  Polymer-sorted high-density carbon nanotube (CNT) arrays have shown great
potential to extend the silicon-based Moore's law. Imaging the CNT arrays on
insulators like SiO2/Si using low-voltage scanning electron microscopy (LVSEM)
to acquire array information like the alignment, density, and distribution of
residual polymers is necessary. Such a task remains challenging due to the
nanoscale CNT body (1-2 nm in diameter), nanoscale tube-to-tube separation
(1-10 nm), the broadening of the apparent diameter, and the complex image
contrast caused by the insulating substrate and polymer residues. In this
study, the imaging mechanism for this system is investigated. Two methods are
developed to separate the three dominant contrasts, i.e. topographic contrast,
charge contrast, and material contrast, by selecting the take-off angle and
energy of the emitted electrons as enabled by changing the working distance or
the deceleration voltage. The contrast formation and separation mechanism is
further confirmed by the dynamic contrast evolution due to the
electron-beam-induced deposition of amorphous carbon. The contrast separation
method is further applied to an individual CNT, reducing its apparent diameter
from 36 nm to 6 nm. This result hints at the potential for LVSEM to count the
density exceeding 150 CNTs/um of CNT arrays. Finally, a comparative study of
LVSEM and transmission electron microscopy confirms the failure of LVSEM to
resolve CNTs in a bundle. The results suggest that the density of CNT arrays
may be underestimated in reported SEM data. The proposed method can serve as a
useful tool for further study and application of arrayed CNTs.
