Newly Created Molecules Drive Lithography Technique

New two-photon absorbing molecules that are sensitive to light at short wavelengths are helping to produce 3-D polymer line structures as small as 65 nm wide. The 3-D multiphoton lithography technique could compete with existing processes for making nanoscale electronic, photonic and microfluidic devices.

A Georgia Institute of Technology research team led by Joe Perry (left, with researcher Vincent Chen) has produced 3-D polymer line structures as small as 65 nm wide using new two-photon absorbing molecules. (Georgia Tech Photo: Gary Meek)

Fabricating such small features (a nanometer is one-billionth of a meter) normally requires expensive electron beam or extreme ultraviolet lithography equipment. However 3-D multiphoton lithography simplifies the process and reduces the cost, researchers said, by using two-photon absorbing molecules.

"Being able to obtain line widths down to 65 nanometers, which is substantially below prior published work of 100 nanometers, opens up new applications for multiphoton lithography," said Joseph Perry, a professor in the Georgia Institute of Technology School of Chemistry and Biochemistry and the Center for Organic Photonics and Electronics.

The technique scans a laser beam across a substrate coated with a polymer resin containing a unique dye to create a desired hardened polymer structure. The laser writing process takes advantage of the fact that the chemical reaction of cross-linking occurs only where molecules have absorbed two photons of light. Since the rate of two-photon absorption drops off rapidly with distance from the laser's focal point, only molecules at the focal point receive enough light to absorb two photons.

Seth Marder and Stephen Barlow, also researchers in the School of Chemistry and Biochemistry and the Center for Organic Photonics and Electronics, synthesized the unique molecule called DAPB, 4,4'-bis(di-n-butylamino)biphenyl, to initiate the chemical reaction leading to the hardening of the polymers when exposed to laser light.

"We needed a dye with good two-photon absorption at a wavelength of 520 nanometers, so we tried DAPB," said Perry. "DAPB proved to be very effective in this kind of lithography."

The molecule developed by Marder and Barlow is about 10 times more efficient at absorbing light by two-photon absorption than commercial ultraviolet photoactive materials. That efficiency allowed Perry and graduate students Wojciech Haske and Vincent Chen, research scientist Joel Hales and postdoctoral associate Wenting Dong to create 3-D patterns with nanoscale lines at light intensities low enough to avoid damaging the polymers.

Scanning electron microscope images of woodpile-type photonic crystal structures fabricated with 520-nm excitation at (a) higher power and at (b) lower power using DABP. Magnified images of the structures are shown below their respective overview images. (Image: Wojciech, H. et al.)

For the experiments, a film of the polymer resin containing DAPB was formed. When the film was exposed to the focused laser, DAPB was excited and triggered cross-linking, leaving the insoluble scanned structure on the surface of a substrate when placed in a developer solution.

Since Perry controls where the titanium:sapphire pulsed laser scans with a computer program, the polymers can be cross-linked in any pattern including 3-D stacks of straight lines that are connected and sturdy. The laser beam is turned on to expose lines of polymer and off when no line should be drawn.

Conventional lithography involves creating a specific pattern on a mask for each new layer and exposing each layer to light and developing it. With this new technique, 3-D layered nanostructures can be created simply by having a computer program scan a different pattern for each layer. Mask templates become unnecessary and the coating, exposing and developing processes only have to be conducted once.

"We can create essentially any pattern we want. For this work, some of the patterns look like walls or lines suspended across walls and some are like a tall stack of crisscrossed lines," said Perry.

"We can write very small lines and create stacked-up grids of lines called photonic crystals," Perry said. "This work shows that we can fabricate functional photonic microdevices with tailored transmission capabilities."

It takes only 10 minutes to create a 20 µm by 20 µm structure with 30 layers, Perry said. He envisions using this technology to create compact microspectrometers on a chip for use in telecommunications and sensors. It may also be used as a compact way to separate the multiple wavelengths traveling through a fiber-optic cable.

This type of simple, table-top technology may also be useful to fabricate customized types of circuits with many layers, which would be extremely expensive with standard methods because each layer would require a special mask.

"With the combination of the right molecule and short wavelength light, we've demonstrated that we can obtain nanoscale features. We're at 65 nanometers now and we're still trying to go smaller," Perry said.

The fabrication method and dye were described in the March 19 issue of Optics Express. The research was supported by the Office of Naval Research APEX Consortium and the National Science Foundation, through the Science and Technology Center for Materials and Devices for Information Technology Research. Perry and Marder co-founded a company in 2003 called Focal Point Microsystems that is working to commercialize the technology.

For more information, visit: www.gatech.edu