Incident angle dependence of nanogap size in suspended carbon nanotube shadow lithography

Suspended nanotubes or nanowires can be used in line-of-sight depositions to produce nanometre-scale lines. Reported here is the experimental quantification of line-of-sight shadow widths for multiwalled carbon nanotubes that are suspended (400 nm) over a silicon nitride (Si3N4) membrane (transmission electron microscopy) TEM grid by a lithographically defined resist line array. Aluminium evaporation was performed at incident angles of 0.7◦–2.0◦ by use of the slit (0.81 mm diameter). Shadow widths and carbon nanotube diameters were directly observed by scanning transmission electron microscopy (STEM). Observed gaps were less (0–7 nm) than that predicted from simple line-of-sight geometry. Thus, surface migration assisted by the momentum of incident metal evaporant may be an important mechanism in nanoscale shadow lithography processes. High resolution lithography techniques are required to incorporate isolated molecules into functional, scalable electronic devices. Significant progress has been made in UV and e-beam lithography for reliable deca-nanometre-scale feature sizes [1–3] but scaling to the 1–2 nm molecular size range remains a significant challenge, especially concerning the serial nature of electron beam and scanning probe techniques. Notable success has been achieved in making 2–5 nm junctions to isolate single molecules [4] and nanoparticles [5] for transport studies. These techniques relied on break junctions and direct e-beam writing that have significant challenges in their reliable scaling for device integration. Although carbon nanotubes (CNTs) and nanowires have nanometre-scale diameters, they also have micron-scale lengths; thus, in principle they can be manipulated with current microelectronic fabrication techniques, i.e. photolithographic based patterns. By utilizing randomly dispersed CNTs [6–8] or with electric field alignment [9–11] as shadow masks, nanometre-scale lines have been demonstrated. In addition to CNTs, semiconductor or oxide nanowires can be applied to nanometre-scale gap formation through lift-off [12] or ion beam milling [13]. In both cases it is critical to control placement and diameter control of the nanowires and nanotubes. Another promising line-of-sight technique has also been demonstrated in depositions between hexagonal closest packing of dispersed polystyrene beads [14]. Shadow lithography also makes it possible to readily incorporate self-assembled nanostructures into VLSI processes, in contrast with break junctions [15] and other sequential scanning-probe nanolithography techniques. Major challenges associated with CNT shadow mask fabrication include control of the diameter and spatial placement of the CNTs. These can be achieved by precisely patterning the catalyst support layer in a chemical vapour deposition growth of multi-wall CNTs (MWCNTs), where the catalytic film thickness controls the resultant MWCNT diameter [16, 17]. Moreover, individually suspended CNTs attached to the tops of photolithographically defined posts have been demonstrated [18]. CNTs have also been aligned by electric field generated between electrodes, and then used to form shadows [9, 11]. However, previous experiments pointed to the difficulty of determining the precise size of 0957-4484/05/010133+04$30.00 © 2005 IOP Publishing Ltd Printed in the UK 133