Recently, vertically aligned carbon nanofiber based dual-gate field emission structures have been fabricated for use in parallel electron-beam lithography, and their electron-beam focus adjustment capability has been demonstrated. This article summarizes the results of a numerical investigation of device behavior due to geometry variation in an effort to better understand device characteristics and performance. The effect of electrode thickness on minimum beam diameter and depth of field (DOF) were investigated for axially symmetric structures using a two-dimensional simulation package. The results indicate both a decrease in minimum spot size from similar to 30 to similar to 20 nm, and an increase in DOF from similar to 1 to similar to 6 mu m when the focus electrode thickness is increased from 100 to 500 nm. The impact of misalignments of the focus electrode and the carbon nanofiber on the beam behavior was investigated with a three-dimensional simulation package. Results show that reasonably well-converged beams can be achieved even with considerable offset to either the focus electrode or carbon nanofiber, though large beam deflections are produced. In addition, the deflections due to these offsets appear to be linear. (c) 2007 American Vacuum Society.