Photolithography Improved Without UV Light

by Rebecca C. Jernigan

COLLEGE PARK, Md. – As computer chips become smaller and more powerful, it is important that their features remain precise. The fact that smaller light wavelengths create smaller features has demanded that developers and manufacturers use expensive and difficult to work with ultraviolet light as the basis for such photolithographic nanofabrication – until now.

All images are schematic depictions of RAPID lithography, the technique developed by University of Maryland professor John Fourkas and colleagues which enables the creation of features 2500 times smaller than the width of a human hair. (Images courtesy John T. Fourkas, professor of chemistry and biochemistry, University of Maryland)

Researchers at the University of Maryland College of Chemical and Life Sciences, in the pursuit of improving the resolution of multiphoton absorption polymerization, have developed a tabletop technique called RAPID (Resolution Augmentation through Photo-Induced Deactivation) photolithography that removes ultraviolet light from the equation. Though inadvertent, the discovery could have applications in many areas, including electronics, optics and biomedical devices.

The investigators used 800-nm light from two tunable titanium:sapphire lasers to both harden a substance and to prevent it from hardening, which enabled them to sculpt features 2500 times smaller than the width of a human hair.

Multiphoton absorption of a pulsed 800-nm light is used to initiate crosslinking in a polymer photoresist, hardening it in place in a way that is similar to that used by dentists when filling a cavity, according to John T. Fourkas, one of the researchers. At the same time, one-photon absorption of continuous-wave 800-nm light prevents the photoresist from hardening.

The researchers used spatial phase shaping of the latter beam to control the area that was affected, in a manner similar to that used in stimulated emission depletion (STED) fluorescence microscopy. The use of continuous-wave light for shaping simplified the process, as users would not need to develop precise timing between the exitation and deactivation beams like they would if both beams were pulsed.

In traditional photolithography, the resolution is limited to around one-fourth of the wavelength, due to diffraction. In the university team's approach, it is possible to make patterns that are one-twentieth the size (along the optical axis) of the wavelength used, enabling the creation of much smaller features with larger light wavelengths. This was achieved using a specific phase mask to manipulate the deactivation beam. The scientists believe that by choosing a different phase mask, the same size feature could be achieved along the transverse axis, and by using two phase-masked beams the measurement could be achieved in all dimensions.

In principle, the technique could enable users to create 3-D nanostructures with features ranging from tens of microns down to 40 nm. Fourkas, who led the research, believes that with further developments and refinements RAPID could shrink the feature resolution to as low as 20 nm, which could make nanopatterning useful in a variety of new applications.

Though there is still room for improved materials and better two-dimensional spatial light modulators could make the technique easier to implement, Fourkas is pleased with the team's discovery. "In the way that we have implemented it, RAPID is far easier than we had ever imagined it could be."

Rebecca C. Jernigan
rebecca.jernigan@laurin.com