By Liz Stevens, writer, UV+EB Technology
Smart Material Solutions, Inc. (SMS) is a precision engineering company in Raleigh, North Carolina, with expertise in precision manufacturing, nanofabrication and metamaterials. The company is a member of the RadTech RadLaunch Class of 2024. SMS is backed by the National Science Foundation, the US Army, the US Air Force and NASA to develop nanocoining – the rapid nanopatterning process that the company uses to create seamless cylindrical molds for roll-to-roll nanoimprint lithography (NIL). SMS has an exclusive license for the patented nanocoining process from North Carolina State University, as well as a patent portfolio of its own. UV+EB Technology spoke with Robin McDonald, a materials scientist, to learn more.
What makes the nanopatterning done by SMS noteworthy is the company’s development of rapid design and at-scale production of large-area coatings/surfaces with new optical and wetting functionalities. A focused ion beam is used to create a diamond die that can micro- or nanopattern cylindrical molds. These molds can drop into UV-cured or thermal embossing roll-to-roll systems to enhance the functionality of polymer and metal mesh materials.
Traditionally, nanotextured surfaces are created on and replicated from silicon wafers. The pattern can be replicated into polymers via a plate-to-plate process that involves thermal embossing or UV nanoimprint lithography. The team of materials scientists and mechanical engineers at SMS created a technology which instead makes a cylindrical mold that can be used in roll-to-roll processes to form a continuous patterned surface with trillions of micro and nanostructures per square meter. A nanostructure is on a different scale than a microstructure, but it also requires different production techniques.
“Nanostructures are much smaller than microstructures,” said McDonald. “Just as a meter-wide cube is 1,000 times wider than a millimeter-wide one, nanopatterning creates features that are 1,000 times smaller than those created by micropatterning.” Nanopatterning requires higher precision in alignment and tooling, as it creates structures that are smaller than bacteria or the wavelength of light. “While micropatterning can be achieved using a variety of techniques,” McDonald explained, “nanopatterning requires high-energy electrons, ions, special lasers or unique chemistries to create the miniscule geometries that have rarely been scaled up to commercial levels.”
The Three-Step Nanopatterning Process
Nanopatterning of a diamond die blank with a focused ion beam is the first step of the process at SMS. A pitch of 200 nm to 8 microns typically is used, although larger and smaller pitches can be made. The aspect ratio is up to about 0.8 (height:pitch). The features can be arranged in different lattices, including square, hexagonal and stochastic. A variety of shapes can be incorporated, such as pillars, microlenses, pyramids and cavities.
“Hierarchical micro/nanopatterning creates complex structures comprised of multiple features at different size scales,” McDonald said. “Unlike simpler patterning methods that produce binary structures such as gratings or pillars, hierarchical patterns incorporate at least two different feature sizes.” An example would be using nanoparticle resins to create structures with small nanofeatures on top of larger nanocoined nanofeatures, which in turn would sit atop even larger nanocoined microfeatures. “Our nanocoining process allows us to create multi-scale geometries in a way that drastically reduces the number of fabrication steps typically required,” said McDonald. “We have developed a hierarchical pattern that both decreases dust adhesion and improves light capture – two important attributes, for example, for the top surface of a solar panel.”
Nanocoining is the patented process that SMS uses with a nanopatterned diamond die to seamlessly indent micro or nanopatterns around the outside of a cylindrical drum or sleeve. The company offers dies with tunable 3D features, as opposed to the “binary” structures that standard photo or electron-beam lithography produces. SMS starts with a flawlessly smooth surface to be nanocoined. “For the majority of our cylindrical and flat molds,” said McDonald, “we start by diamond turning to get the surface roughness below 3 nm, which gives a mirror-like finish. However, other transparent molds made of glass cannot be diamond turned and thus retain any form (shape) errors. For all molds, we use a proprietary surface tracking ability during our nanocoining process to ensure uniform patterning.”
SMS’s nanocoining is a rapid mold-making process that operates 500 times faster than electron beam lithography through the use of an ultrasonic actuator. McDonald described the process. “Nanocoining was first developed at North Carolina State in their Precision Engineering Center,” she said. “From that initial work, we vastly expanded the library of patterns and materials we work with, with increasing levels of temperature and stitching control.” The company’s patterning method of nanocoining is unconventional. “Instead of traditional electron beam or lithography methods to pattern wafers,” said McDonald, “we use a tough diamond die that indents or ‘coins’ a cylindrical surface.” The micro or nanopattern in an SMS diamond die first is written using an ultra-precise ion beam. The diamond die then is driven by ultrasonic piezo actuators that indent a spinning cylindrical mold.
For now, SMS can create seamless cylindrical drums, sleeves and photomasks up to 6.5 inches in diameter and 6.5 inches long. It also can produce flat shims or photomasks for initial testing. Molds can be made of high-phosphorous nickel, copper and aluminum while the photomasks are comprised of metal-coated quartz. Once a mold has been produced, SMS can use it to make thousands of meters of patterned film.
Large-area nanofabrication is the last step in which SMS uses roll-to-roll UV nanoimprint lithography (UV NIL), UV photolithography or thermal embossing to transfer the pattern from the mold onto a film. “We primarily pattern polymer substrates by adding a UV-cured patterned coating on top or by thermally embossing the film with heat,” said McDonald. These films then can be adhered to the surface of interest – whether that be the top of a solar panel to improve light capture or a decorative item to create colorful effects. “Through our grant-funded work,” she said, “we also have developed methods to directly transfer patterns into metal and glass through etching.”
“For UV curing,” said McDonald, “we primarily use a dose of 200 mJ/cm2, with 365 nm and 395 nm LED lamps due to their higher efficiency and operation without ozone generation. While we have some broad-spectrum lamps, we typically make our own LED lamps, so we have the flexibility to configure and collimate the light as needed.”
Applications
Transforming a surface with billions of individual nano and microscale features drastically changes its surface area, which can impart advanced wetting and optical properties. With certain plasmonic metamaterial structures, the interaction between light and the electrons on the surface can give rise to new effects not seen in nature. The advanced wetting properties can yield benefits such as anti-microbial surfaces, drag reduction, dust mitigation, self-cleaning windows and solar cells, as well as superhydrophobic properties. Advanced optical characteristics offer anti-reflective and anti-glare properties, improved light transmission, enhanced light-trapping, structural color and surface mimicry.
Designing, producing and ramping up coatings with these important traits sometimes is easier said than done. “While there are many impressive patterns described in academic journals,” said McDonald, “scaling them up to the commercial level has been a major hurdle due to their inherent complexity. Some functionalities are easier to translate into practical applications by running simulations and making tweaks to the patterns and materials involved.” One of the toughest applications that SMS has addressed is reducing lunar dust adhesion to surfaces for NASA. “The conditions in space,” McDonald explained, “such as microgravity, high levels of static electricity and intense solar radiation, drastically are different from those on Earth, making it difficult to test and refine our solutions under realistic conditions. For particularly challenging applications like reducing lunar dust adhesion, we heavily collaborate with University of Texas Austin and NASA.”
“We are ever expanding the library of patterns and materials we work with,” McDonald said, “such as creating more durable patterns in composite and inorganic materials. In addition to this, we are developing decorative patterns that produce structural colors.”
For more information, visit www.smartmaterialsolutions.com.
