We report the successful application of a new approach, ice lithography (IL), to fabricate nanoscale devices. The entire IL process takes place inside a modified scanning electron microscope (SEM), where a vapor-deposited film of water ice serves as a resist for e-beam lithography, greatly simplifying and streamlining device fabrication. We show that labile nanostructures such as carbon nanotubes can be safely imaged in an SEM when coated in ice. The ice film is patterned at high e-beam intensity and serves as a mask for lift-off without the device degradation and contamination associated with e-beam imaging and polymer resist residues. We demonstrate the IL preparation of carbon nanotube field effect transistors with high-quality trans-conductance properties.
The team is now trying to improve the process by making 3D nanodevices and adding etching clusters during fabrication. “We also need to better understand the detailed mechanisms behind ice lithography, about which we know very little.”
One of the ways in which the ice method provides scaling potential is by replacing atomic force microscopes (AFM) with scanning electron microscopes (SEM) in the mapping of the carbon nanotubes.
“Although not generally spelled out in publications, it is well known among the cognoscenti that nanotubes are contaminated or damaged by mapping their location in an electron microscope” Han is quoted as saying in the Nanowerk article. “To avoid contaminating or damaging, CNTs are often mapped by atomic force microscopes (AFM). But AFM is extremely slow. SEM mapping through ice is much faster and could be automated.”
Ice lithography also eliminates a problem when using polymer-based resists, the resist residue affectionately known in the semiconductor industry as ‘scum’. While the scum is often less than one or two nanometers thick, that’s enough to obscure partially a carbon nanotube or to bosure completely a single layer for graphene. The process of getting rid of the scum (oxygen plasma) also manages to remove any carbon-based nanocomponents, according to Han.
The ice lithography technique doesn’t leave any residue, which will likely improve the quality of the resulting nanodevices.
Now that the researchers have moved their research from patterning of simple nanowires to nanodevices, they see the next steps to be in the direction of making 3D nano- and graphene-based devices with this ice lithography technique. But the fundamental research into understanding the mechanisms behind ice lithography remains incomplete.