1.	Artistic schematic of extreme light-focusing bowtie nanoantennas and corresponding cascade domino lithography.
1. Artistic schematic of extreme light-focusing bowtie nanoantennas and corresponding cascade domino lithography.
2.	Scanning electron microscopy image of collapsed photoresist pillars created during CDL to fabricate ultra-sharp bowtie nanoantenna with sub-1 nm radius of curvature and 5 nm gap size. As a proof of concept, a single-molecular-level sensitive SERS sensor has been proposed.
2. Scanning electron microscopy image of collapsed photoresist pillars created during CDL to fabricate ultra-sharp bowtie nanoantenna with sub-1 nm radius of curvature and 5 nm gap size. As a proof of concept, a single-molecular-level sensitive SERS sensor has been proposed.

Squeezing photons into tiny nano-sized spaces encourages a wealth of unusual electronic phenomena, including enhanced interactions with light that are essential to plasmonics. The tiny volumes of plasmonic nanostructures or nanoantennas lead to huge enhancements of electromagnetic field. These ‘hotspots’, which depend in part on the nanoantenna size and shape, turn out to be particularly strong in bowtie-like structures. The small gap between the sharp tips of the bowtie produce the greatest local field enhancement. But electron beam lithography, which is currently used to fabricate these structures, is limited in the sharpness of the tips it can produce and smallness of the gaps between them.

Now researchers from Pohang University of Science and Technology (POSTECH) in have developed a new form of lithography that can produce extremely sharp bowtie nanoantennas – essentially the size of a gold nanocluster – with single-digit gaps between the tips [Kim et al., Materials Today (2020), https://doi.org/10.1016/j.mattod.2020.06.002].

“To overcome the flaws of existing nanofabrication techniques such as limited resolution and blunt edges of fabricated nanostructures… we have developed a new lithography technique, called cascade domino lithography (CDL), for single-digit-nanometer scale nanoantennas,” says Junsuk Rho, who led the work.

During conventional electron beam lithography, interactions between electrons, resist materials, and the substrate result in patterns larger than the actual scanned area. This ‘proximity’ effect limits the chance of producing very small structures. To get around this restriction, Rho and colleagues were inspired by the way in which a trail of falling dominos cascade into each other. If one domino falls onto a misaligned domino, the angle between them is very sharp. This idea led to an unconventional approach to nanopatterning whereby the ‘fall’ or collapse of an electron-beam photoresist mask against another defines a tip-shape than would otherwise be impossible to achieve.

“Our unconventional nanofabrication approach exploits photoresist collapse, which [we] found in a failed patterning sample,” explains Rho.

By using two electron-beam resists with different solubilities and over-exposing them, the researchers can control the collapse process. Gold atoms can then be evaporated onto the sample, where the collapsed mask makes a sharp junction with the mask below. In this way, CDL enables the consistent, repeated formation of extremely pointed bowties with single-digit nanometer gaps between tips.

“Not only bowtie-shaped structures, but other diverse single-digit-nanometer scale structures can be made [in this way], such as nanowire and arc-shaped nanoantennas,” points out Rho. “We expect CDL can be used in state-of-the-art nanopatterning research towards the single-digit-nanoscale regime because the process is fully compatible with CMOS fabrication.”

As an example, the researchers fabricated an ultra-sensitive surface-enhanced Raman scattering (SERS) sensor, which could achieve single molecular level sensitivity.