Laser Process Simplifies Fabrication of Nanoimprint Molds

Daniel S. Burgess

Investigators at the Industrial Materials Institute of the National Research Council of Canada in Boucherville, Quebec, have developed a laser-based technique for the fabrication of metallic nanostructures on prepatterned substrates that promises to speed production and reduce the cost of constructing 3-D molds for nanoimprint lithography as well as nanostructured substrates for biochemical sensing.

Since its development in 1996, nanoimprint lithography has been studied as a means of producing ultradense integrated circuits, fluidic micromachines for the sorting of biological molecules, and various photonic components such as gratings, filters and polarizers.

In this process, explained research associate Boris Le Drogoff, a mold is pressed into a thermoplastic polymer heated above its glass transition temperature. The topography of the mold is transferred to the polymer, which serves as a resist for the subsequent lithographic patterning of an underlying substrate.

For all its potential, nanoimprint lithography has one limitation: Producing a mold can be a complex endeavor.

“The fabrication of complex, multilevel molds is one of the challenges that need to be solved for applications requiring more than 2-D surface patterning,” Le Drogoff said.

He explained that photolithography and electron-beam lithography can be used to produce the molds, but that they involve expensive projection optics, advanced illumination sources or specialized resists. Moreover, he noted, photolithography tends to be limited by its resolution, and electron-beam lithography is relatively slow.

The new 3-D patterning technique, which the scientists have called laser-assisted nanotransfer printing, dispenses with resists and effectively replaces the patterning, metal deposition and liftoff steps of standard lithography with one step, Le Drogoff said. In a process similar to laser-induced forward transfer, laser pulses selectively heat a source of donor material so that it melts and bonds to the desired locations on an acceptor substrate. Unlike that method, however, both substrates in laser-assisted nanotransfer printing are prepatterned so that some areas will not accept the donor material. As a result, the resolution of the approach is limited not by the spot size of the laser, but by the size of the features on the donor material.

In laser-assisted nanotransfer printing, metal nanodots on a quartz plate are melted and fuse to a prepatterned substrate. The process has the potential to be used to fabricate molds for nanoimprint lithography. Inset shows a close-up. Courtesy of Boris Le Drogoff.

In their proof-of-principle demonstration, the scientists used photolithography and reactive-ion etching to produce a series of 100-μm-diameter, 40-nm-deep microwells into a 500-nm-thick SiO₂ layer on a silicon wafer. A fused quartz plate coated with 80-nm-across, 35-nm-high chromium nanodots was pressed against the silicon acceptor and exposed through the back side with 266-nm radiation from a frequency-quadrupled Nd:YAG laser from Continuum of Santa Clara, Calif. At a laser fluence of 350 mJ/cm², the nanodots were transferred to those regions of the substrate in contact with the quartz plate.

Le Drogoff said that the researchers also have performed laser-assisted nanotransfer printing using a KrF excimer laser but that the process does not necessarily require a UV source. What is important is that the laser has a short pulse width for spatially well-defined surface melting, with negligible heating or damage of the bulk substrates.

“We have recently started experiments using a ‘homemade’ Q-switched Nd:YAG laser system in its fundamental frequency, which offers a better spatial beam profile, close to a top-hat profile,” he said. As expected (because of the lower excitation energy), successful transfer of the metal nanodots was achieved with higher laser fluence, he added.

The scientists are experimenting with various donor materials and acceptor substrates to learn more about the phenomena behind the transfer process. Le Drogoff noted that other potential applications of laser-assisted nanotransfer printing include the preparation of substrates for surface-enhanced Raman scattering spectroscopy.

Applied Physics Letters, Sept. 11, 2006, 113103.