- High-Speed Nanoimprinting Gets on a Roll
Successive coating and imprinting help achieve patterns with infinitesimal features.
Se Hyun Ahn and L. Jay Guo of the University of Michigan, Ann Arbor, are on a nanopatterning roll. They recently demonstrated a high-speed roll-to-roll technique for imprinting that enables continuous fabrication of nanometer-scale structures on a flexible plastic substrate.
This schematic illustrates a roll-to-roll process for patterning films with nanoscale structures at high speed. Images reprinted with permission from Advanced Materials.
Their method could benefit a variety of areas, including liquid crystal displays, solar cells, data storage and biochips.
“There are many nontraditional microelectronics applications that require the use of nanoscale structures, and they demand low-cost nanopatterning technology,” said Guo, an associate professor of electrical engineering and computer science.
Nanoimprint lithography has the potential for high throughput and high resolution. In nanoimprinting, a master is pressed into a polymer resist coating on a substrate, leaving an impression that faithfully reproduces the mold. The problem has been that existing nanoimprint lithography techniques use silicon wafers and require a few minutes per wafer — a process that is too slow for many applications.
Researchers used their technique — dubbed roll-to-roll nanoimprint lithography — to imprint gratings with 700-nm period and 300-nm linewidth in PDMS. The length of the sample is 570 mm.
To overcome such drawbacks, the researchers devised a roll-to-roll process consisting of coating and imprinting steps. In the first step, they cover a web of moving material with a resist coating, into which they imprint a pattern. In the next step, the coated substrate travels around a roller that contains the desired pattern. Because of pressure, the pattern transfers from the mold into the coating. Infrared or ultraviolet radiation cures the resist, solidifying the pattern. Subsequent steps can follow, such as coating the pattern with metal.
The key to the technique, Guo noted, lies in combining the right materials with the proper imprinting conditions. For fast roll-to-roll nanoimprinting, it is best to use a solvent-free resist material that remains liquid after coating the flexible substrate. That condition allows quick formation of nanopatterns using low pressure.
He noted that previous investigations had provided two such materials from which to choose. The first was a fast thermally curable PDMS that cross-linked within a few seconds at 120 °C. The second was a UV-curable low-viscosity liquid epoxysilicone. The low viscosity enabled the material to be imprinted at room temperature and at low pressure.
Because they had to attach a mold to a cylindrical roller, the researchers needed a flexible material for the mold also. They found a suitable candidate in a commercial fluoroco-polymer, ethylene-tetrafluoroethylene (ETFE). They used a silicon master to create the ETFE mold, patterning the master via interference lithography accomplished with a 325-nm HeCd laser from Kimmon Electric US LP of Centennial, Colo.
In a demonstration of the technique, the researchers made a metal wire grid polarizer by depositing a thin layer of aluminum over a nanoimprinted grating pattern. The largest patterns had a period of 700 nm, with 300-nm lines. The smallest had a 100-nm period and 70-nm lines. They measured the transmittance of polarized light through the structures using a UV-visible spectrometer from Ocean Optics Inc. of Dunedin, Fla.
The results proved that the devices — and the fabrication technique — worked. The production of other devices could follow, Guo said. “We are applying nanoimprinting to organic solar cell fabrication, and the approach is potentially scalable to roll-to-roll nanoimprinting process in the future.”
Advanced Materials, Published online April 24, 2008, doi: 10.1002/adma.200702650.
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