"In our previous work we used strictly lithographic techniques," team member Michael Guillorn told nanotechweb.org. "While this process was successful at creating working devices it required complicated photolithographic equipment. In our latest work, we use a self-aligned fabrication technique that employs CMP. This has allowed us to simplify our device-production strategy and increase our yield significantly."

To make the devices, the researchers treated an n-type silicon wafer with electron beam lithography and physical vapour deposition to create catalyst sites for the growth of vertically aligned carbon nanofibres (VACNF) and alignment marks for lithographic patterning. Then they deposited VACNF material by plasma-enhanced chemical-vapour deposition, producing carbon nanofibres of around 1 μm in height with tip diameters of less than 30 nm.

The next step was to deposit a 1.2 μm-thick layer of silicon dioxide onto the substrate. This formed mounds over the VACNF emitters. Then the scientists used photolithography to define the gate electrode, and metallized the gate pattern by depositing molybdenum. Finally, they added an extra layer of silicon dioxide and carried out CMP. The CMP removed the mounds made during the initial deposition of silicon dioxide, creating self-aligned gate electrode apertures.

"While the CMP self-alignment technique has been known for more than a decade, it had never been used to synthesize devices with nanostructured carbon cathodes," added Guillorn. "This work shows that the carbon nanofibre can replace silicon or molybdenum in these types of structures without losing the ability to perform self-aligned device fabrication."

According to Guillorn, the main challenge was to learn the types of microfabrication processes that the carbon nanofibre was capable of surviving. "Once we knew the techniques that were compatible with the fibres it became just a matter of integrating them together into this device-fabrication technology," he added.

Because the position of the emission site within the gun structure can be relatively well controlled, the devices suit applications as electron sources in high-performance microscale electron guns for electron-beam lithography and electron microscopy.

The researchers reported their work in Applied Physics Letters.