(Left to right) Random parts – nanowires, gears, scalpels, etc.- fabricated in bulk metallic glass molds. (Credit: Yale)
Yale engineers are “shaping” the future of nanodevice manufacturing by developing nanoscale molds made from amorphous metals – a.k.a. bulk metallic glasses. Opening the door to mass nanofabrication processes, their work may also enable higher-density computer chips, faster microprocessors, better biosensors and more. Research is led by Jan Schroers, a professor of mechanical engineering in Yale’s School of Engineering and Applied Science. Reporting in the February 12 edition of Nature, Schroers says the “success and proliferation” of nanotechnology fabrication depends on “robust and durable master molds.”
Currently, however, most nanomolds are comprised of silicon or metal – two materials that offer significant mold-making limitations. For example, silicon molds can capture fairly fine detail, but they’re brittle and seldom last longer than 100 uses. Metal molds last longer, but the grains in their crystalline structures interfere with imprinting details on their surfaces. Where silicon and metal fail, however, bulk metallic glasses or “BMGs” succeed. Cooled rapidly, they don’t form crystal structures and, so, don’t have “grains” as metals do. “Although they seem solid,” Schroers says, “they are more like a very slow-flowing liquid that has no structure beyond the atomic level – making them ideal for molding fine details.” BMGs also have the “pliability of plastics at moderately elevated temperatures, but they are stronger and more resilient than steel or metals at normal working temperatures,” Schroers reports.
BMG molds can be “can be used millions of times to pattern materials, including polymers like those used to make DVDs,” he says, noting the Yale team had used them to fabricate a range of three-dimensional microparts, including tweezers, gears, scalpels and more. He reports confronting only one major hurdle – finding a way to get material to release intact from a BMG mold. “Surfaces of liquid metals exhibit high surface tension and capillary effects that can interfere in the molding,” he explains. Golden Kumar, a postdoctoral team member, found the solution. “By altering the mold-BMG combination,” Kumar was able to “create surfaces so that the atoms take advantage of their favorable interaction with the mold – to both fill the mold and then release the product,” according to the Yale Bulletin. This solution reportedly has enabled the nano-patterning of details as small as 13 nanometers. Schroers believes even finer detail can be replicated in the future. “Theoretically,” he says “the size limit is the size of a single atom.”