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Novel Nanolithography Approach Combines Interference Lithography and Grayscale-Patterned Secondary Exposure

The precise control of feature sizes exhibits great importance in fabricating nanodevices for optoelectronics, plasmonics, meta-optics, and biosciences, just to name a few. Some applications require nanostructures with uniform feature sizes while others rely on spatially varying morphologies.

However, fine manipulation of the feature size over a large area remains a substantial challenge because mainstream approaches of precise nanopatterning are based on low-throughput pixel-by-pixel processing, such as those utilizing focused beams of photons, electrons, or ions. Other high-throughput approaches either heavily rely on molds or can only fabricate specific patterns.

No existing methods can simultaneously satisfy the requirements of high throughput, large area, and precise feature size control in nanopatterning.

In a new paper published in Light Science & Application, a research team led by Professor Wen-Di Li from Mechanical Engineering, the University of Hong Kong (HKU), China, developed a novel nanolithography approach combing interference lithography and grayscale-patterned secondary exposure (IL-GPSE).

This new approach enables high-throughput nanopatterning on a wafer-scale area with the feature sizes spatially modulated on demand. Different from the mainstream pixel-by-pixel serial writing process, the research team first uses interference lithography (IL) to efficiently exposure a large-area periodic nanoscale pattern on the photoresist.

Then they employ a secondary exposure of ultraviolet (UV) light, carrying a designed intensity distribution of a grayscale pattern, projected on the IL-exposed photoresist to spatially modulate the feature sizes of individual nanostructures. This new process portfolio greatly enhances the efficiency of fabricating nanostructures that require spatially varying dimensions.

Typical interference lithography uses the sinusoidal intensity distribution generated by two coherent laser beams to expose the photoresist, thereby creating periodic nanogratings after development. Using positive photoresist as the example, the region that receives an exposure dose above the clearing threshold would be "washed out" during the development while that below the threshold remains.

The linewidth of the developed gratings mainly correlates with the portion of the photoresist that receives a dose below the threshold. Employing the non-linear characteristic of the photoresist, the researchers apply a secondary exposure to provide an additional dose that superimposes on the sinusoidal IL dose distribution to modulate the effective dose applied on the photoresist, therefore increasing the portion of the photoresist that receives above-threshold dose and leaving less photoresist after development.

Thus, a linewidth modulation can be achieved. Notably, this method strongly relies on the high contrast and high stability in the initial IL patterning. The HKU team has long and profound experience in developing high-performance IL systems and innovative nanofabrication processes, which lays the cornerstone of the proposed technique.

"Our technique is reliably verified by experimental results and numerical simulation. We implement UV contact photolithography, maskless projection photolithography, and direct laser writing as the secondary exposure approaches, respectively, to demonstrate feature size modulation with sub-micrometer resolution and at a wafer-scale area."

"As demonstrations, we successfully fabricated highly uniform 125-nm linewidth nanogratings with only 5% variation across a 4-inch wafer. We also fabricated structural-color painting of Along the River During the Qingming Festival on a whole 3-inch wafer by modulating the filing ratios of nanostructures," they added.

"Wafer-scale nanodevices fabricated by the presented technique can benefit a wide variety of applications. For example, large-area uniform nanogratings can be used in spectroscopy, astronomy, lasers, etc. Large-area structural color attracts broader applications in high-definition displays, anti-counterfeiting, sensing, etc. In addition, this breakthrough could open a new venue for fabricating emerging nanophotonic devices, such as metalenses and augmented reality glasses. With the reduction in fabrication difficulties and R&D cost, our invention will greatly benefit the realization of emerging nanodevices." the researchers forecast.

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