June 14, 2017

Nano eraser developed at SIU could fix microchip errors

by Tim Crosby

CARBONDALE, Ill. – Watching his young daughter’s painstaking efforts to master the ability to write letters got Punit Kohli to thinking in the way that only scientists tend to think.

The act of writing information down – lithography – is as ancient as civilization. But in modern times, lithography has been taken to extremes: as in extremely small. In fact, lithography at the nanoscale – a nanometer is one-billionth of a meter – is used in applications such as microchips and making certain types of sensors.

As Kohli, a professor of chemistry and biochemistry at Southern Illinois University Carbondale, watched his daughter struggle to learn how to write, she often would have to erase what she did and try again. That’s when he realized there was a huge need for the ability to erase mistakes in lithography at the nanoscale, too.

“She was 5 years old and I’d watch her write and erase,” Kohli recalled. “But correcting errors at the nanoscale, that’s very difficult and there really wasn’t a good way to do it at that time.”

So Kohli took up the challenge in his lab, working with Pradeep R. Rajasekaran, a doctoral student at the time, on the idea. After years of hard work and a series of National Science Foundation grants, Rajasekaran, now a post-doctorate researcher at the Institute of Systems Research at the University of Maryland, would perfect the idea he began working on at SIU.

The journal Science Advances, issued by the American Association for the Advancement of Science, published his work in early June. Scientists from GSI Helmholtz Centre for Heavy Ion Research in Germany also contributed.

Just as in handwriting, one needs an instrument to make a mark. Usually, it is pointed, like a pencil. In the tiny world of nanoscale, erasers need to take on that conical shape, as well.

The writing part has been understood for some time, at least since the early 2000s. In some sense, it often worked like one of the oldest writing instrument used by humans: a quill. Hard, pointed nanostructures are dipped into whatever “ink” was needed and then moved across a substrate as directed.

But the hurdle Rajasekaran and Kohli had overcome was making a conically shaped structure that did the opposite of the writing instrument: erasing. Current materials and technology at the time did not lend themselves well to this function. So the secret, it turned out, was all in the material used to create tiny, conically shaped erasers: Something soft, porous and sponge-like instead of hard and solid.

One of the three NSF grants funding the research provided $490,000 for an electron microscope, which allowed the researchers to actually see with their own eyes the nature and textures of the materials they were creating and testing. Rajasekaran recalled a moment when everything changed as he examined newly created erasers made from agarose, a cousin of cellulous.

“When I first glanced at those beautiful conical nanostructures, which were porous, filled with water and flexible, I was extremely excited about the potential,” he said. “I realized that this material looked exactly like a sponge. So it could be potentially used for soaking up and releasing any kind of material.”

The porous material making up the erasers can hold any type of liquid material and can clean or erase a nanoscale surface the way people typically use wet sponges to clean a surface in their kitchens. The liquid also acts as a lubricant, allowing the sponge to slide along that surface free of friction while also soaking up byproducts and debris left over from the cleaning process. And, just as a painter utilizes a sponge paint roller to transfer paint to walls, conical nano-sponges in this process can also deposit any material on any surface.

Another key was the ability of the researchers to maneuver the materials with a piezo-electric motor while monitor the movement live through a microscope, Rajasekaran said.

 “It just made me think of the limitless places it could be used, from the semiconductor industry to biotechnology,” he said. “As a scientist, I was very happy that I was able to make some real and significant contribution to the scientific community.”

Miniaturization has revolutionized the world, Rajasekaran said, which means efficiently making small structures is among the most important missions scientists have today. Being able to correct errors in this environment, therefore, has become even more important.

“Thanks to the developments in the semiconductor industry, computers that occupied an entire room can now be confined to the tip of our fingers. But even as of today, photo lithography is the industry standard when it comes to making those silicon processors used in computers,” he said. “There is no easy way to correct for errors that occur during the multiple steps of fabrication, as there is no method to selectively and precisely erase and correct errors at the nanoscale. The defective products are simply discarded that amounts to multi-million dollar losses annually to those industries.”

Reza Ghodssi, who holds the Herbert Rabin Distinguished Chair of Engineering and is the director of the Systems Research Center at the University of Maryland, said the technology could have a tremendous impact on a variety of applications.

“For instance, it could potentially hold very small regents and samples in a clinical setting for high throughput, programmable screening of bio-markers of different diseases in the future." Ghodssi said. 

Ryan D. Sochol, assistant professor of mechanical engineering at the A. James Clark School of Engineering at the University of Maryland, College Park, said the technology provides a promising route to fix manufacturing errors for micro3D printing applications. 

“In particular, the ability to integrate their nanoporous probe with micro3D printers and or bioprinters could allow researchers to erase printing mistakes during print runs,” he said. “Such developments could greatly extend the capabilities of today's micro3D printers.”

Rajasekaran said he hopes the process will revolutionize the semiconductor industry, leading to the development of high-tech devices and gadgets previously thought impossible because of the limitations of fabrication.

Another way it might be exploited is in medical sensors that might, for example, be able to diagnose multiple conditions or diseases using a microscopic drop of blood, increasing patient care, comfort and efficiency in treatment. The agarose material can handle sample quantities at the picoscale – one one-trillionth of a meter/liter.

“This would allow us to run multiple diagnostic tests with very low sample volumes and very low reagent needs,” Rajasekaran said. “That in turns would bring down the cost of laboratory tests, allow for simultaneous testing for multiple conditions and also use less blood or other fluids in the process.”

Kohli said the findings are just the beginning, and more work is needed to bring the process into play in an industrial setting.

“It could have a great impact on the society and the everyday life of people,” he said.