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Posted: Nov 04, 2010
Nanotechnology on ice - novel lithography approach to nanodevice fabrication
(Nanowerk Spotlight) A few years back, researchers at Harvard University showed that nanostructures can be patterned with focused electron or ion beams in thin, stable, conformal films of water ice grown on silicon ("Nanometer Patterning with Ice"). They demonstrated ice lithography as a lithographic technique for patterning e.g. metal wires down to 20 nm wide.
What's interesting about this technique is that patterning with ices of any condensed gas is a straightforward and practical process. Ice resist does not require spinning or baking. All processing and patterning steps can occur in a single evacuated chamber and be monitored at high resolution. The final removal of unexposed resist leaves minimal residues. Environmentally harmful solvents are not required and complete dry removal of the ice layer can be performed by in situ sublimation. Also, ice lithography makes it possible to nanopattern chemical modifications into silicon and other substrates.
In a new report in the November 1, 2010 online issue of Nano Letters ("Ice Lithography for Nanodevices"), the team has now reported the successful application of ice lithography to the fabrication of nanoscale devices.
"We have applied our ice lithography technology to make nanodevices, which involves a different set of challenges than the previously made simple structures like nanowires," Daniel Branton from the Harvard Nanopore Group tells Nanowerk. "We weren't sure if we could actually make nanodevices. We tried this because we wanted to solve contamination problems we had when fabricating nanodevices for DNA sequencing – the research area our group is focused on."
In their new work, the team has used ice lithography to make single-walled carbon nanotubes (SWCNT) nanodevices.
"Making nanodevices today is a very tedious and error-prone process, involving many different machines and processes" explains Anpan Han, the lead author of the Nano Letters paper. "We demonstrated that using ice as a resist it is possible to make nanodevices quickly with the possibility to do quality control at mid-point. If adapted by the field, more reliable data and structures would be possible."
What makes this novel technique so interesting for practical applications is that it holds the promise of scaling up CNT nanodevice applications.
Han points out that scanning electron microscope (SEM) imaging through ice makes it possible to map randomly located CNT grown by chemical vapor deposition (CVD) without damage or contamination.
"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" he says. "To avoid contaminating or damaging, CNT are often mapped by atomic force microscopes (AFM). But AFM is extremely slow. SEM mapping through ice is much faster and could be automated."
Another important aspect of ice lithography is that water does not leave any contamination compared to the state-of-the-art polymer based resists. The semiconductor industry jargon 'scum' is the resist residue, often less than one or two nanometers thick.
"Normally, scum is removed by a short oxygen plasma which unfortunately also removes any carbon based nanocomponents" says Han. "A nanometer of scum is sufficient to partially bury carbon nanotubes, and completely bury a single layer of graphene which is less than 1 nm thick. It has been shown by others that the properties of nanodevices are highly influenced by these contaminants. We therefore believe that ice lithography will generally improve the quality of nanodevices."
The ice lithography process. (a) The sample with preformed Mo microleads and SWCNT on the SiO2-coated Si substrate is loaded into the SEM via the load-lock and cooled down to ∼110 K on the SEM cryo-stage. Water vapor is leaked into the SEM through a nozzle just above the sample and condenses as amorphous ice on the cold sample. Typically, 80 nm of ice is deposited in 30 seconds. The location of a SWCNT under the ice is mapped. (b) An intense e-beam draws patterns for the contacts (white dotted line) and removes ice, forming a mask for metal electrodes contacting the SWCNT. (c) The sample with nanopatterned ice resist is transferred onto the metal deposition chamber, and Pd is sputtered over the entire sample. The sample is removed from the metal deposition chamber and, while still frozen, immediately immersed into 2-propanol held at room temperature, whereupon the Pd film on top of the ice resist drifts away, leaving the preformed Mo leads connected to the SWCNT with Pd interconnections only where the ice had been removed by the e-beam. (Reprinted with permission from American Chemical Society)
Branton notes that – because it is known that metal films deposited onto cold surfaces tend to be nanoporous – the team was concerned that the metal contacts and leads formed on cold device structures by ice lithography would not be of the highest electrical quality. They compared their ice lithography fabricated devices with conventionally fabricated nanotube devices on silicon dioxide coated silicon. By annealing their samples in argon, they were able to achieve source-drain resistance (Rsd) values comparable to those obtained using standard resist based e-beam lithography: metallic FET devices with Rsd between 60 and 90 kΩ, and semiconducting tubes with Rsd down to 100 kΩ.
"We attribute the improved contacts to high-temperature densification of the cold-deposited palladium contacts" says Han.
The results by the Harvard team demonstrate a new approach to nanodevice fabrication that solves several problems often faced in designing and producing devices, in particular imaging and contamination issues. They are confident that ice lithography can be incorporated into more complex cluster tool environments for making graphene nanoribbons and nanopore devices.
Branton sees three general directions for the team's future research, which would progress in parallel: "We described the first nanodevice, and we are now perusing other applications such as making 3D nano- and graphene devices. Secondly, adding other functionalities and technological sophistication such as etching clusters would expand the applicability of ice lithography. Finally, we need to achieve a deeper understanding of the mechanisms behind ice lithography, about which we still know very little."