2. Carbon Nanotube X-ray Sources
2.3 CNT cathode design for a Micro-CT X-ray Source
While the first CNT x-ray source created in our lab was capable of capturing x- ray projection images of some biological materials, significant modification was
necessary before such a source could be used in medical applications The flux demands and operating conditions for such applications require a high current density on the order of 102–103 mA cm-2 and electron acceleration voltages ranging from 30 to 220 kV for various imaging and radiation therapy applications.
In order to meet these high demands, carbon nanotube fabrication methods needed to be optimized to maintain desired film thicknesses, emitter densities and adhesion. Electrophoretic deposition (EPD), an automated industrial process with high throughput,
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is capable of efficiently depositing even CNT coatings on conducting substrates [13]. Our method of fabricating field emission cathodes combines electrophoresis with
photolithography of functionalized CNTs to exert fine control over film thickness and morphology.
Figure 2-4: The procedure used to create a CNT cathode through EPD. On inset figures, optical microscope images of a cathode after (a) photolithography, (b) CNT deposition, and (c) liftoff with NMP and vacuum annealing. [14]
The general method of CNT cathode fabrication is as follows [15], illustrated in Figure 2-4. First the substrate is spin coated with a uniform layer of OmniCoat™ SU-8 release (MicroChem, Inc.) and then spin coated again with a layer of epoxy-based SU-8 negative photoresist (Step 1). The photoresist is patterned to the desired cathode shape and size using UV contact photolithography and the exposed layer is then removed to reveal the surface beneath (Step 2). When a glass substrate is used, a thin silver contact
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lines must be thermally evaporated onto the surface with photoresist to provide electrical contact. Then the CNTs are applied using EPD. The CNT coating thickness is controlled by adjusting both the applied voltage and deposition time. After soft baking, sonication in a stripping solution of N-Methyl-2-Pyrrolidone stripped the photoresist, and excess stripping solution was removed by acetone rinsing before performing the vacuum anneal.
The adhesion between the CNT composite film and the substrate is strong enough that few CNTs are removed from the substrate surface after photoresist liftoff. With SEM imaging (Figure 2-5) we confirm that CNTs are randomly oriented after EPD and
vacuum annealing. An activation process of mechanically removing a top layer of the composite film using an adhesive tape causes the surface CNTs to align vertically with one end firmly embedded inside the matrix and the other end protruding from the surface (Figure 2-5c). This vertical morphology is optimal both for adhesion and for field
enhancement.
Figure 2-5: SEM images showing the top surface of the composite CNT film both: (a) before and (b) after vacuum annealing. The CNTs are randomly oriented on the surface. (c) Cross-sectional SEM image of the CNT cathode after the activation process. The surface CNTs are now vertically aligned in direction perpendicular to the substrate surface. Cross-sectional SEM images of two cathodes fabricated under the same conditions except different CNT concentrations in the EPD inks. Cathode shown in (e)
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was made using an ink with 4× the CNT concentration than the cathode shown in (d). [14]
The cathode emission is evaluated in triode mode and with applied high anode voltage (50 kV) illustrated in Figure 2-6a inset. After a conditioning process, the cathode operated stably as part of a field emission source with a 50 kV anode voltage, appropriate for an x-ray source in a micro-CT imaging system. For a 0.50 mm x 2.35 mm elliptical cathode and focusing layouts used in the micro-CT x-ray source (Figure 2-6a), a stable cathode current of 3 mA is easily achieved with an applied gate voltage (Vg) of ∼1800 V. This source has a transmission rate through to the anode of 60% of cathode current; the rest is lost to the gate and focusing electrodes. A transmission rate of less than 100% is fully expected, however, as the process of focusing to a smaller focal spot on the anode involves some blockage of the electrons that are emitted from the CNT cathode.
In Figure 2-6b, lifetime stability of a CNT cathode in a micro-CT x-ray configuration was examined. The 0.5 mm x 2.35 mm elliptical CNT cathode was operated for 4500 minutes with the appropriate parameters for cardiac micro-CT, including a 50 kV applied anode voltage and gate voltage adjusted to maintain a stable pulsed 3.0 mA cathode current. Pulses were 20 ms in duration and occurred at a frequency of 1 Hz (2% duty cycle). Over this lifetime test there was a gate voltage degradation of approximately 300 V (from 1800 to 2100 V) over the 5400 minute test, which is equivalent to 810 successive cardiac micro-CT image acquisitions.
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Figure 2-6: (a) Field emission current as a function of the applied gate voltage from a 0.50 mm × 2.35 mm elliptical CNT cathode at constant anode voltage. For comparison the data from the same cathode measured in the parallel-plate geometry (cathode-to- anode spacing was 150 μm) is also shown. (b) Emission lifetime measurement of a 0.50 mm × 2.35 mm CNT cathode at constant current mode in triode geometry. [14]