Since the discovery of X-rays over a century ago the techniques applied to the engineering of X-ray sources have remained relatively unchanged. From the inception of thermionic electron sources, which, due to simplicity of fabrication, remain central to almost all X-ray applications at this time, there have been few fundamental technological advances. The emergence of new materials and manufacturing techniques has created an opportunity to replace the traditional thermionic devices with those that incorporate Field Emission electron sources. One of the most important attributes of Field Emission X-ray sources is their controllability, and in particular the fast response time, which opens the door to applying techniques which have formerly been the preserve of optical systems. The work in this thesis attempts to bridge the gap between the fabrication and optimisation of the vacuum electronic devices and image processing aspects of a new approach to high speed radiographic imaging, particularly with a view to addressing practical real-world problems. Off the back of a specific targeted application, the project has involved the design of a viable field emission X-ray source, together with the development of an understanding of the failure modes in such devices, both by analysis and by simulation. This thesis reviews the capabilities and the requirements of X-ray sources, the methods by which nano-materials may be applied to the design of those devices and the improvements and attributes that can be foreseen. I study the image processing methods that can exploit these attributes, and investigate the performance of X-ray sources based upon electron emitters using carbon nanotubes. Modelling of the field emission and electron trajectories of the cathode assemblies has led me to the design of equipment to evaluate and optimise the parameters of an X-ray tube, which I have used to understand the performance that is achievable. Finally, I draw conclusions from this work and outline the next steps to provide the basis for a commercial solution.

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

<p>Thin films of refractory metals and carbides have been studied extensively over many years because of their wide range of application. The two major techniques used are Chemical Vapour Deposition (CVD) and Physical Vapour Deposition (PVD). These can result in the deposition of two-dimensional blanket or patterned thin films. Laser-assisted Chemical Vapour Deposition (LCVD) can provide a maskless alternative for localised deposition in two and three dimensions. This thesis describes LCVD of micrometer-sized tungsten, molybdenum and tungsten carbide rods. The kinetics, phase composition and microstructure have been studied as a function of in situ measured laser induced deposition temperature.</p><p>Tungsten and molybdenum rods were deposited by hydrogen reduction of their corresponding hexafluorides, WF6 and MoF6, respectively. Single crystal and polycrystalline tungsten rods were obtained, depending on the H2/WF6 molar ratio and deposition temperature. The molybdenum rods were either single crystals or dendritic in form depending on experimental conditions. The field emission characteristics of the tungsten single crystals were investigated. The results showed LCVD to be a potential fabrication technique for field emitting cathodes.</p><p>Nanocrystalline tungsten carbide rods were deposited from WF6, C2H4 and H2. TEM analysis showed that the carbide rods exhibited a layered structure in terms of phase composition and grain size as a result of the temperature gradient induced by the laser beam. With decreasing WF6/C2H4 molar ratio, the carbon content in the rods increased and the phase composition changed from W/W2C to WC/WC1-x and finally to WC1-x/C.</p>

<p>In several x-ray related areas there is a need for high-precision elements for x-ray generation and focusing. An elegant way of realizing x-ray related elements with high precision and low surface roughness is by the use of microfabrication; a combination of semiconductor processing techniques and miniaturization. Photolithographic patterning of silicon followed by deposition, etching, bonding and replication is used for batchwise fabrication of small well-defined structures. This thesis describes microfabrication of a miniature x-ray source and a refractive x-ray lens. A miniature x-ray source with diamond electrodes has been tested for x-ray fluorescence. Another version of the source has been vacuum encapsulated and run at atmospheric pressure. One-dimensionally focusing saw-tooth refractive x-ray lenses in silicon, epoxy, and diamond have been fabricated and tested in a synchrotron set-up. Sub-micron focal lines and gains of up to 40 were achieved. The conclusion of the thesis is that the use of microfabrication for construction of x-ray related components can not only improve the performance of existing components, but also open up for entirely new application areas.</p>

Thin films of refractory metals and carbides have been studied extensively over many years because of their wide range of application. The two major techniques used are Chemical Vapour Deposition (CVD) and Physical Vapour Deposition (PVD). These can result in the deposition of two-dimensional blanket or patterned thin films. Laser-assisted Chemical Vapour Deposition (LCVD) can provide a maskless alternative for localised deposition in two and three dimensions. This thesis describes LCVD of micrometer-sized tungsten, molybdenum and tungsten carbide rods. The kinetics, phase composition and microstructure have been studied as a function of in situ measured laser induced deposition temperature. Tungsten and molybdenum rods were deposited by hydrogen reduction of their corresponding hexafluorides, WF6 and MoF6, respectively. Single crystal and polycrystalline tungsten rods were obtained, depending on the H2/WF6 molar ratio and deposition temperature. The molybdenum rods were either single crystals or dendritic in form depending on experimental conditions. The field emission characteristics of the tungsten single crystals were investigated. The results showed LCVD to be a potential fabrication technique for field emitting cathodes. Nanocrystalline tungsten carbide rods were deposited from WF6, C2H4 and H2. TEM analysis showed that the carbide rods exhibited a layered structure in terms of phase composition and grain size as a result of the temperature gradient induced by the laser beam. With decreasing WF6/C2H4 molar ratio, the carbon content in the rods increased and the phase composition changed from W/W2C to WC/WC1-x and finally to WC1-x/C.

In several x-ray related areas there is a need for high-precision elements for x-ray generation and focusing. An elegant way of realizing x-ray related elements with high precision and low surface roughness is by the use of microfabrication; a combination of semiconductor processing techniques and miniaturization. Photolithographic patterning of silicon followed by deposition, etching, bonding and replication is used for batchwise fabrication of small well-defined structures. This thesis describes microfabrication of a miniature x-ray source and a refractive x-ray lens. A miniature x-ray source with diamond electrodes has been tested for x-ray fluorescence. Another version of the source has been vacuum encapsulated and run at atmospheric pressure. One-dimensionally focusing saw-tooth refractive x-ray lenses in silicon, epoxy, and diamond have been fabricated and tested in a synchrotron set-up. Sub-micron focal lines and gains of up to 40 were achieved. The conclusion of the thesis is that the use of microfabrication for construction of x-ray related components can not only improve the performance of existing components, but also open up for entirely new application areas.

In recent years, carbon nanotubes (CNTs) have emerged as one of the best field emitters for a variety of technological applications. The field emitting cathodes have several advantages over the conventional thermionic cathodes: (i) current density from field emission would be orders of magnitude greater than in the thermionic case, (ii) a cold cathode would minimize the need for cooling, and (iii) a field emitting cathode can be miniaturized. In spite of good performance of such cathodes, the procedure to estimate the device current is not straight forward and the required insight towards design optimization is not well understood. In addition, the current in CNT-based thin film devices shows fluctuation. Such fluctuation in field emission current is not desirable for many biomedical applications such as x-ray devices. The CNTs in a thin film undergo complex dynamics during field emission, which includes processes such as (i) evolution, (ii) electromechanical interaction, (iii) thermoelectric heating, (iv) ballistic transport, and (v) electron gas flow. These processes are coupled and nonlinear. Therefore, they must be analyzed accurately from the stability and long-term performance point of view. In this research, we develop detailed physics-based models of CNTs considering the aspects mentioned above. The models are integrated in a systematic manner to calculate the device current by using the Fowler-Nordheim equation. Using the models, we were able to capture the fluctuations in the field emission current, which have been observed in actual experiments. A detailed analysis of the results reveals the deflected shapes of the CNTs in an ensemble and the extent to which the initial state of geometry and orientation angles affect the device current. In addtion, investigations on the influence of defects and impurities in CNTs on their field emission properties have been carried out. By inclusion of defects and impurities, the field emission properties of CNTs can be tailored for specific device applications in future. For stable performance of CNT-based field emission devices, such as x-ray generators, design optimization studies have been presented. It has been found that the proposed design minimizes transience in field emission current. In this thesis, it has been demonstrated that phonon-assisted control of field emission current in CNT based thin film is possible. Finally, experimental studies pertaining to crosstalk phenomenon in a multi-pixel CNT array are presented.

In recent years, carbon nanotubes (CNTs) have emerged as one of the best field emitters for a variety of technological applications. The field emitting cathodes have several advantages over the conventional thermionic cathodes: (i) current density from field emission would be orders of magnitude greater than in the thermionic case, (ii) a cold cathode would minimize the need for cooling, and (iii) a field emitting cathode can be miniaturized. In spite of good performance of such cathodes, the procedure to estimate the device current is not straight forward and the required insight towards design optimization is not well understood. In addition, the current in CNT-based thin film devices shows fluctuation. Such fluctuation in field emission current is not desirable for many biomedical applications such as x-ray devices. The CNTs in a thin film undergo complex dynamics during field emission, which includes processes such as (i) evolution, (ii) electromechanical interaction, (iii) thermoelectric heating, (iv) ballistic transport, and (v) electron gas flow. These processes are coupled and nonlinear. Therefore, they must be analyzed accurately from the stability and long-term performance point of view. In this research, we develop detailed physics-based models of CNTs considering the aspects mentioned above. The models are integrated in a systematic manner to calculate the device current by using the Fowler-Nordheim equation. Using the models, we were able to capture the fluctuations in the field emission current, which have been observed in actual experiments. A detailed analysis of the results reveals the deflected shapes of the CNTs in an ensemble and the extent to which the initial state of geometry and orientation angles affect the device current. In addtion, investigations on the influence of defects and impurities in CNTs on their field emission properties have been carried out. By inclusion of defects and impurities, the field emission properties of CNTs can be tailored for specific device applications in future. For stable performance of CNT-based field emission devices, such as x-ray generators, design optimization studies have been presented. It has been found that the proposed design minimizes transience in field emission current. In this thesis, it has been demonstrated that phonon-assisted control of field emission current in CNT based thin film is possible. Finally, experimental studies pertaining to crosstalk phenomenon in a multi-pixel CNT array are presented.

We have developed a thermionic cathode capable of high emission currents. The structure of this cathode is oxide coated carbon nanotubes (CNTs) on a tungsten (W) substrate. This cathode was superior in emission due to the combination of the field enhancement effect from the CNTs and the lowered work function from the semiconducting oxide surface. Such oxide coated CNTs were excellent electron emitters. Conventional electron emission theories, such as Richardson's and Fowler-Nordheim's, did not accurately describe the field enhanced thermionic emission from such emitters. A unified electron emission theory was adopted and numerical simulations were performed to explain the deviation of electron emission from conventional field and thermionic emission theories. Also, the thermionic measurement system and measurement methods were improved in order to measure and characterize the strong electron emission from this new cathode. Large electron emission current from such structures also made a new thermionic cooling device a possibility. Cooling due to the electron emission was measured in terms of temperature drop, and a large temperature drop was observed from this cathode structure. Finally, applications of this cathode in plasma discharge devices were explored. This new cathode was tested in a plasma environment and initial results were obtained. / Department of Physics and Astronomy

Thesis(M.S.)--Auburn University, 2006. / Abstract. Vita. Includes bibliographic references.