Field emission is emission of electrons induced by a high intensity electrostatic field. A conventional field emission device comprises a cathode and an anode spaced from the cathode. The cathode may be a field emitter array including one or more field emitter elements. A voltage applied between the anode and cathode induces the emission of electrons towards the anode. Field emission devices have been utilized as bright electron sources for high-resolution electron microscopes.
In order to create a high intensity electric field (e.g., about 1×107 volts per centimeter) to extract electrons from emitters, tip/needle emitters are developed. For example, carbon nanotubes (CNTs) are a carbon material with a sharp tip that has a minute structure of a nanometer size with a high aspect ratio. CNTs have increasingly being utilized as electron field emitters because of their high electrical conductivity, high aspect ratio “needle like” shape for optimum geometrical field enhancement, and remarkable thermal stability.
A conventional method of fabricating tip/needle emitters, such as CNTs, involves depositing CNTs on a patterned layer of catalyst at preferred locations of the cathode. CNTs are directly grown on the cathode and ideally perpendicular to the cathode substrate surface, and are therefore well aligned with respect to the electric field applied in a gated field emission structure. However, tip/needle emitters do not necessarily grow straight or to the same length. In order to use these needles as emitters in a microscope or other e-beam systems, the electron beam emerging from needle emitters has to be corrected.
Conventional field electron devices may include several electrodes to control/correct the electron beams. Emission current control may be performed by a suppressor electrode near the emitter. In some systems, the current is adjusted by the extractor electrode and two electrostatic lenses provided relatively far from the emitter while maintaining a focus at the image plane. In addition, blanking is usually performed by deflector elements located downstream of the emitter region. Stigmation and beam steering are also performed downstream of the emitter region. Since these electrodes for optical aberration corrections are provided far from the emitter (e.g., 1 mm), relatively large voltages are required to either steer the beam, perform the blanking operation, or to adjust the beam current. In addition, since the first possible beam steering and stigmation control occurs after the beam condensing, there is no control in the emitter region to center the individual beam to the condensing lens optical center in an emitter array architecture.
It is within this context that aspects of the present disclosure arise.