Multi-Photon Litho Makes 65 nm Polymer Structures

Conventional lithography creates patterns through the use of a mask for each layer of a design, exposing each layer to light and developing it. Researchers at the Georgia Institute of Technology (Atlanta) are using a multi-photon lithography technique that eradicates the need for masks, creating 3-D polymer line structures as small as 65 nm.

With this technique, 3-D layered nanostructures can be created simply by using a computer program to scan a different pattern for each layer. Not only do masks become unnecessary, but the coating, exposing and developing processes only have to be done once. And it takes just 10 minutes to create a 20 × 20 µm structure with 30 layers, according to Joseph Perry, a professor in Georgia Tech's School of Chemistry and Biochemistry and the Center for Organic Photonics and Electronics .

“Being able to obtain linewidths down to 65 nm, which is substantially below prior published work of 100 nm, opens up new applications for multi-photon lithography,” Perry said. Specifically, Perry envisions using this technology to create compact micro-spectrometers 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.

The key to the latest achievements, which were described in the March 19 issue of Optics Express,¹ comes from new two-photon absorbing molecules that are sensitive to laser light at short wavelengths. This allows researchers to create small linewidths without highly sophisticated fabrication methods, such as electron-beam or extreme ultraviolet (EUV) lithography equipment. Using 3-D multi-photon lithography simplifies the process and reduces the cost. The technique could compete with existing processes for fabricating nanoscale electronic, photonic and microfluidic devices.

SEM images of woodpile-type photonic crystal structures fabricated with 520 nm excitation at higher power (left) and lower power (right) using DABP. Magnified images of the structures are shown below their respective overview images. (Source: Georgia Institute of Technology)1

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 4,4-bis(di-n-butylamino)biphenyl (DABP) molecule 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 nm, so we tried DABP,” Perry explained. “DABP proved to be very effective in this kind of lithography.”

The molecule is about 10× more efficient at absorbing light by two-photon absorption than commercial UV 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.

For the experiments, a film of the polymer resin containing DABP was formed. When the film was exposed to the focused laser, DABP 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 Ti: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.

“We can create essentially any pattern we want,” Perry said. “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.”

Perry and Marder co-founded a company in 2003 called Focal Point Microsystems (Atlanta) that is working to commercialize this fabrication technology. In addition to the applications mentioned above, this type of simple, tabletop technology may also be useful to fabricate customized types of circuits with many layers, which would be extremely expensive with standard methods.