Scientists using moulds derived from carbon nanotubes have approached the ultimate resolution – defined by molecular scale dimensions – in a widely used polymer nanoimprinting technique.
By accurately replicating features with nanometre dimensions, the technique could play future roles in fabricating structures in fields as diverse as microelectronics, nanofluidics and biotechnology.
Polymer nanoimprint lithography works by pressing a mould with embossed relief structures against a thin polymer film. Little is known, however, of the basic physics and chemistry that operate between the two surfaces at the molecular level, let alone how these interactions relate to resolution.
John Rogers, a professor of materials science and engineering at the University of Illinois at Urbana-Champaign and colleagues at Illinois and Dow Corning Corporation explored the fundamental resolution limits of polymer nanoimprint lithography. The researchers began by growing single-walled carbon nanotubes on a silicon wafer. Then they prepared a mould of the nanotubes by pouring a thermal-setting polymer over the wafer.
After curing the mould, they gently pressed it against a thin layer of photocurable polyurethane. Passing light through the transparent mould caused the material to cross-link and harden. The researchers then used atomic force microscopy to measure the heights of the resulting relief structures and transmission electron microscopy to determine their widths.
“Our approach allowed us to reach a critical size regime never explored before,” Rogers said. “From a detailed analysis of the microscope images, we were able to demonstrate reliable patterning at the two nanometre scale, and even some capability down to one nanometre. These dimensions are comparable to the sizes of individual macromolecules.”
To obtain features with a resolution of two nanometres, both the average distance between polymer cross-links (approximately one nanometre) and the lengths of individual chemical bonds (approximately 0.2 nanometres) become important in the moulding process.
“We normally wouldn’t be concerned with the molecular structure of the polymer,” Rogers said, “but at these dimensions we have feature sizes that are only a few times larger than the length of individual bonds in the polymer. In addition, we have a countable number of polymer bond lengths that are available to replicate the relief structure.”
By varying the density of cross-links in the polymer, the researchers also established a connection between resolution limit and molecular structure of the polymer. “The ultimate resolution is correlated to the ability of the prepolymer to conform to the surface and the ability of the cross-linked polymer to retain the moulded shape,” Rogers said.
The ability to mould nanoscale features can benefit many fields, from semiconductor device manufacturing to emerging areas of biotechnology. For example, polymer nanoimprint lithography could help the electronics industry achieve the resolution requirements needed for next-generation devices. By structuring materials with dimensions smaller than the wavelength of light, the technique also could create photonic devices whose optical properties are defined by the geometry of the relief structures embossed on them.
In other applications, polymer moulds with molecular scale channels could prove useful in nanofluidics, where the tiny tunnels would transport fluids or separate materials based on size, Rogers said. By allowing for the nanoimprinting of individual macromolecules, the technique might open new paths to molecular recognition, drug discovery and catalysis.