“Two-photon absorption” technique shows promise for 3D microstructures

Georgia Tech researchers say they’ve developed a “3D multiphoton” technique, a process involving two-photon absorbing molecules that are sensitive to laser light at short wavelengths, that could be applied to simplify and cut costs for patterning 65nm structures.

Conventional lithography techniques involve creating specific mask patterns for each layer and exposing and developing each one. This new two-photon technique, the scientists say, lets them create 3D layered nanostructures just by programming a different pattern for each layer — meaning no more mask templates, and just one pass for coating, exposing, and developing.

“In conventional lithography 3D structures have to built up in a layer by layer process, each layer requiring coating, exposure and washing,” explained Joseph Perry, a professor in the Georgia Tech School of Chemistry and Biochemistry and the Center for Organic Photonics and Electronics. “The multiphoton technique is a “one-cycle” method for fabricating 3D structures so there is also a huge savings in terms of volumes of materials and solvents used,” he told SST, adding that another benefit is reduced environmental impact.

In their technique, a laser beam is written across a substrate coated with a film of polymer resin (a blend of acrylic monomers) containing a special dye, “DAPB” (4,4′-bis[di-n-butylamino]biphenyl) as a two-photon photoactivator, with molecules capable of two-photon absorption at 520nm light wavelength that are about 10x more efficient than commercial ultraviolet photoactive materials, which are engineered for single photon absorption. “Essentially it is a negative resist that can be activated by multiphoton laser excitation,” Perry told SST. This particular research involved a glass substrate, though Perry says they’ve also fabricated with silicon, aluminum, and plastic.

The film of DAPB polymer resin is excited when exposed to a Ti: Sapphire pulsed laser, triggering crosslinking that occurs only where molecules have absorbed two photons of light — and only molecules at the laser’s focal point receive enough light to absorb two photons. The remaining insoluble scanned structures are left behind on the surface of the substrate when placed in a developer solution. Controlling the pulsed laser scans can direct the crosslinking reaction in any pattern, including 3D stacks of straight lines; simply turning the laser on and off allows researchers to expose lines of polymer and avoid areas where no lines should be drawn.

Perry, who with fellow GA Tech researcher Seth Marder founded startup Focal Point Microsystems to commercialize the technology, claims they can create a 20×20µm structure with 30 layers in about 10 minutes. “We used laser scan speeds of 60µm/sec,” which is close to what would be needed commercially for benchtop, research lab, or prototyping (perhaps within two years), he suggested. “We are now fabricating at speeds of 20 cm/s and higher speeds are possible,” he added, with production scale systems likely three or four years out depending upon demand and better instruments to increase the throughput.

Others have applied two-photon absorption to fabrication as early as the 1990s (e.g. Cornell U. of California/Irvine applied it to 3D memory and fluorescence, and 3D microfabrication was explored by Satoshi Kawata et. al. at Osaka U.), and work in related area continues today in the US, Europe, Japan, and Australia, Perry noted. Potential applications include simplifying the double-patterning lithography methods contemplated for 32nm. (Pixelligent Technologies LLC, for example, has been developing a nanocrystal-based reversible contrast-enhancement layer material, a coating spun directly onto the photoresist that enhances resolution by absorbing low-intensity light in image regions intended to be dark.)

GA Tech’s two-photon absorption technique enables fabrication of true 3D structures of almost any shape, including those with interlocking or moveable parts, Perry explained — e.g., functional photonic micro-devices with tailored transmission capabilities such as photonic crystals (stacked-up grids of lines). Possible future applications include building compact microspectrometers-on-a-chip for telecommunications and sensors, or for separating multiple wavelengths in fiber-optic cables. Ultimately he sees this technique with application in moderate-scale production of special-purpose ICs, “especially where there is a high diversity of patterns to be fabricated batch to batch.” — J.M.

IMAGE CAPTION: Scanning electron microscope images of woodpile-type photonic crystal structures fabricated with 520nm excitation at (a) higher power and at (b) lower power using DABP. Magnified images of the structures are shown below their respective overview images. (Source: Georgia Tech)

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