Design and Fabrication of Carbon Nanotube Array based Field Emission Cathode for X-ray Tube
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Field emission cathodes have proven themselves to be excellent candidates for some special medical X-ray applications. Spindt cathode and CNT (carbon nanotube) based field emission cathode have been widely studied for many years. Spindt cathode has the near perfect structure, but the material property limits its applications. On the other hand, low density vertically aligned CNT array has been proved the best candidate of field emission material. Several attempts have been made to combine the advantages of the Spindt cathode and CNT array, but some most important advantages of Spindt cathode have not been successfully utilized in CNT emitter design, for example: ballast resistor, self-aligned fabrication process, sub-micron scale gate electrode, and low control voltage. In this thesis, the design, fabrication and test of CNT based field emission cathode with a novel ballast resistor and coaxial cylinder shape gate electrode is reported. A connection pad has been reported for the first time. This structure makes the ballast resistor can be utilized in a CNT field emitter array. Therefore, the uniformity and stability of field emission current is improved significantly. In addition, the stabilized emission current heated up the sample to a high temperature and changes the emission from field emission to Schottky emission regime. This is the first report of the self-heating Schottky emission from a CNT emitter array. Coaxial cylinder shape gate electrode is another important improvement in the CNT emitter design. The gate electrode adopts the self-alignment property of the Spindt cathode, and adjusted to fit the structure and synthesis process of CNT array. This new design and fabrication process has all the advantages of both the Spindt cathode and CNT emitter. In addition to the field emission cathode design, a novel PMMA (poly methyl methacrylate) based FEM (field emission microscope) is designed to evaluate the emission uniformity and capture high resolution images of the distribution of field emitted electrons. Compare to the conventional phosphor screen based FEM, the PMMA based FEM has a much higher resolution and sensitivity. Images of this new FEM have a resolution of as high as 0.34 nm and clearly show the boundary of crystal facets.