Optimizing a “pillar” of modern technology: REAP Lab streamlines nanotechnology development

Many modern technologies, such as computer chips and sensors rely on tiny silicon structures invisible to the naked eye. These structures, called nanopillars, are smaller than the wavelength of light, which allows them to control how light behaves. Because many technologies store, sense, and transmit information using light, the ability of nanopillars to affect light behavior plays a crucial role in developing smarter, faster, and more efficient technologies. However, manufacturing nanopillars often relies on expensive materials or involves many complicated, expensive steps. 

Researchers in Tufts’ Renewable Energy and Applied Photonics Lab were able to create silicon nanopillars through a simpler, more cost-friendly process using an ultra-thin, light sensitive coating that guides where and how nanopillars are created. Published in the Journal of Vacuum Science and Technology B., this research could help to streamline the complicated and costly process of creating nanopillars that are critical for accelerating important technology advancements. 

Creating simpler, more affordable nanopillars 

To create something as small as a silicon nanopillar (one strand of hair is about 80,000 nanometers thick), scientists cover silicon in a coating and use an electron beam to “draw” the locations and sizes of the nanopillars to be created, like an extremely precise pen. Then, the electron beam etches down the silicon to create the pillars, which are protected by the coating. 

A common, inexpensive coating, also known as an “electron-beam resist,” is Poly-methyl methacrylate (PMMA), a clear, plastic-like polymer. However, this electron-beam resist is known to degrade easily during dry etching and is only used indirectly to help make other resists—a timely process that increases costs.  

“Optimizing the creation of things that you can’t see with the naked eye can be challenging,” said Kareena Guness, EG25 and Ph.D. candidate in electrical and computer engineering who led the research for her dissertation. “Despite the small size of nanopillars, they are vital to the success and efficiency of technology. If we can fabricate nanopillars more effectively, we can develop better technologies in more affordable, faster ways.” 

Other contributors to the research included Pan Menasuta, Ph.D. candidate in electrical and computer engineering, Zachary Kranefeld, EG25, Basil Vanderbie, Ph.D. student in materials science, and Thomas Vandervelde, professor and chair of electrical and computer engineering and principal investigator of the Renewable Energy and Applied Photonics Lab. 

The team's research created and demonstrated the success of using PMMA directly in a streamlined, simpler nanopillar fabrication process. Instead of trying to work around the limits of PMMA, the team changed elements of the fabrication process itself, altering the etching step to reduce stress on the coating.  

During the fabrication process, the team created shorter, more robust nanopillars. This required less etching, preventing PMMA degradation. They also increased the density of the nanopillars and arranged them uniformly, which allowed the etching to be more uniform across the surface, reducing destruction to the PMMA mask. 

Additionally, the team altered the etching chemistry to slow down the speed at which PMMA was consumed throughout the etching process. This combination of changes allowed the team to successfully use PMMA as a direct etch mask.

A tiny structure with a wide-ranging impact 

“Our study shows that PMMA does not have to be replaced with more rare, expensive materials if the fabrication process itself is optimized,” Guness said. “This could provide a new guideline for the future of nanopillar fabrication, saving time, money, and materials that often slow down technology creation and advancement.”     

Because nanopillars are used in a wide range of technologies from solar panels to biosensors, these results could potentially be applied to a variety of fields, helping to streamline the creation of technologies needed for energy, medical diagnosis, quantum information processing, and more. 

The Renewable Energy and Applied Photonics Lab studies how light interacts with matter and then applies that knowledge to technology development. From solar cells to cameras, Vandervelde's team is committed to research that can accelerate next-generation imaging technology and energy independence.