Because diffraction limits the resolution of conventional photolithography to roughly one-quarter of the wavelength of light used, alternate techniques such as nanoimprint lithography and dip-pen lithography are being explored. Besides the need for smaller integrated-circuit features, lithography also plays a role in the fabrication of two- and three-dimensional nanostructures using such processes as two-photon polymerization, enabling feature sizes on the order of 100 nm and a higher resolution of approximately λ/10. Now, researchers at the University of Maryland (College Park, MD) have developed a new process called resolution augmentation through photo-induced deactivation (RAPID) lithography that can create nanostructures with feature sizes as small as 40 nm, or approximately λ/20 resolution, using a typical 800 nm ultrafast-source wavelength.
Inspired by STED
The researchers say the RAPID technique was inspired by the technology behind stimulated-emission-depletion (STED) microscopy, in which an initial laser pulse excites fluorescent molecules and a second laser pulse, tuned to a much longer wavelength, de-excites the molecules through stimulated emission everywhere except in a region at the center of the focal volume of the pulses. Spatial phase shaping of the depletion beam therefore localizes fluorescence in a zone much smaller than the excitation wavelength.
In like manner, RAPID lithography uses one laser beam to initiate polymerization in a photoresist and then uses a second beam to deactivate the photoinitiator and halt the polymerization process. Spatial shaping of the phase of this deactivation beam can create structures much smaller than the excitation wavelength.
However, unlike STED microscopy, which requires lasers with two significantly different wavelengths, the research team experimented with various dye molecules or photoinitiators and found that malachite green carbinol base could be both polymerized with 800 nm, 200 fs fast pulses and deactivated with longer 800 nm, 50 ps pulses. This means that the RAPID lithography process could be implemented using just one ultrafast laser source if desired.
RAPID fabrication
To demonstrate the RAPID lithography process, a series of polymer lines were fabricated using excitation and deactivation beams that were either offset or coincident in space, and with different timing and pulse lengths. The efficiency of deactivation did not change noticeably between excitation/deactivation delays between 0 and 13 ns; however, polymerization was complete after 13 ns and could not be deactivated. It was also observed that deactivation could be accomplished using a continuous-wave laser.
Next, the contribution of spatial phase shaping of the deactivation beam was explored. A phase mask was introduced in the lithography setup and a 10 mW excitation power was used along with different deactivation-beam powers to fabricate an array of voxels. As deactivation power increased, the axial voxel dimension decreased and voxels as small as 40 nm could be created using a 93 mW, 800 nm deactivation beam.
With optimization of the phase mask, laser wavelength, and photoinitiator materials, the researchers think they could develop nanostructures with dimensions as small as 10 nm. “The enhanced resolution available with RAPID significantly expands the capabilities and potential applications of two-photon polymerization,” says John Fourkas, professor of chemistry at the University of Maryland, College Park. “We are pursuing the use of RAPID in the nanopatterning of surfaces for applications in nanophotonics and nanobiotechnology, and as we continue to increase the resolution and improve the properties of the materials, we expect RAPID to be an enabling technology for a broad range of new nanoscale applications.”
REFERENCE
- Linjie Li et al., Science online (www.sciencemag.org/cgi/content/abstract/1168996), DOI: 10.1126/science.1168996 (April 4, 2009).
