15 November 2021

[Nanotechnology] – Calligraphy at the nanoscale.

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Nanotechnology device fabrication – from electronics to photonics, security, biotechnology and medicine – often requires some form of nanopatterning technique in order to achieve the structures and surfaces required for the device. And although they may not be visible to the naked eye, the nanometer-sized trenches, ridges, curves and grooves of these patterns and surfaces have a very visible impact on if the device works as designed.

Researchers are employing a variety of nanotechnology fabrication and patterning techniques such as electron-beam lithography, photolithography, microcontact printing, nanoimprinting and scanning probe lithography (SPL).

Scanning probe lithography techniques rely on the use of cantilevers to pattern sub-100 nm structures generated by the mechanical contact between a cantilever tip and a surface. SPL, with its high resolution, is a popular method for prototyping nanoscale structures. However, the method is difficult to scale up and not able to produce multiple copies of a device or structure at low cost.

“Although SPL-based patterning shows promising results for 0D (dots) and 1D (lines) patterning, tip-induced artifacts tend to arise frequently when defining 2D geometries,” Nikolaos Farmakidis, a doctoral student at the Advanced Nanoscale Engineering lab at the University of Oxford, tells Nanowerk. “Furthermore, the patterning speed associated with SPL becomes a significant obstacle when transitioning to 2D patterning, where the lithography time scales approximately inversely with the square of the patterning resolution.”

The concept of this new technique is based on the ancient art of calligraphy, where an asymmetric writing tool produces a line width which varies according to the direction the tool is being moved in.

In this work, the team replaced the calligrapher’s pen with a custom atomic force microscope tip where the rotational symmetry of the tip is broken by sculpting the desired shapes with focused ion beam milling.

“We focused on two novel types of asymmetric tips, which we termed ‘chisel-tip’ and ‘double-tip’, that show particular promise for nc-SPL,” Farmakidis explains. “By controlling the writing angle, we show that the distance between the lines can be tuned to produce nanowires with sizes which vary from several nanometers to microns.”

In their paper, the researchers demonstrate how, owing to the asymmetry of the chisel-tip probe, complex lithographic patterns of different dimensions can be produced, allowing for intricate patterns without scanning artifacts or resist buildup at the borders of the lithography path.

The double-tip configuration is suitable for nanopatterning applications down to and below 50 nm that require a nanoscale gap such as two conducting lines close to each other without overlap.

As the researchers point out, by using tips that do not necessarily have nanoscale dimensions themselves, they were able to pattern gaps that are effectively much smaller than the minimum probe dimensions.

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