Chemical Vapor Deposition,? Jpn. J. Appl. Phys., vol. 51, no. 1, p. 01AH01, Jan. 2012. [119] N. R. Franklin, Y. Li, R. J. Chen, A. Javey, and H. Dai, ?Patterned growth of single-walled carbon nanotubes on full 4-inch wafers,? Appl. Phys. Lett., vol. 79, no. 27, p. 4571, Dec. 2001. [120] D. N. Hutchison, N. B. Morrill, Q. Aten, B. W. Turner, B. D. Jensen, L. L. Howell, R. R. Vanfleet, and R. C. Davis, ?Carbon Nanotubes as a Framework for High-Aspect-Ratio MEMS Fabrication,? J. Microelectromechanical Syst., vol. 19, no. 1, pp. 75?82, Feb. 2010. [121] Y. Hanein, ?Carbon nanotube integration into MEMS devices,? Phys. status solidi, vol. 247, no. 11?12, pp. 2635?2640, Dec. 2010. [122] N. R. Franklin, Q. Wang, T. W. Tombler, A. Javey, M. Shim, and H. Dai, ?Integration of suspended carbon nanotube arrays into electronic devices and electromechanical systems,? Appl. Phys. Lett., vol. 81, no. 5, p. 913, Jul. 2002. [123] C. Wang, K. Takei, T. Takahashi, and A. Javey, ?Carbon nanotube electronics - moving forward.,? Chem. Soc. Rev., Dec. 2012. [124] B. R. Burg, T. Helbling, C. Hierold, and D. Poulikakos, ?Piezoresistive pressure sensors with parallel integration of individual single-walled carbon nanotubes,? J. Appl. Phys., vol. 109, no. 6, p. 064310, Mar. 2011. [125] K. Chikkadi, C. Roman, L. Durrer, T. S?ss, R. Pohle, and C. Hierold, ?Scalable Fabrication of Individual SWNT Chem-FETs for Gas Sensing,? Procedia Eng., vol. 47, no. null, pp. 1374?1377, Jan. 2012. [126] J. Cao, C. Nyffeler, K. Lister, and A. M. Ionescu, ?Resist-assisted assembly of single-walled carbon nanotube devices with nanoscale precision,? Carbon N. Y., vol. 50, no. 5, pp. 1720?1726, Apr. 2012. [127] A. Arun, H. Le Poche, T. Idda, D. Acquaviva, M. F.-B. Badia, P. Pantigny, P. Salet, and A. M. Ionescu, ?Tunable MEMS capacitors using vertical carbon nanotube arrays grown on metal lines.,? Nanotechnology, vol. 22, no. 2, p. 025203, Jan. 2011. [128] J. Choi, J.-I. Lee, Y. Eun, M.-O. Kim, and J. Kim, ?Aligned carbon nanotube arrays for degradation-resistant, intimate contact in micromechanical devices.,? Adv. Mater., vol. 23, no. 19, pp. 2231?6, May 2011. 116  [129] Y. Wang, Z. Li, D. T. McCormick, and N. C. Tien, ?A micromachined RF microrelay with electrothermal actuation,? Sensors Actuators A Phys., vol. 103, no. 1?2, pp. 231?236, Jan. 2003. [130] W. Shi, N. C. Tien, and Z. Li, ?A Highly Reliable Lateral MEMS Switch Utilizing Undoped Polysilicon as Isolation Material,? J. Microelectromechanical Syst., vol. 16, no. 5, pp. 1173?1184, Oct. 2007. [131] M. Glickman, P. Tseng, J. Harrison, T. Niblock, I. B. Goldberg, and J. W. Judy, ?High-Performance Lateral-Actuating Magnetic MEMS Switch,? J. Microelectromechanical Syst., vol. 20, no. 4, pp. 842?851, Aug. 2011. [132] H.-C. Lee, J.-Y. Park, and J.-U. Bu, ?Piezoelectrically actuated RF MEMS DC contact switches with low voltage operation,? IEEE Microw. Wirel. Components Lett., vol. 15, no. 4, pp. 202?204, Apr. 2005. [133] P. Krulevitch, A. P. Lee, P. B. Ramsey, J. C. Trevino, J. Hamilton, and M. A. Northrup, ?Thin film shape memory alloy microactuators,? J. Microelectromechanical Syst., vol. 5, no. 4, pp. 270?282, 1996. [134] M. S. Mohamed Ali and K. Takahata, ?Frequency-controlled wireless shape-memory-alloy microactuators integrated using an electroplating bonding process,? Sensors Actuators A Phys., vol. 163, no. 1, pp. 363?372, Sep. 2010. [135] M. Kohl, B. Krevet, and E. Just, ?SMA microgripper system,? Sensors Actuators A Phys., vol. 97?98, pp. 646?652, Apr. 2002. [136] M. S. Mohamed Ali and K. Takahata, ?Wireless microfluidic control with integrated shape-memory-alloy actuators operated by field frequency modulation,? J. Micromechanics Microengineering, vol. 21, no. 7, p. 075005, Jul. 2011. [137] D. Xu, L. Wang, G. Ding, Y. Zhou, A. Yu, and B. Cai, ?Characteristics and fabrication of NiTi/Si diaphragm micropump,? Sensors Actuators A Phys., vol. 93, no. 1, pp. 87?92, Aug. 2001. [138] C. Megnin, J. Barth, and M. Kohl, ?A bistable SMA microvalve for 3/2-way control,? Sensors Actuators A Phys., vol. 188, pp. 285?291, Dec. 2012. [139] B. Bhattacharya and O. P. Patel, ?A New Shape Memory Alloy Based Smart Encoder for Sensing of Direction and Angular Motion,? Sensors Transducers J., vol. 130, no. 7, pp. 103?117, 2011. [140] J. Barth, B. Krevet, and M. Kohl, ?A bistable shape memory microswitch with high energy density,? Smart Mater. Struct., vol. 19, no. 9, p. 094004, Sep. 2010. 117  [141] J.-E. Wong, J. H. Lang, and M. A. Schmidt, ?An electrostatically-actuated MEMS switch for power applications,? in Proceedings IEEE Thirteenth Annual International Conference on Micro Electro Mechanical Systems (Cat. No.00CH36308), 2000, pp. 633?638. [142] C. T. Wirth, S. Hofmann, and J. Robertson, ?Surface properties of vertically aligned carbon nanotube arrays,? Diam. Relat. Mater., vol. 17, no. 7?10, pp. 1518?1524, Jul. 2008. [143] S. K. Thakur, K. A. SumithraDevi, and I. Ranjitha, ?Performance of low loss RF MEMS Fixed - Fixed capacitive switch characterization,? in 2009 Applied Electromagnetics Conference (AEMC), 2009, pp. 1?4. [144] F. Wakaya, K. Katayama, and K. Gamo, ?Contact resistance of multiwall carbon nanotubes,? Microelectron. Eng., vol. 67?68, no. null, pp. 853?857, Jun. 2003. [145] B. D. Jensen, L. L.-W. Chow, K. Huang, K. Saitou, J. L. Volakis, and K. Kurabayashi, ?Effect of nanoscale heating on electrical transport in RF MEMS switch contacts,? J. Microelectromechanical Syst., vol. 14, no. 5, pp. 935?946, Oct. 2005. [146] B. C. Bayer, S. Sanjabi, C. Baehtz, C. T. Wirth, S. Esconjauregui, R. S. Weatherup, Z. H. Barber, S. Hofmann, and J. Robertson, ?Carbon nanotube forest growth on NiTi shape memory alloy thin films for thermal actuation,? Thin Solid Films, vol. 519, no. 18, pp. 6126?6129, Jul. 2011. [147] D. C. Lagoudas, D. A. Miller, L. Rong, and P. K. Kumar, ?Thermomechanical fatigue of shape memory alloys,? Smart Mater. Struct., vol. 18, no. 8, p. 085021, Aug. 2009. [148] M. Dahmardeh, M. S. Mohamed Ali, T. Saleh, T. M. Hian, M. V. Moghaddam, A. Nojeh, and K. Takahata, ?High-power MEMS switch enabled by carbon-nanotube contact and shape-memory-alloy actuator,? Phys. Status Solidi, vol. 210, no. 4, pp. 631?638, Apr. 2013. [149] A. Cao, P. L. Dickrell, W. G. Sawyer, M. N. Ghasemi-Nejhad, and P. M. Ajayan, ?Super-compressible foamlike carbon nanotube films.,? Science, vol. 310, no. 5752, pp. 1307?10, Nov. 2005. [150] H. Huang, C. H. Liu, Y. Wu, and S. Fan, ?Aligned Carbon Nanotube Composite Films for Thermal Management,? Adv. Mater., vol. 17, no. 13, pp. 1652?1656, Jul. 2005. [151] K. K. S. Lau, J. Bico, K. B. K. Teo, M. Chhowalla, G. a. J. Amaratunga, W. I. Milne, G. H. McKinley, and K. K. Gleason, ?Superhydrophobic Carbon Nanotube Forests,? Nano Lett., vol. 3, no. 12, pp. 1701?1705, Dec. 2003. 118  [152] Y. Murakami, E. Einarsson, T. Edamura, and S. Maruyama, ?Polarization Dependence of the Optical Absorption of Single-Walled Carbon Nanotubes,? Phys. Rev. Lett., vol. 94, no. 8, p. 087402, Mar. 2005. [153] K.-C. Hsieh, T.-Y. Tsai, D. Wan, H.-L. Chen, and N.-H. Tai, ?Iridescence of patterned carbon nanotube forests on flexible substrates: from darkest materials to colorful films.,? ACS Nano, vol. 4, no. 3, pp. 1327?36, Mar. 2010. [154] M. Vahdani Moghaddam, M. S. Sarwar, Z. Xiao, M. Dahmardeh, K. Takahata, and A. Nojeh, ?Field-Emission from Carbon Nanotube Cones Fabricated by Micro-Electro-Discharge Machining,? in 26th international Vacuum Nanoelectronics Conference (IVNC), 2013, pp. 1?2. [155] Z. Xiao, M. Saquib Sarwar, M. Dahmardeh, M. Vahdani Moghaddam, A. Nojeh, and K. Takahata, ?Cone-shaped forest of aligned carbon nanotubes: An alternative probe for scanning microscopy,? Appl. Phys. Lett., vol. 103, no. 17, p. 171603, 2013. [156] P. Joseph, C. Cottin-Bizonne, J.-M. Beno?t, C. Ybert, C. Journet, P. Tabeling, and L. Bocquet, ?Slippage of Water Past Superhydrophobic Carbon Nanotube Forests in Microchannels,? Phys. Rev. Lett., vol. 97, no. 15, p. 156104, Oct. 2006. [157] J. Kong, N. Franklin, C. Zhou, M. Chapline, S. Peng, K. Cho, and H. Dai, ?Nanotube molecular wires as chemical sensors,? Science, vol. 287, no. 5453, pp. 622?5, Jan. 2000. [158] S. Ammu, V. Dua, S. R. Agnihotra, S. P. Surwade, A. Phulgirkar, S. Patel, and S. K. Manohar, ?Flexible, all-organic chemiresistor for detecting chemically aggressive vapors.,? J. Am. Chem. Soc., vol. 134, no. 10, pp. 4553?6, Mar. 2012. [159] T. Saleh, M. Dahmardeh, A. Nojeh, and K. Takahata, ?Dry micro-electro-discharge machining of carbon-nanotube forests using sulphur-hexafluoride,? Carbon N. Y., vol. 52, pp. 288?295, Feb. 2013. [160] M. Dahmardeh, M. Vahdani Moghaddam, M. Hian Tee, A. Nojeh, and K. Takahata, ?The effects of three-dimensional shaping of vertically aligned carbon-nanotube contacts for micro-electro-mechanical switches,? Appl. Phys. Lett., vol. 103, no. 23, p. 231606, 2013.   119  Appendices Appendix A  Carbon Nanotube Forest Growth System  The CNT forests used for the experiments presented in section  2.2 and some experiments of Chapter 3 were produced by an atmospheric pressure chemical vapour deposition (CVD) system. The CVD growth system was developed by the author and Anas Bsoul. Figure A1 shows the CVD growth systems. 10 nm of alumina (Al2O3) and 1.5 nm of iron (Fe) were deposited on the silicon wafer with e-beam deposition technique as catalyst. The chips that had been placed on the local silicon heater (Figure A2) in the reaction zone were annealed at 800 ?C while H2 and Ar gases flow at the rates of 800 and 1500 sccm, respectively. A power supply located under the system provided the current needed to heat up the silicon wafer. The flow-rates were controlled using manual or automatic flow controllers. All gases passed through a pre-heater system which was heated to 850 ?C before arriving at the reaction zone. After 1 minute of annealing, 400 sccm of ethylene (C2H4) was introduced while maintaining the H2 and Ar gases flow rates. The growth process began from the time ethylene was introduced to the reaction tube and could be from few minutes up to one hour, depending on the length that is needed. As the growth was finished, the silicon heater was turned off by shutting down the power supply and stopping to flow of all gases except for Ar. The chips with carbon nanotube forests were cooled down for few minutes under the flow of Ar gas, before opening the chamber. It was very important to close the H2 and C2H4 gas valves before opening the chamber to prevent any explosions.  120   Figure A1: Atmospheric Pressure CVD growth system of carbon nanotube   Figure A2: Assembly of the silicon heater of the CVD growth system    121  Appendix B  Field-Emission from Carbon Nanotube Cones Fabricated by Micro-Electro-Discharge Machining  In this section7, field emission properties of micropatterned and non-patterned carbon nanotube forests are presented as the second application of the developed machining technique. Micropatterning is carried out using dry micro-electro-discharge machining.   Introduction CNTs, with their extremely high-aspect-ratio geometries, were considered to be a good choice for applications in vacuum nanoelectronics. In some applications, for example in creating large-area field-emission sources, one needs to have a three-dimensionally engineered cathode or a cathode with a flat surface that can produce a large, flat electron beam. Here, it was shown that using ?EDM, one can manufacture cathodes with uniform surfaces or 3D cathodes in an array of vertically aligned CNTs to produce more uniform and/or confined electron beams from the array.  Experiment and Results The catalyst is patterned on a highly p-doped silicon wafer with the shape of uniform circles with a diameter of ~2.3 mm. A CVD reactor was used to grow the vertically aligned multiwalled CNTs with the shape of pillars. The growth process is similar to  Appendix A  . Figure B1 shows the schematic view of the experimental setup. An external voltage (V1) was applied to a stainless-steel mesh with an opening size of ~279 ?m to extract the electrons from                                                  7 Portion of this section is presented in a proceeding (Reused with permission from ?M. Vahdani Moghaddam, M.S. Sarwar, Z. Xiao, M. Dahmardeh, K. Takahata, A. Nojeh, ?Field-emission from carbon nanotube cones fabricated by micro-electro-discharge machining?, 26th Int'l Vacuum Nanoelectronics Conf. (IVNC 2013), Roanoke, USA, 2013?, Copyright ? 2013 IEEE)   122  the machined and non-machined CNT forests. A phosphor screen, coated on ITO glass, was used as anode.  Figure B1: Schematic view of the experimental setup  As the first experiment, we tested the effect of machining on the uniformity of electron emission from the top surface of the CNT forest pillar and compared it with the shape of electron emission from a non-machined pillar. For this, we flattened the top surface of one CNT forest pillar by removing its surface layer with ?EDM. Figure B2 show images of the electron beams resulting from the machined and non-machined CNT forest pillars, respectively, on the phosphor screen.   Figure B2: Field-emission from (a) ?EDMed pillar and (b) original pillar. 123  As can be seen, the electron beam from the non-machined pillar is not uniform. This is due to the variations in the height of the nanotubes in the array; growing an array of CNTs with identical heights is very difficult. The result clearly indicates that the uniformity was improved by the ?EDM surface treatment. ?EDM cuts long nanotubes in the CNT forest pillar and creates a smooth surface. Interestingly, we also noticed that electron field-emission from the ?EDMed surfaces occurred at lower electric fields. For example, we studied the electron emission from a chip that had three pillars, one machined with the height of ~62 ?m and two bare CNT pillars with the heights of ~123 ?m and ~145 ?m, respectively (Figure B3). We observed that at 1100 V the electron beam only comes from the machined pillar.    Figure B3: Machined and non-machined CNT forest pillars.  Cutting the nanotubes along the z-direction (along the height of nanotubes) with ?EDM enables to study the electron emission irradiation for pillars with various heights under identical electric fields. For this, we imaged the electron emission pattern from two machined pillars with different heights and located on one chip (Figure B4). As we see, the electron emission from the 124  taller pillar is much brighter than the shorter pillar. This level of engineering can be achieved only through height control with a process such as ?EDM and is not possible with lithography.   Figure B4: Image of electron emission from two machined CNT forest pillars with different heights: 120 ?m (dark circle) and 150 ?m (bright circle) at (a) E=5.5?105 (V/m) and   (b) E=7?105 (V/m).      Another unique enabling aspect of ?EDM ? that of creation of angled surfaces - is demonstrated in Figure B5: we fabricated a 3D cone with a diameter of ~100 ?m. As opposed to the flat CNT pillars that generate a wide electron beam, in the case of the cone, the electron beam originates only from the region around the tip, resulting in a much smaller spot on the phosphor screen (compare Figure B2(a) and Figure B5(b)). This opens up the possibility of combining the advantages of nanotube field-emitters with the mechanical robustness of the cone structure. 125   Figure B5: SEM image of cone made by (a) ?EDM, and (b) photograph of field electron emission from that at E=12?105 (V/m).  Conclusion Compared to bare CNT forests, field-emission from CNT forests treated by ?EDM occurs at lower electric fields and results in a more uniform electron beam. In addition, ?EDM would allow creating 3D shapes in nanotubes, such as a cone. The electron beam resulting from such structures is significantly more localized compared to the beam originating from a flat CNT forest. This unique ability of ?EDM in terms of height control and creation of angled surfaces opens the door to engineering a new class of field-emitters based on carbon nanotubes.    126  Appendix C  Cone-Shaped Forest of Aligned Carbon Nanotubes: An Alternative Probe for Scanning Microscopy  To demonstrate another application of the developed machining technique, this section8 reports a scanning microscopy probe based on three-dimensionally shaped CNT forests and its application to atomic-force microscopy (AFM). Micro-scale CNT forests directly grown on silicon cantilevers are patterned into cone shapes with the tips of a few individual nanotubes. The CNT-forest-based probes provide significantly higher mechanical stability/robustness than the common single-CNT probes. AFM imaging using the fabricated probes reveals their imaging ability comparable to that of commercial probes. The patterning process also improves the uniformity of the CNT forests grown on each cantilever. The results suggest a promising future for CNT scanning probes and their production approach.  Introduction Atomic force microscopy (AFM) nowadays plays a core role in materials science, surface physics, and biology, to name a few disciplines. The technique is considered as one of the most important inventions in materials science. Generally speaking, AFM is one form of the scanning probe microscope, which, as the name explains, uses a scanning probe to map the surface topography of samples. The probe typically has a micro-scale cantilever with a sharp tip formed at its end. The cantilever serves as a transducer that is used to detect the force signal exerted by the interaction between the probe tip and the sample surface. Typical materials used for AFM                                                  8 Part of this chapter has been published in a peer-reviewed journal (Reused with permission from ?Z. Xiao, M.S. Sarwar, M. Dahmardeh, M. Vahdani Moghaddam, A. Nojeh, and K. Takahata, ?Cone-shaped forest of aligned carbon nanotubes: An alternative probe for scanning microscopy?, Appl. Phys. Lett., 103, 171603, 2013?, Copyright ? 2013, AIP Publishing LLC)  127  probes are silicon (Si) and silicon nitride (Si3N4). These cantilever probes with desired spring constants and resonant frequency, morphology, and sharp tips can be mass-produced using MEMS technology. However, manufacturing AFM probes with high reproducibility in dimensional control of 10 nm or below at reasonable costs still remains a major challenge. In the full/intermittent contact mode, the geometry of the tip changes during the scanning process due to mechanical wear of the tip, deteriorating the quality of AFM images. Therefore, alternative materials are required for AFM probes with improved performance. CNTs have well-defined nano-scale geometry and, with three times the stiffness of Si, exhibit much higher wear resistance than that of Si. CNTs are therefore considered as one of the most promising candidates for AFM probe materials. CNT probes are commercially available and are currently produced through two main methods: assembly of a CNT at the tip of a micro-machined Si structure and direct growth of a CNT on a Si tip using chemical vapor deposition (CVD). The former is not only time-consuming but also needs expensive auxiliary instruments, such as a scanning electron microscope (SEM), to perform the nano-scale assembly in a precise and systematic manner. The latter method eliminates the need for assembly, which is promising in achieving relatively fast and low-cost fabrication of the probes. However, the control of CNT growth, including the orientation, the number of CNTs formed on each cantilever, and the wafer-scale uniformity of grown CNTs, still remains an essential issue.  In this section, an alternative, potentially low-cost, and wafer-scale method for manufacturing of CNT-based AFM probes is presented. Micro-scale arrays or ?forests? of vertically aligned CNTs are grown directly on tipless Si cantilevers. The CNT forests are then three-dimensionally (3D) patterned into cone shapes to be used as AFM probe tips. Dry micro-electro-discharge machining (?EDM) is employed for this 3D patterning. 128   Experiment and Results To create the CNT-forest probe and ensure its compatibility with AFM systems, we utilized a commercially available tipless Si cantilever probe (ACL-TL, Applied NanoStructures, Inc., CA, USA) and grew the CNT forest at the free end of the cantilever using CVD system. For this, first, the tipless probe was covered by a shadow mask except for the end of the cantilever where the catalyst layer (1-nm-thick iron on 10-nm-thick alumina) was deposited using electron-beam evaporation (Figure C1). The length of the exposed cantilever was adjusted to be 50 ?m to 150 ?m.  Figure C1: Catalyst deposition on the Si cantilever.  CNT forests with a height of ~100 ?m were grown on the area covered with catalyst. Dry ?EDM was performed in the CNT forest to fabricate cone-shaped probes. To create a cone shape in the forest material, a tungsten electrode with a diameter of 300 ?m was shaped into a 200-?m-long, circularly truncated cone (Figure C2) using wire electro-discharge grinding (WEDG) technique. The patterning process was then initiated in air using a discharge voltage of 50V. This discharge voltage was observed to be optimal in terms of the quality of machined forest surfaces in the present case. The sample was continuously moved along a circular orbit with a fixed 129  radius of 45 ?m using the X-Y stage of the system while feeding the electrode with the Z stage until it reached a target depth. In this process, the CNTs along the electrode path were removed, forming a cone shape in the forest.    Figure C2: (a) Tapered tungsten electrode used for ?EDM. Scale bar is 50 ?m. Illustration of the cone-shaping process at (b) the final stage of the process in which the forest is shaped to a sharp-tip cone.  Figure C3(a) displays the result of ?EDM patterning, showing a cone created in the CNT forest on the Si cantilever. The apex radius of the cone was measured to be approximately 2.5 ?m. Compared with commercial CNT AFM probes, in which a single CNT is commonly used as the probe tip fixed on a cantilever, the cone-shaped CNT-forest probes have higher mechanical stability/robustness due to their unique structure. The CNTs, whose tips make direct contact with the samples to be probed, are in the central region of the cone. The outer CNTs that surround the central CNTs have densities (estimated to be ~1015/m2 with an average distance of a few tens of nanometers between neighboring CNTs) high enough to physically support and protect the 130  central ones. Furthermore, those CNTs are entangled with each other, leading to constraining forces among the CNTs that tie them as a whole and further enhance the physical supporting effect. Although the contact between an individual CNT and the substrate may be weak, the contact of the CNT forest as a whole with the substrate (cantilever in this case) is strong. Therefore, the mechanical stability and robustness of these CNT probes are expected to be much higher than those available with conventional CNT probes fabricated with other methods described earlier, potentially enabling significantly improved reliability and longevity in CNT-based AFM probes. Theoretically, the developed process can produce a cone tip containing a single CNT only. However, as the CNT forest is machined in the dry ?EDM process, some of the debris of the removed CNTs tends to adhere to the electrode potentially due to less flushing effects compared with traditional wet ?EDM that is performed in liquid. As the carbon debris is electrically conductive, those stuck on the electrode essentially can serve as parts of the electrode and contribute to the removal of the forest in an unpredictable manner. This removal effect can also occur at the apex of the cone, deteriorating the sharpness of the cone?s tip. In order to answer this limitation, we used a multi-step machining process, in which machining is paused and the tool is cleaned after machining every few microns (3-5 ?m at the beginning and 0.5-1 ?m for finishing). The results are shown in Figures C3 (b) and (c).  Precise control of the height of CNT forests in CVD growth still remains an unsolved essential issue. Even if we initiate the synthesis of CNTs on multiple probes at the same time in the same reaction tube, the heights of the forests grown on the probes could be different from each other. 131   Figure C3: SEM images of CNT probes; (a) single-step (b), (c) multi-step machining.  Thus, achieving high uniformity of individual CNTs on the cantilever in terms of their final height is a great challenge if direct CVD growth is used to define the CNT probes. Post-growth treatments can be introduced to improve the uniformity. However, these treatments not only are time-consuming but also need rather expensive systems to perform, posing practical issues in product manufacturing. Here, the developed ?EDM process for cone patterning is directly applicable for customizing the final height of the CNT probes, to be precisely determined as part of the cone shaping process, addressing the non-uniformity issue. 132  AFM imaging was performed using probes 2 and 3 (Figures C3(b) and C3(c), respectively) in a commercially available AFM system (Easyscan 2, Nanosurf, Liestal, Switzerland). In order to evaluate the imaging ability of the fabricated CNT probes, we conducted the same imaging using a commercial Si probe (ACL-A, Applied NanoStructures, Inc., CA, USA; typical tip radius ~6 nm, apex half cone angle 11?, resonant frequency ~184 kHz). The tapping mode was used in these AFM tests, in which each probe was scanned over a standard calibration sample that has an array of 5-?m square dimples with a depth of 98 nm. In the developed probes, the presence of the CNTs on the cantilever decreased the resonant frequency of the cantilever. The rated nominal resonant frequency of the tipless probe used is 190 kHz, and this frequency was measured to be modified to 136 kHz and 170 kHz for probes 2 and 3, respectively. The difference between these resonant frequencies of probes 2 and 3 is presumed to be mainly associated with the difference in the thickness of the bottom layers of the forests (i.e., probe 2 has a thicker bottom layer and thus a higher mass, leading to the lower resonant frequency compared with probe 3). The AFM results are presented in Figures C4(a)?C4(f). Comparing between the results with the developed CNT probes and that with the commercial one, it is clear that the CNT probes were able to trace the contour of this square patterns similarly to the commercial probe. The root mean squared area/line roughness of the bottom surface of a single dimple and the top surface of the sample (between two dimples) was characterized. As shown in Table CI, the results obtained with probe 2 are somewhat larger than those with the commercial probe. We can see that the roughness obtained by probe 3 is close to that of the commercial probe. 133   Figure C4: AFM images of calibration microstructures measured using (a) probe 2, (c) probe 3, and (e) commercial Si probe. Corresponding cross-sectional pro-files of multiple dimples are shown in (b), (d), and (f), respectively.   The difference in these outcomes could be associated with that in the radius of the two probes. Nevertheless, the results demonstrate that the fabricated CNT probes can produce scanned data comparable to those that the commercial probe does in AFM imaging of micro-134  scale patterns with nanoscale steps. The machining process is being improved to achieve finer probe tips and higher lateral resolutions in AFM imaging.  Table C1: RRMS values of calibration dimple structures measured using fabricated CNT probes and commercial probe RRMS (nm) Probe-2 Probe-3 Commercial Probe Area Bottom 5.9 2.7 4.3 Top 4.6 3.4 2.5 Line Bottom 2.7 2.5 2.0 Top 3.1 2.5 1.6   Conclusions It has been demonstrated that dry ?EDM is highly effective in producing CNT-forest AFM probes with custom cone-shaped designs. The probe shaping process was performed on the CNT forests that were grown directly on the tips of Si cantilevers compatible with commercial AFM systems. This approach potentially enables CNT-based AFM probes to have remarkably higher mechanical stability and robustness, as well as to be batch manufactured with higher dimensional precisions at low costs through a path of parallel ?EDM. The results of AFM imaging obtained with the fabricated probes encourage further improvement of the probe and AFM imaging with it through nano-meter level dimension control and finishing of the probe tips.  

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