self-assembly

Ink-jet printing creates ‘soft’ nanostructures

A new way to make nanostructures combines advanced ink-jet printing technology with block copolymers that spontaneously form ultra-fine structures.

The new approach could have applications for the semiconductor and magnetic storage industries.

Researchers were able to increase the resolution of their intricate structure fabrication from approximately 200 nanometers to approximately 15 nanometers. A nanometer is a billionth of a meter, the width of a double-stranded DNA molecule.

The ability to fabricate nanostructures out of polymers, DNA, proteins, and other “soft” materials has the potential to enable new classes of electronics, diagnostic devices, and chemical sensors.

block copolymer
This atomic force microscope image shows two different printed lines of block copolymers, colored and scaled by the microscope’s software. Each block copolymer has a different molecular weight and therefore has a different size and spacing of its structural components. The repeating structures at left have a spacing of 41 nanometers, while those at right have spacing of 27 nanometers. (Credit: Serdar Onses/U. Illinois at Urbana-Champaign)

The challenge is that many of these materials are fundamentally incompatible with the sorts of lithographic techniques that are traditionally used in the integrated circuit industry.

Recently developed ultra high-resolution ink jet printing techniques have some potential, with demonstrated resolution down to 100-200 nanometers, but there are significant challenges in achieving true nanoscale dimension.

“Our work demonstrates that processes of polymer self-assembly can provide a way around this limitation,” says John Rogers, professor of materials science and engineering at University of Illinois at Urbana-Champaign.

Combining jet printing with self-assembling block copolymers enabled the engineers to attain the much higher resolution, as suggested by lead author Serdar Onses, a postdoctoral scientist at Illinois. Onses earned his doctorate at the University of Wisconsin under Paul Nealey, now professor of molecular engineering at the University of Chicago and a co-author of the paper in Nature Nanotechnology.

“This concept turned out to be really useful,” Rogers says.

Engineers use self-assembling materials to augment traditional photolithographic processes that generate patterns for many technological applications. They first create either a topographical or chemical pattern using traditional processes. For the new paper, this was done at imec in Belgium, an independent nanoelectronics research center.

Nearing the limits

The resolution of the chemical pattern nears the current limit of traditional photolithography, notes Lance Williamson, a graduate student in molecular engineering at University of Chicago and co-author of the article. “Imec has the capability to perform the photolithography at this scale over large areas with high precision,” Williamson says.

Back at the University of Illinois, engineers place a block copolymer atop this pattern. The block copolymer self-organizes, directed by the underlying template to form patterns that are at much higher resolution than the template itself.

Previous work has focused on the deposition and assembly of uniform films on each wafer or substrate, resulting in patterns with essentially only one characteristic feature size and spacing between features. But practical applications may need block copolymers of multiple dimensions patterned or spatially placed over a wafer.

“This invention, to use ink-jet printing to deposit different block copolymer films with high spatial resolution over the substrate, is highly enabling in terms of device design and manufacturing in that you can realize different dimension structures all in one layer,” Nealey says. “Moreover, the different dimension patterns may actually be directed to assemble with either the same or different templates in different regions.”

The track system coats, bakes, and develops thin polymer films on silicon wafers. (Credit: © IMEC)
The track system coats, bakes, and develops thin polymer films on silicon wafers. (Credit: © IMEC)

High-res printing

The advanced form of ink-jet printing the engineers use to locally deposit block copolymers is called electrohydrodynamic, or e-jet printing. It operates much like the ink-jet printers office workers use for printing on paper.

“The idea is flow of materials from small openings, except e-jet is a special, high-resolution version of ink-jet printers that can print features down to several hundred nanometers,” Onses says.

And because e-jet can naturally handle fluid inks, it is exceptionally well suited for patterning solution suspensions of nanotubes, nanocrystals, nanowires, and other types of nanomaterials.

“The most interesting aspect of this work is the ability to combine ‘top down’ techniques of jet printing with ‘bottom up’ processes of self-assembly, in a way that opens up new capabilities in lithography—applicable to soft and hard materials alike,” Rogers says.

“The opportunities are in forming patterned structures of nanomaterials to enable their integration into real devices. I am optimistic about the possibilities.”

Researchers from Hanyang University in Korea also contributed to the study, which received funding from the National Science Foundation and National Research Foundation of Korea.

Source: University of Chicago

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