Record resolution for electron beam lithography


Rebecca Pool

Tuesday, May 2, 2017 - 13:30
Image: Focused electron beam (green) shining through a polymeric film.
In a breakthrough for electron-beam lithography, researchers at Brookhaven National Laboratory, US, have unveiled an electron microscope-based lithography system that can write nearly one trillion features per square centimetre.
Performing electron beam lithography with a scanning transmission electron microscope, Vitor Manfrinato and colleagues from the Center for Functional Nanomaterials patterned thin films of polymethyl methacrylate with individual features as small as a single nanometre, with a spacing between features of 11 nm.
“Our goal to study how the optical, electrical, thermal, and other properties of materials change as their feature sizes get smaller,” explains Manfrinato. “Until now, patterning materials at a single nanometre has not been possible in a controllable and efficient way.”
Lihua Zhang, Vitor Manfrinato, and Aaron Stein are part of the team at Brookhaven Lab's Center that pushed the resolution limits of electron-beam lithography to the 1 nm length scale.
Commercial electron beam lithography instruments typically pattern materials at sizes between 10 and 20 nm.
And while some techniques produce higher-resolution patterns, these require conditions that either limit their practical utility or dramatically slow down the patterning process.
Here, Manfrinato and colleagues pushed back the resolution limits of electron beam lithography by installing a pattern generator into an aberration-corrected STEM.
This electronic system precisely moves the electron beam over a sample to draw patterns designed with computer software.
“We converted an imaging tool into a drawing tool that is capable of not only taking atomic-resolution images but also making atomic-resolution structures,” highlights Manfrinato's colleague, Aaron Stein.
Measurements revealed a nearly 200 percent reduction in feature size, from 5 to 1.7 nm, compared to previous scientific reports.
What's more, areal pattern density was increased by 100 percent increase, from 0.4 to 0.8 trillion dots per square centimetre, or from 16 to 11 nm spacing between features.
The researchers' patterned PMMA films can be used as stencils for transferring the drawn single-digit nanometre feature into any other material.
In this work, the researchers created structures smaller than 5 nm in both metallic- gold palladium - and semiconducting - zinc oxide - materials. The fabricated gold palladium features were as small as six atoms wide.
According to Manfrinato and Stein, an exciting result of this study was the realisation that polymer films can be patterned at sizes much smaller than the 26 nm effective radius of the PMMA macromolecule.
“The polymer chains that make up a PMMA macromolecule are a million repeating monomers long; in a film, these macromolecules are all entangled and balled up,” points out Stein. “We were surprised to find that the smallest size we could pattern is well below the size of the macromolecule and nears the size of one of the monomer repeating units, as small as a single nanometre.”
“This technique opens up many exciting materials engineering possibilities, tailoring properties if not atom by atom, then closer than ever before,” he adds.
Research is published in Nano Letters.
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