University of Maryland chemistry Professor John Fourkas and his research group recently introduced a technique called RAPID lithography that makes it possible to use visible light to attain lithographic resolution comparable to (and potentially even better than) that obtained with shorter wave length radiation.
"Our RAPID technique could offer substantial savings in cost and ease of production," Fourkas said. "Visible light is far less expensive to generate, propagate and manipulate than shorter wavelength forms of electromagnetic radiation, such as vacuum ultraviolet or X-rays. And using visible light would not require the use of the high vacuum conditions needed for current short wavelength technologies."
The key to RAPID is the use of a special "photoinitiator" that can be excited, or turned on, by one laser beam and deactivated by another. In new work just published online by Nature Chemistry, Fourkas and his group report three broad classes of common dye molecules that can be used for RAPID lithography.
In earlier work, Fourkas and his team used a beam of ultrafast pulses for the excitation step and a continuous laser for deactivation. However, they say that in some of their newly reported materials deactivation is so efficient that the ultrafast pulses of the excitation beam also deactivate molecules. This phenomenon leads to the surprising result that higher exposures can lead to smaller features, leading to what the researchers call a proportional velocity (PROVE) dependence.
"PROVE behavior is a simple way to identify photoinitiators that can be deactivated efficiently," says Fourkas, "which is an important step towards being able to use RAPID in an industrial setting."
By combining a PROVE photoinitiator with a photoinitiator that has a conventional exposure dependence, Fourkas and co-workers were also able to demonstrate a photoresist for which the resolution was independent of the exposure over a broad range of exposure times.
"Imagine a photographic film that always gives the right exposure no matter what shutter speed is used," says Fourkas. "You could take perfect pictures every time. By the same token, these new photoresists are extremely fault-tolerant, allowing us to create the exact lithographic pattern we want time after time."
According to Fourkas, he and his team have more research to do before thinking about trying to commercialize their new RAPID technology. "Right now we're using the technique for point-by-point lithography. We need to get it to the stage where we can operate on an entire silicon wafer, which will require more advances in chemistry, materials and optics. If we can make these advances -- and we're working hard on it -- then we will think about commercialization."
Another factor in time to application, he explained, is that his team's approach is not a R&D direction that chip manufacturers had been looking at before now. As a result, commercial use of the RAPID approach is probably at least ten years down the road, he said.
Nature Chemistry - Multiphoton photoresists giving nanoscale resolution that is inversely dependent on exposure time
Recent advances in materials science have made it possible to perform photolithography at the nanoscale using visible light. One approach to visible-light nanolithography (resolution augmentation through photo-induced deactivation) uses a negative-tone photoresist incorporating a radical photoinitiator that can be excited by two-photon absorption. With subsequent absorption of light, the photoinitiator can also be deactivated before polymerization occurs. This deactivation step can therefore be used for spatial limitation of photopatterning. In previous work, continuous-wave light was used for the deactivation step in such photoresists. Here we identify three broad classes of photoinitiators for which deactivation is efficient enough to be accomplished by the ultrafast excitation pulses themselves. The remarkable properties of these initiators result in the inverse scaling of lithographic feature size with exposure time. By combining different photoinitiators it is further possible to create a photoresist for which the resolution is independent of exposure over a broad range of fabrication speeds.
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