Originally Published MPMN July 2009
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Lithography Technique 'Waves' Goodbye to UV Light
The image on the left shows features created with conventional MAP techniques, while the picture on the right illustrates the smaller features made possible by RAPID technology.
Photolithography has enabled manufacturers in recent years to achieve the diminishing features required for chip fabrication, shrinking implants, and microelectromechanical systems (MEMS)–based applications. But as features decrease, photolithography becomes increasingly difficult to work with—not to mention much more expensive—owing to limitations of feature sizes to about a ¼ of a wavelength of the ultraviolet (UV) light employed to expose the photoresist. Aiming to pattern smaller features and overcome these limitations, a research team from the University of Maryland (UMD; College Park, MD; www.umd.edu) has developed an alternative lithography technique that could help cut costs and facilitate fabrication of minuscule parts without the use of UV light.
Dubbed resolution augmentation through photoinduced deactivation (RAPID) by the researchers, the tabletop technique allows for the patterning of features that are 2500 times smaller than a human hair. Enabling such a feat is a new approach to multiphoton absorption polymerization (MAP), according to John Fourkas, a professor of chemistry at UMD’s department of chemistry and biochemistry and lead author of a resulting paper on the research. Capable of yielding features down to roughly 100 nm in two directions, MAP entails focusing short laser pulses on a liquid to harden it into a polymer, a process that Fourkas likens to getting a composite filling at the dentist’s office.
Not satisfied with the minute feature sizes attainable through MAP, however, the researchers sought to generate even smaller patterns. But rather than turning to UV light, they found inspiration in stimulated emission depletion (STED) fluorescence microscopy, in which molecules are excited with one beam of light but are turned off by another beam of light prior to fluorescing. Spatial shaping of the second beam is crucial, Fourkas notes, so that the center of the beam is dark and the molecules in that region only are not turned off. By doing so, much higher resolution can be achieved, he adds.
“We thought that we ought to be able to use something similar in MAP. That is, start this stuff polymerizing but then add the second beam of light that inhibits the polymerizaton before it really gets going,” Fourkas recalls. “If you do that properly, you can make much smaller features.”
Although a number of researchers have been exploring this avenue, the UMD group was the first team that found the chemistry to enable this process. This, Fourkas says, was the true breakthrough. The team was able to identify a molecule that could initiate polymerization, but was slow to do so after being exposed to the light. This substantial window of time provided the scientists with ample time to turn the molecule off with the second, same-color beam of light.
“This ended up better than we had ever hoped,” Fourkas says. “In STED and what we tried first, you would have to use pulses of light to do this. It ended up that with the molecules we found, you initiate polymerization with the pulse of light, but you can actually turn it off with the [second] laser that’s on all the time.”
Applicable for use with polymers and possibly biomolecules, RAPID could have potential application for medical devices. Although the researchers don’t foresee commercialization until five to 10 years from now, the technique could be used for nanopatterning of implant surfaces to promote integration and acceptance of the device by the body, according to Fourkas. He also hypothesizes that the lithography method could come in handy for exploring cell behavior on the nanoscale.
“We’re really interested in exploring those types of phenomena,” he says. “The advantage we have is that we can fabricate arbitrary structures with this really high resolution. So, we have the ability to look at 3-D structures, or things that are essentially 2-D but have unique geometries, and see how cells behave on them, and really try to start to control that.”
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