If you’ve ever seen the inside of a smart phone or a piece of wearable tech like an Apple Watch, you may have been amazed at the tiny size of the semiconductor chips that power the device. They aren’t small enough, though, for the electronics industry. The industry seeks to produce components at ever decreasing size in an effort to increase speed and efficiency, and at the same time, lower power consumption and weight. Working with materials at the microscopic level, Christopher K. Ober, Materials Science and Engineering and his lab may have the answer. They produce an array of thin films and surfaces that have the potential to revolutionize the world of electronics. In addition, the Ober lab creates innovative surface materials for other industries, such as the biomedical and even the shipbuilding industries.
Creating the Tiniest Chip, Smallest Features
Recently, Ober joined with department colleague Emmanuel P. Giannelis to invent the world’s most sensitive (fastest) extreme ultraviolet (EUV) photoresist. A photoresist is a coating used in photolithography to map out a pattern on a surface—say, of a silicon chip. The chip is coated with the photoresist and exposed to a light shone through a mask. The photoresist’s solubility changes where the light strikes. A solvent then removes the more soluble sections, before the silicon chip is subjected to a subsequent process, such as etching or dopant diffusion, which transforms the silicon wherever the photoresist is absent, creating patterns on the chip.
Ober and Giannelis’s breakthrough photoresist is made of metal oxide nanoparticle complexes only two nanometers across. (By comparison, the width of a human hair is around 80,000 nanometers.) The coating is designed to change solubility when exposed to light at the EUV wavelength of 13 nanometers. “A crude rule of thumb for photolithography is that the wavelength you use for imaging is roughly the dimension of the patterned structures you can make,” Ober explains. “With EUV imaging, that means you can make structures on the order of 13 nanometers, so down to 10 nanometers. That is awesomely tiny.”
For the past 20 years, the electronics industry used various tricks to fabricate ever smaller structures using ultraviolet light with a wavelength of 193 nanometers, but they are finally stumped.
“They chose the EUV wavelength as their next target goal in part because the smaller wavelength makes smaller features,” Ober says, “but the revolution in wavelength requires a revolution in materials. Most photoresists today are polymers (plastics) that are almost invisible at EUV wavelengths. They can’t absorb the radiation, so the needed solubility change we want can’t be induced in the coating. To capture the EUV radiation, we needed to make our photoresist from heavier elements in the periodic table. It took my polymer background and Emmanuel’s inorganic materials background to actually make it work. Separately, I don’t think we could have done it.”
“Right now, we’re working with a company to improve these materials. The next step is to get the photoresist to consistently show resolution below 15 nanometers at a quickness that is acceptable for potential manufacturing.”
The new photoresist has shown good test results, and the semiconductor industry is very interested. “Right now, we’re working with a company to improve these materials,” Ober says. “The next step is to get the photoresist to consistently show resolution below 15 nanometers at a quickness that is acceptable for potential manufacturing.”
The Cornell Nanoscale Facility—Essential to Inventing New Materials and to Biological Studies
Ober, who is the director of the Cornell Nanoscale Facility (CNF), points out the crucial role CNF played in the development of the new photoresist. “Without CNF, we could not have done this,” he says. “We have the best facility in the country for nanoscale research. All the basic semiconductor processing tools we need are here.”
He mentions in particular the facility’s JEOL (Japan Electron Optical Laboratory) 9500 scanning electron beam lithography tool, the only one of its kind at an American university. “We’re lucky to have a tool like that at Cornell,” he says. “This is one of the few places in the U.S. you can test at a size scale that small.”
Working at the nanoscale, the Ober Lab also uses other techniques to modify surfaces, including polymer brushes—polymer chains grown from a surface. “We start the growth of the polymer chain by attaching it chemically to a surface,” Ober explains. “Depending on what we attach to this polymer backbone, we can make it hydrophilic (attracting water) or hydrophobic (repelling water), or we can attach chemical groups that will bind proteins, antibodies, or other biological components to that surface. Over the thickness of just a few nanometers, we can transform glass or silicon so that a living cell thinks it’s in contact with another cell, for instance.”
Using the tools at CNF, the researchers take the brushes and pattern them into shapes that can help them control how biological components interact with the surface, allowing them to study cell surface interactions. For instance, in one project they used pattern brushes to direct the growth of nerve cells on the surface of the brushes. “We were working with researchers from the George Malliaras group [University of Cambridge], who were designing brain electrodes,” Ober says, “We wanted to provide a surface the nerves were comfortable with, which then led the nerves to the electrode location. We gave the nerve cells a physical cue that directed them to go to the place we wanted them to grow.”
The Tough Problem of Boat Hull Fouling
For years Ober has also been applying his work with polymer brushes to a particularly thorny problem: the fouling of boat hulls by ocean life forms, such as barnacles, seaweed, and bacteria. Fouling is a natural process with a huge cost both environmentally and monetarily. “All that stuff growing on the hull makes the surface very rough, and ships have to burn extra fuel to push their way through the water,” Ober explains. “For an ocean-going ship, that can mean several million dollars extra in fuel costs a year. And that fuel creates carbon dioxide, which pollutes the atmosphere.”
The traditional solution has been to poison the sea creatures by loading toxic copper metal into the paint used on ships’ hulls. The paint is designed to wear away, constantly exposing fresh copper, and the copper ends up accumulating in the ocean. Ober is part of a multidisciplinary international team funded by the United States Navy seeking to solve the problem through surface engineering. “It’s incredibly hard because, given all the years of evolution, there’s always something that will stick to your surface,” Ober says.
So far, the researchers have come up with materials that resist fouling in the lab but fall short when tested in the ocean, especially against sea creatures known as hard foulers—barnacles, zebra mussels, and the like. Hard foulers attach themselves to surfaces with a liquid adhesive that cures to a material similar to an industrial strength epoxy. “We’ve figured out the optimal surface tension a material needs to prevent most life forms from sticking,” Ober says. “Now if we can figure out how to interfere with the process hard foulers use to cure their liquid adhesive into a solid, we can engineer the material we need. Our goal is to coat the hull with the material. Then as the ship pushes through the water, the critters will just fall off.”