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Electrolithography: A Breakthrough in Nano-patterning Process from Indian Scientists.

31 July 2016

Bangalore: Nestling amidst 400 acres filled with over 100 exotic and indigenous trees and plants, the swanky new modern complex which houses the Centre for Nano Science and Engineering (CeNSE) is in sharp contrast to the classically styled old stone architectural building of the Indian Institute of Science ( IISc), Bangalore.

Prof Rudra Pratap, chairperson of the Centre for Nano Science and Engineering (CeNSE) Prof Rudra Pratap, chairperson of the Centre for Nano Science and Engineering (CeNSE)

And, the kind of work being done here by Professor Rudra Pratap, who leads a team of about 140 Ph.D scholars, is also in sharp contrast to the regular academia study and research done by the other 40 or more departments at the Institute.

Centre for Nano Science and Engineering (CeNSE) Centre for Nano Science and Engineering (CeNSE)

Pratap, chairperson of the Centre for Nano Science and Engineering (CeNSE) holds a Ph.D. degree from Cornell University, a Master’s degree from the University of Arizona and an engineering degree from the Indian Institute of Technology, Kharagpur. He has also taught at the Sibley School of Mechanical and Aerospace Engineering at Cornell University, for two and a half years.

Indian Institute of Science ( IISc), Bangalore Indian Institute of Science ( IISc), Bangalore

A team of three -- Pratap, Ph.D scholar Santanu Talukder and Professor Praveen Kumar-- have found a way to etch nano-circuits on silicon chips at room temperature, which could potentially accelerate research efforts at the fundamental chip level. This technique could lower production cost tremendously and revolutionize the semiconductor industry.

“We turned an accidental discovery into a technique that could save billions of dollars for the semiconductor industry," says Pratap. "Today, it is a highly expensive process using electron beam (e-beam) lithography to create the tiny structures that run the chips inside our computers. But our technique called electro-lithography can do this at a fraction of that cost. We believe this to be is a game-changer because this makes the whole patterning technique affordable to a larger audience. And it came from our discovery that it you take chromium and if you apply voltage on chromium with a probe then chromium melts and moves along the electric field. This is something that we discovered in our research.”

The team has been working on this for the past five years and has finally come out with this discovery and has explained the technique in its paper titled “Electrolithography- A new and versatile process for nano patterning and published in Scientific Reports. They have filed a patent in India and are in the process of global patent filing for this technique.

“Another Ph.D scholar had worked on gold line patterning (gold is used for electrical connections) for sensor technology and had noticed a rare phenomenon while it was part of his research," Pratap explained. "We wanted to make a technology out it and that’s when we asked Talukder to work on this and take it forward. When Talukder started working on this, he discovered this effect (which we hadn’t discovered before) – the melting of chromium and moving along the electrical field. Once he found that and showed it to us, then we had to figure out why that was happening - to understand the science and figure out why it is melting and moving along those particular lines and what were the control parameters it was following. Once we understood all of that, then we became very excited and worked on this technique and realized that we can actually use it for nano-circuits patterning. We did it and more important we realized that we can do it a fraction of the cost it takes to do patterning using e-beam lithography.”

Navigating into the Nano-world Navigating into the Nano-world

Simply put, it is a new lithography technique based on electro-migration driven material transport for drawing patterns at nanometer scales in ambient conditions. A sharp metal tip is used to bombard electrons onto a metal. The resulting heating, due to electric current, melts the chromium (actually a low melting temperature compound of chromium that forms under the high electric field) and the "electron wind" makes it flow along electric field lines.

“We used a thin metal film as a masking layer and a polymer layer beneath it as a pattern transfer layer," says Talukder. "The desired pattern is drawn in the metal layer by etching the metal with a conducting scanning probe assisted by liquid electro-migration. The pattern drawn on the metal layer is transferred to the polymer layer by etching the polymer with an appropriate solvent. Subsequently, the pattern is transferred to the desired material layer using a film deposition technique followed by conventional lift-off process. Using this simple technique, we have achieved pattern resolutions of 9 nm on the polymer and 40 nm on transferring the pattern to another material."

Based on the ease of use and process costs, this technique appears to be very promising. On the basis of process costs, it is far less when compared to e-beam lithography. It is also competitive in terms of throughput (speed of patterning) and almost there in terms of resolution.

“You don’t need the high vacuum or higher voltage for our technique. It can be done at room temperature and normal atmospheric pressure," says Pratap. "Of course, you do it in a clean room because you don’t need dust to interfere with the process but other than that the process itself is very cheap. And it came from our discovery that it you take chromium and if you apply voltage on chromium with a probe then chromium melts and moves along the electric field. This is something that we discovered in our research."

The next step is to improve the reliability of the method and commercialize the product.

“We are talking with a couple of other companies about creating a machine that will do this," adds Pratap. "When you do these things in a lab you are doing it in bits and pieces. It is on a smaller scale – there are various components hanging here and there - but when you want others to use this technique you need to make a machine to use it. Only then it becomes a commercial product and researchers across the world can use it."

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