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IR Two-Photon Process Creates Acids

Photonics Spectra
Jul 2002
Gary Boas

Two-photon absorption techniques show promise for such applications as microfabrication. But although researchers have examined ultraviolet-sensitive photoacid generators for use in two-photon microfabrication, these agents typically exhibit low two-photon sensitivity, so manufacturing approaches that employ them require long exposure times and high excitation intensities that can damage the structures. To overcome these problems, a team at the University of Arizona in Tucson and Cornell University in Ithaca, N.Y., has developed a two-photon process in which the acid is generated by infrared radiation.

Specially engineered molecules absorb two near-infrared photons simultaneously and activate chemical reactions that lead to acid production at any point in three dimensions. To employ the molecules in microfabrication, the photoactive compounds are added to polymer materials. Once excited by near-infrared pulses from a Ti:sapphire laser, the compounds chemically transform the materials; for example, making them transparent or increasing their mechanical strength.

Perhaps the most significant transformation is making the materials soluble in water. Using the new photoacid generators, it is possible either to write structures in the resin and then dissolve it, leaving only the structures, or to create microchannels in the resin that dissolve in a basic water solution. Previous work at the University of Arizona resulted in two-photon processes that could do the former, but the latter is the more practical solution to some problems.

As a demonstration of the technique, the researchers fabricated a microchannel resist that featured 100-µm-long, 20-µm-deep cavities connected by 12 channels that were 50 µm long and 4 µm deep. Because the photoacid generators display a two-photon-absorption cross section 80 times greater than typical, UV-sensitive ones, an average power of only 40 µW was required to initiate the reaction.

The development marks the first time that two-photon absorption is demonstrably useful outside the laboratory. "There has been some nice work using two-photon processes of other types of polymers, but they have terrible shrinkage problems," explained Christopher K. Ober, director of materials science and engineering at Cornell and one of the researchers on the project. "It's one thing to write the pattern in three dimensions. It's another to maintain the same dimensions in the polymer. When you take a plastic precursor to polymerize, there's about a 10 to 20 percent shrinkage."

Biotech applications

There are a number of potential applications of the technique beyond microfabrication; perhaps the most exciting are in medicine. For photodynamic therapy, for example, the absorption of two infrared photons could trigger the release of a drug capsule. Moreover, because the method uses infrared instead of ultraviolet wavelengths, it could be used to shape polymers while they are in living cells. "You can even imagine building a functioning kidney," Ober said.

The researchers are working to commercialize some of the processes that they have developed, but still must achieve better control of the associated imaging processes.

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