The field emission electron sources using carbon nanotubes (CNTs) are being targeted for low-power vacuum microelectronic applications for harshenvironment operation (high temperature, pressure, and corrosive atmosphere). While CNTs have demonstrated excellent properties in terms of low threshold field, low-power operation, and high current densities, one problem with vacuum electronic applications is poor adhesion of CNTs to the substrate on which they are synthesized. The chemical vapor deposition (CVD) process used to grow CNTs on silicon or other metallic substrates using an iron catalyst with a thin oxide diffusion barrier layer has consistently provided reproducible growth. The CNTs are only surface- adhering in these cases, and are easily removed from the surface with the application of minor forces — typically pressures of 20 to 60 kPa. This causes catastrophic failures of CNT field emitters since the applied field could exceed the adhesion strength of CNTs to the substrate.
An in situ process was developed that allows welding of CNT bundles to the support substrate. An efficient field emission architecture of CNTs has been reported and patented by JPL. This architecture consists of arrays of CNT bundles of 1 to 2 micrometers in diameter, spaced 5 micrometers apart. These bundles can be grown on any substrate that is conducive for lithographic patterning and catalyst deposition for CNT growth (Fe, Ni, etc.). A diffusion barrier layer, typically an oxide layer, is deposited underneath the metal catalyst.
Growth of CNTs on titanium substrate with a diffusion barrier at 575 to 600 °C has been demonstrated. Following the growth process, the CNT bundle arrays are welded into Ti by heating the Ti substrates to 1,050 °C such that the surface of Ti softens and the CNTs get rooted inside. This welding process provides CNT field emission samples that are robust and are tightly bound to the substrate. They have been shown to withstand high electric fields without getting dislodged.
The novelty here is the single process step that allows the growth and in situ welding to produce vertically aligned, patterned CNTs on a metallic substrate. Even though the process was developed for titanium substrate, it is possible to implement the same with other metallic substrates and catalyst combinations. It is possible to deposit any of the metal layers on any other metal substrates to create a multi-metal substrate on which CNT bundle arrays or simply CNTs are grown. The main advantage is that it allows creation of array patterns of CNTs, and welds them into place as opposed to previously reported processes that required depositing loose CNTs on metallic films followed by a welding process. It is not possible to achieve vertically oriented and patterned CNTs with such a welding process.
This work was done by Harish Manohara, Valerie Kristof, and Risaku Toda of Caltech for NASA’s Jet Propulsion Laboratory.
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