A study conducted by researchers at Santa Clara University and the NASA Ames Research Center explored the various failure modes of individual nanotube field emission cathodes. High electric fields (≥109 V m-1), needed to induce cold field emission current, as well as turbulence from high vacuum pumping cause tremendous stresses that can result in mechanical breakdown at the CNT support structure interface. Furthermore, Joule heating at this nanoscale interface due to field emission current can lead to irreversible damage.
The study introduced and analyzed the reliability of a novel microelectromechanical systems (MEMS) CNT cathode fabrication technique to address these issues. The new cathode consists of a single multiwalled CNT attached to a silicon structure coated with a layer of nickel that is optimized for mechanical and electrical reliability. A thin nickel layer led to high mechanical stability but high contact resistance between the nanotube and support structure. A thickness of 25 nm is optimal for good mechanical stability and low interfacial resistance. The researchers also demonstrated that each nanotube field emitter can be shortened using a thermally assisted field evaporation technique, with a precision of 40 nm or less. The novel MEMS fabrication method was demonstrated for complex structures as well, such as a cathode consisting of two parallel CNTs with highly controlled lengths and inter-nanotube spacing.
Multiscale device integration, down to the stable nanoscale CNT field emitter and the microscale MEMS structure, is now possible with this novel fabrication technique. The group is presently developing an individual CNT field emission gun for a miniaturized MEMS-based scanning electron microscope as well as an electron gun array for maskless parallel e-beam lithography.