Carbon Nanotube Bundle Array Cold Cathodes for THz Vacuum Tube Sources
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- Manohara, H.M., Toda, R., Lin, R.H. et al. J Infrared Milli Terahz Waves (2009) 30: 1338. doi:10.1007/s10762-009-9547-x
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We present high performance cold cathodes composed of arrays of carbon nanotube bundles that routinely produce > 15 A/cm2 at applied fields of 5 to 8 V/µm without any beam focusing. They have exhibited robust operation in poor vacuums of 10-6 to 10-4 Torr- a typically achievable range inside hermetically sealed microcavities. A new double-SOI process was developed to monolithically integrate a gate and additional beam tailoring electrodes. The ability to design the electrodes for specific requirements makes carbon nanotube field emission sources extremely flexible. The lifetime of these cathodes is found to be affected by two effects: a gradual decay of emission due to anode sputtering, and catastrophic failure because of dislodging of CNT bundles at high fields ( > 10 V/µm).
KeywordsField emissionCarbon nanotubeCNTsNanoklystronVacuum tubeHigh frequency sources
Efficient electron sources are the fundamental components of- (i) miniature vacuum tube sources for high frequency applications , (ii) many analytical instruments used for elemental and mineralogical analyses , and (iii) vacuum microelectronic devices that are radiation-insensitive and high-temperature tolerant for extreme environments electronic applications. Each application requires an electron source with specifically designed beam forming optics and emission densities that span a range of tens to hundreds of amperes per sq. cm. State-of-the art thermionic cathodes are ill-suited for miniature instruments because of their bulkiness, high temperature operation, and high power consumption. Conversely, state-of-the-art cold cathodes  based on atomically sharp micromachined tips [4, 5] are highly susceptible to poisoning when operated in non-UHV (10-8 to 10-9 Torr) environments. This is an important limitation because the micromachining techniques such as lithography, etching, and wafer bonding used to create vacuum cavities in miniature devices are not able to sustain vacuums better than 10-6 Torr because of small volumes containing relatively large outgassing surfaces. This vacuum tends to get worse during operation as well as due to aging because of its static nature (without continuous pumping). Moreover, presence of microcomponents inside these cavities creates a high potential for virtual leaks and surface contamination. While incorporating getters help to some extent, it does not address the fabrication process-induced inability to achieve UHV. Interest in high current density electron sources that are robust to poor vacuums and are capable of operation at low voltages is motivated by the re-emerging vacuum tube technology for high frequency sources. Intense research efforts continue with the goal to realize portable, narrow band, coherent CW sources of tunable RF power in the submillimeter wave frequency bands (300–3000 GHz) for active (and passive) sensor systems based on vacuum tube technology. One such approach based on the reflex klystron principle, called the Nanoklystron  merges cold cathode and silicon micromachining techniques. A simplified beam analysis performed using relations well known in previously published work  indicated current density requirement of ~ 1 kA/cm2 for a 500 V beam to produce ~ 3 mW output power at 1200 GHz. The device dimensions such as the beam tunnel diameter of 20 μm, the cavity diameter of 80 μm, and the overall device footprint of < 1 mm2 posed new challenges to develop a cathode technology suitable for such small devices. In addition to being small they are required to deliver tens to hundreds of amperes per sq. cm and be robust to poor vacuums.
1.2 Carbon nanotube field emitters
2 Carbon nanotube bundle array cathodes
2.1 Carbon nanotube bundle arrays
2.2 Monolithic electrode integration
2.3 Lifetime issues
We have developed CNT bundle array cathodes that show promise as high-current density cold cathodes, robust to poor vacuums and capable of operating at low voltages. A monolithic electrode integration technique using a double SOI process was presented that enables multiple level electrode integration to realize a miniature electron gun. Additionally, two failure modes of these CNT bundles were presented. The first is a gradual decay of emission due to CNT tip failure as well as the anode sputtering effect. The second corresponds to dislodgement of CNT bundles due to force exerted by the anode at high fields. Enhanced adhesion of CNT to substrates can overcome the latter failure.
This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with National Aeronautics and Space Administration (NASA). This work was funded by JPL’s Research and Technology Development Fund and partly by Defense Advanced Research Project Agency. We thank Dr. Paula Grunthaner, Dr. Barbara Wilson, Dr. James Cutts, and Dr. Timothy Krabach for their advice on the cathode development as part of the internal research and development effort. We thank Dr. Mark Rosker of Defense Advanced Research Projects Agency for his support of this work. We thank Dr. Ken Dean of Motorola, Dr. John Hong of JPL (at the time of this work), and Dr. Dev Palmer of US Army Research Office for useful discussions.