Tag Archives: lithography

How to prevent your scanning tunneling microscope probe’s ‘tip crashes’

The microscopes used for nanoscale research were invented roughly 35 years ago and as fabulous as they’ve been, there is a problem (from a February 12, 2018 news item on Nanowerk),

A University of Texas at Dallas graduate student, his advisor and industry collaborators believe they have addressed a long-standing problem troubling scientists and engineers for more than 35 years: How to prevent the tip of a scanning tunneling microscope from crashing into the surface of a material during imaging or lithography.

The researchers have prepared this video describing their work,

For those who like text, there’s more in this February 12, 2018 University of Texas at Dallas news release,

Scanning tunneling microscopes (STMs) operate in an ultra-high vacuum, bringing a fine-tipped probe with a single atom at its apex very close to the surface of a sample. When voltage is applied to the surface, electrons can jump or tunnel across the gap between the tip and sample.

“Think of it as a needle that is very sharp, atomically sharp,” said Farid Tajaddodianfar, a mechanical engineering graduate student in the Erik Jonsson School of Engineering and Computer Science. “The microscope is like a robotic arm, able to reach atoms on the sample surface and manipulate them.”

The problem is, sometimes the tungsten tip crashes into the sample. If it physically touches the sample surface, it may inadvertently rearrange the atoms or create a “crater,” which could damage the sample. Such a “tip crash” often forces operators to replace the tip many times, forfeiting valuable time.

Dr. John Randall is an adjunct professor at UT Dallas and president of Zyvex Labs, a Richardson, Texas-based nanotechnology company specializing in developing tools and products that fabricate structures atom by atom. Zyvex reached out to Dr. Reza Moheimani, a professor of mechanical engineering, to help address STMs’ tip crash problem. Moheimani’s endowed chair was a gift from Zyvex founder James Von Ehr MS’81, who was honored as a distinguished UTD alumnus in 2004.

“What they’re trying to do is help bring atomically precise manufacturing into reality,” said Randall, who co-authored the article with Tajaddodianfar, Moheimani and Zyvex Labs’ James Owen. “This is considered the future of nanotechnology, and it is extremely important work.”

Randall said such precise manufacturing will lead to a host of innovations.

“By building structures atom by atom, you’re able to create new, extraordinary materials,” said Randall, who is co-chair of the Jonsson School’s Industry Engagement Committee. “We can remove impurities and make materials stronger and more heat resistant. We can build quantum computers. It could radically lower costs and expand capabilities in medicine and other areas. For example, if we can better understand DNA at an atomic and molecular level, that will help us fine-tune and tailor health care according to patients’ needs. The possibilities are endless.”

In addition, Moheimani, a control engineer and expert in nanotechnology, said scientists are attempting to build transistors and quantum computers from a single atom using this technology.

“There’s an international race to build machines, devices and 3-D equipment from the atom up,” said Moheimani, the James Von Ehr Distinguished Chair in Science and Technology.

‘It’s a Big, Big Problem’

Randall said Zyvex Labs has spent a lot of time and money trying to understand what happens to the tips when they crash.

“It’s a big, big problem,” Randall said. “If you can’t protect the tip, you’re not going to build anything. You’re wasting your time.”

Tajaddodianfar and Moheimani said the issue is the controller.

“There’s a feedback controller in the STM that measures the current and moves the needle up and down,” Moheimani said. “You’re moving from one atom to another, across an uneven surface. It is not flat. Because of that, the distance between the sample and tip changes, as does the current between them. While the controller tries to move the tip up and down to maintain the current, it does not always respond well, nor does it regulate the tip correctly. The resulting movement of the tip is often unstable.”

It’s the feedback controller that fails to protect the tip from crashing into the surface, Tajaddodianfar said.

“When the electronic properties are variable across the sample surface, the tip is more prone to crash under conventional control systems,” he said. “It’s meant to be really, really sharp. But when the tip crashes into the sample, it breaks, curls backward and flattens.

“Once the tip crashes into the surface, forget it. Everything changes.”

The Solution

According to Randall, Tajaddodianfar took logical steps for creating the solution.

“The brilliance of Tajaddodianfar is that he looked at the problem and understood the physics of the tunneling between the tip and the surface, that there is a small electronic barrier that controls the rate of tunneling,” Randall said. “He figured out a way of measuring that local barrier height and adjusting the gain on the control system that demonstrably keeps the tip out of trouble. Without it, the tip just bumps along, crashing into the surface. Now, it adjusts to the control parameters on the fly.”

Moheimani said the group hopes to change their trajectory when it comes to building new devices.

“That’s the next thing for us. We set out to find the source of this problem, and we did that. And, we’ve come up with a solution. It’s like everything else in science: Time will tell how impactful our work will be,” Moheimani said. “But I think we have solved the big problem.”

Randall said Tajaddodianfar’s algorithm has been integrated with its system’s software but is not yet available to customers. The research was made possible by funding from the Army Research Office and the Defense Advanced Research Projects Agency.

Here’s a link to and a citation for the paper,

On the effect of local barrier height in scanning tunneling microscopy: Measurement methods and control implications by Farid Tajaddodianfar, S. O. Reza Moheimani, James Owen, and John N. Randall. Review of Scientific Instruments 89, 013701 (2018); https://doi.org/10.1063/1.5003851 Published Online: January 2018

This paper is behind a paywall.

Silky smooth tissue engineering

Virginia Commonwealth University (VCU) researchers have announced a new technique for tissue engineering that utilizes silk proteins. From a May 13, 2014 news item on Nanowerk,

When most people think of silk, the idea of a shimmering, silk scarf, or luxurious gown comes to mind.

But few realize, in its raw form, this seemingly delicate fiber is actually one of the strongest natural materials around – often compared to steel.

Silk, made up of the proteins fibroin and sericin, comes from the silkworm, and has been used in textiles and medical applications for thousands of years. The [US] Food and Drug Administration has classified silk as an approved biomaterial because it is nontoxic, biodegradable and biocompatible.

Those very properties make it an attractive candidate for use in widespread applications in tissue engineering. One day, silk could be an exciting route to create environmentally sound devices called “green devices,” instead of using plastics. However, forming complex architectures at the microscale or smaller, using silk proteins and other biomaterials has been a challenge for materials experts.

Now, a team of researchers from the Virginia Commonwealth University School of Engineering has found a way to fabricate precise, biocompatible architectures of silk proteins at the microscale.

A May 12, 2014 VCU news release by Sathya Achia Abraham, which originated the news item, describes the research underlying two recently published papers by the research team

    Kurland [Nicholas Kurland, Ph.D.] and Yadavalli [Vamsi Yadavalli, Ph.D., associate professor of chemical and life science engineering] successfully combined silk proteins with the technique of photolithography in a process they term “silk protein lithography” (SPL). Photolithography, or “writing using light,” is the method used to form circuits used in computers and smartphones, Yadavalli said.

According to Yadavalli, SPL begins by extracting the two main proteins from silk cocoons. These proteins are chemically modified to render them photoactive, and coated on glass or silicon surfaces as a thin film. As ultraviolet light passes through a stencil-like patterned mask, it crosslinks light-exposed proteins, turning them from liquid to solid.

The protein in unexposed areas is washed away, leaving behind patterns controllable to 1 micrometer. In comparison, a single human hair is 80-100 micrometers in diameter.

“These protein structures are high strength and excellent at guiding cell adhesion, providing precise spatial control of cells,” Yadavalli said.

“One day, we can envision implantable bioelectronic devices or tissue scaffolds that can safely disappear once they perform their intended function,” he said.

The team’s current research focuses on combining the photoreactive material with techniques such as rapid prototyping, and developing flexible bioelectronic scaffolds.

Study collaborators included S.C. Kundu, Ph.D., professor of biotechnology at the Indian Institute of Technology Kharagpur in India, and Tuli Dey, Ph.D., postdoctoral associate, at the Indian Institute of Technology Kharagpur in India, who provided the silk cocoons used in the study and assisted with cell culture experiments. VCU has recently filed a patent on this work.

Here’s a link to and a citation for both papers,

Silk Protein Lithography as a Route to Fabricate Sericin Microarchitectures by Nicholas E. Kurland, Tuli Dey, Congzhou Wang, Subhas C. Kundu and Vamsi K. Yadavalli. Article first published online: 16 APR 2014 DOI: 10.1002/adma.201400777

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Precise Patterning of Silk Microstructures Using Photolithography by Nicholas E. Kurland, Tuli Dey, Subhas C. Kundu, and Vamsi K. Yadavalli. Advanced Materials Volume 25, Issue 43, pages 6207–6212, November 20, 2013 Article first published online: 20 AUG 2013 DOI: 10.1002/adma.201302823

© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Both papers are behind a paywall.

I have written about silk proteins in a Nov. 28, 2012 post (Producing stronger silk musically) that briefly mentioned tissue engineering with regard to a new technique for biosynthesising  materials.

Replacing the lithographic process for semi-conductor chips with self-assemby; one step closer in Canada

A news release from Canada’s National Institute of Nanotechnoloy (NINT), their first this year,  about researchers at NINT and at the University of Alberta claiming that a,

Decrease in self-assembly processing time creates viable alternative to conventional lithography

Thanks to a microwave oven, the fundamental nanotechnology process of self assembly may soon replace the lithographic processing use to make the ubiquitous semi-conductor chips. By using microwaves, researchers at Canada’s National Institute for Nanotechnology (NINT) and the University of Alberta have dramatically decreased the cooking time for a specific molecular self-assembly process used to assemble block copolymers, and have now made it a viable alternative to the conventional lithography process for use in patterning semi-conductors. When the team of chemists and electrical engineering researchers replaced convective heat with a microwave oven, nano-sized particles were encouraged to organize themselves into very regular patterns extremely quickly – reducing the processing time from days to less than one minute. [emphasis mine]

The processing time is very important if this self-assembly process is to be introduced to industrial semi-conductor fabrication. In the International Technology Roadmap for Semiconductors, the promise of self-assembly to address the need to put more and more functionality onto chips was recognized. The block co-polymer method, which directs nanomaterials to create molds and then fills them in with a target material, was known to be capable of creating very detailed patterns many times smaller than current technology. But previously the time needed for molecules to organize themselves was too long to be useful for the industry. The change of the heat source has brought that processing time well under the suggested target of 4 minutes.

“This is one of the first examples of the self-assembly process being used to address a real world problem for the semi-conductor industry,” said Dr. Jillian Buriak “We’ve got the process; the next step is to exploit it to make something useful.”

The process for quicker assembly is outlined in new paper in the American Chemical Society’s ACS Nano, posted on-line October 21, 2010. http://pubs.acs.org/doi/abs/10.1021/nn102387c

The news release can be found Eureka Alert but is not yet available on NINT’s website.