Breakthrough in scalable metasurface manufacturing: POSTECH team proposes nanoimprint lithography solutions with efficiency near electron beam lithography

EurekAlert! - Metasurfaces, ultra-compact optical devices capable of "precisely manipulating light," have shown great potential in augmented reality (AR) glasses, holographic projection, biosensing, and other fields. However, traditional manufacturing technologies face a dilemma: either they are costly and inefficient, or they lack sufficient performance, making large-scale mass production difficult. Recently, a team led by Yujin Park, Donghoe Kim, and Junsuk Rho from Pohang University of Science and Technology (POSTECH), South Korea, published a review paper in Optics and Photonics Research (Opt. Photonics Res.), systematically proposing two innovative strategies based on nanoimprint lithography (NIL). These strategies successfully address the "low refractive index" limitation of traditional processes, bringing the transition of metasurfaces from laboratory research to industrial production within reach.

Metasurfaces are composed of nanoscale "micro-optical units" (meta-atoms). Similar to traditional lenses and filters, they can control the direction, color, and phase of light, but they are hundreds of times thinner and lighter than conventional optical devices. To achieve such performance, two key conditions must be met during manufacturing: first, high-precision nanoscale patterning, and second, the use of high-refractive-index materials (which can "bend" light more strongly to enhance manipulation efficiency).

For a long time, electron beam lithography (EBL) has been the "gold standard" for metasurface manufacturing. It can create patterns with a precision of up to 80 nm, and when combined with deposition technologies like plasma-enhanced chemical vapor deposition (PECVD), it can produce metasurfaces using high-refractive-index materials, achieving an optical efficiency of up to 89%. However, EBL has obvious shortcomings: it draws patterns point by point like "writing by hand with a pen," and can only process small areas at a time. Not only is it extremely costly (the cost of a single batch production is 5-10 times that of NIL), but its throughput is also surprisingly low—it takes several days to process a single 12-inch wafer, making it completely unable to meet the needs of industrial mass production.

To solve the scalability issue, researchers once tried nanoimprint lithography (NIL). This technology replicates nanoscale patterns in batches using a prefabricated mold, similar to "stamping." Its throughput is more than 100 times that of EBL, while the cost is only 1/20 of EBL. However, a new problem emerged: the refractive index of the resin used in traditional NIL is only about 1.5 (close to ordinary glass). Metasurfaces made directly from this resin have an optical efficiency of less than 10%, so they can only be used as "temporary templates" and require additional processing to improve performance, which instead increases manufacturing complexity.

To address the refractive index problem of NIL, the team focused on proposing two solutions in the review. These solutions not only retain the advantages of NIL—high speed and low cost—but also enable metasurfaces to achieve performance comparable to that of EBL processes.

The first strategy is the "hybrid material method": first, nanoscale patterns are imprinted on low-refractive-index resin using NIL, and then a high-refractive-index thin film is coated on the surface of the patterns using "atomic layer deposition (ALD)" technology. For example, titanium dioxide (with a refractive index of 2.3-2.5) is used for visible light scenarios, and zirconium oxide (with a refractive index of over 2.2) is used for ultraviolet (UV) light scenarios. This thin film acts like a "high-refractive-index coat," significantly improving the optical efficiency of the resin patterns. The team's experimental data shows that visible-light metalenses manufactured using this method achieve a maximum focusing efficiency of 89.6% at wavelengths of 450-635 nm, which is almost the same as that of the EBL process. More importantly, this method can already be used for batch production on 12-inch wafers, reducing the manufacturing time per wafer to less than 2 hours.

The second strategy is the "particle-embedded resin (PER) method": high-refractive-index nanoparticles (such as titanium dioxide and silicon particles) are directly mixed into NIL resin, similar to "adding crystal powder to glue," turning the resin itself into a high-refractive-index material. This method does not require subsequent coating, allowing metasurfaces to be manufactured in one step, further simplifying the process. The team also optimized the PER method to address its shortcomings: to solve the problem of "structural damage during demolding," they developed a water-soluble polyvinyl alcohol (PVA) mold—during demolding, the mold only needs to be dissolved in water to obtain undamaged nanostructures; to solve the problem of "residual layers affecting light transmittance," they used a specially designed tape to precisely peel off the residual layers, reducing light scattering of the metasurfaces by more than 30%. Currently, the PER method can manufacture high-precision structures with an aspect ratio of 6, and infrared metalenses manufactured using this method achieve a focusing efficiency of 47% at a wavelength of 940 nm, which can be used for human blood vessel imaging.

The breakthroughs of the two strategies have also enabled metasurface applications to break free from the limitations of "flat surfaces and single wavelengths."

In terms of wavelength coverage, the zirconium oxide coating used in the hybrid material method is suitable for UV scenarios (such as optical components for deep UV lithography), while the silicon particles in the PER method are suitable for infrared scenarios (such as thermal imaging and LiDAR sensors). Titanium dioxide, whether used as a coating or particles, can work efficiently in the visible light range and can be applied to optical lenses for AR glasses and high-definition holographic displays.

In terms of substrate adaptability, the team also extended the PER method to biodegradable materials and curved surfaces: they mixed high-refractive-index particles into hydroxypropyl cellulose (HPC) resin to create metasurface labels that can be directly attached to the surface of fruits like apples. These labels not only have anti-counterfeiting functions but also can be dissolved in water, avoiding packaging pollution. At the same time, the PER method can also imprint patterns on curved glass surfaces, providing a low-cost manufacturing solution for the "curved optical windows" of automotive LiDAR.

Despite achieving significant progress, the team also objectively pointed out in the paper that there are still two major challenges to be solved in the large-scale manufacturing of metasurfaces: first, the peeling of residual layers in the PER method currently relies on manual adjustment of tape adhesion, and automated equipment needs to be developed in the future to ensure consistency in mass production; second, the ALD coating technology used in the hybrid material method currently requires multiple "precursor injection-cleaning" cycles to coat each thin film, resulting in slow speed, and the process needs to be optimized to improve throughput.

"Metasurfaces are expected to transform optical devices from being 'bulky' to 'thin and lightweight'—for example, reducing the thickness of camera lenses from several centimeters to the micrometer level," said Professor Junsuk Rho, the leader of the team. "Next, we will focus on promoting the combination of these two strategies with roll-to-roll (R2R) manufacturing technology. Our goal is to achieve 'hundred-meter-level' continuous production of metasurfaces, further reducing costs and enabling more consumer electronics and medical devices to use this new type of optical device."

Park Y, Kim D, Rho J. Nanoimprint lithography for scalable manufacturing of optical metasurfaces. Opt. Photonics Res. 2025(1):0001, https://doi.org/10.55092/opr20250001