Suspended carbon nanotube shadow lithography: Incident evaporation angle dependence

Due to micron scale lengths, carbon nanotubes can be manipulated with current microelectronic fabrication techniques yet result in nm-scale line widths. Examples of CNT manipulation include self assembly or catalytically supported CNT growth in areas defined by sub-micron-scale lithography. One promising geometry is to suspend CNTs by their attachment to the tops of photo lithographically defined posts. Coupled with a line of site deposition, this geometry will result in nm-scale line underneath the CNT. The width of the resulting shadow is a function of CNT diameter, incident evaporation angle and height of CNT above substrate. Reported here is that CNTs were suspended (500nm) over Si ₃N ₄ membrane TEM grid by lithographically defined PMMA lines. Al evaporation was performed at well controlled incident angles by use of slits. Shadows were directly observed by scanning transmission electron microscopy (STEM). CNTs of ∼20nm diameter, 500 nm above Si ₃N ₄ membrane and incident angle of 0.95° result in shadows of 6-8nm. Simple line of site geometry would predict 14nm line widths, thus significant surface migration from the momentum of incident metal evaporate is observed. Significantly, this incident angle effect requires relatively large CNTs (10-20nm) to form nm-scale lines. This requirement is necessary since the post height must be greater than that of post separation for suspended CNT growth. These nm-scale lines can be the basis of precisely controlled self assembly processes that would occur only in nm-scale shadow areas.