Metal metasurfaces, as artificially engineered two-dimensional structures with subwavelength features, enable precise electromagnetic wave manipulation and demonstrate unique application potential across cutting-edge fields. Conventional fabrication approaches for metallic nanostructures, including electron beam lithography, UV lithography, interference lithography, and nanoimprint lithography combined with lift-off processes, present inherent limitations. While UV lithography offers high-throughput production, it requires multiple patterning steps for high-precision features and demands sophisticated equipment. Interference lithography generates periodic patterns through coherent light interference but lacks design flexibility for complex structures. Nanoimprint lithography, despite its high-resolution replication capability and mass-production advantages, faces technical challenges in large-area applications. Traditional large-area nanoimprint techniques include full-wafer nanoimprint (limited by high mold costs), step-and-repeat nanoimprint (with alignment accuracy and efficiency constraints), and roll-to-roll nanoimprint (challenged by pattern fidelity and mold durability during high-speed operation). Furthermore, conventional nanoimprint processes typically employ negative photoresists incompatible with lift-off, necessitating additional metal deposition, residual layer removal, and wet etching steps, a process prone to anisotropic etching effects and pattern distortion risks.
The work aims to develop a high-throughput fabrication method for metal metasurfaces. Firstly, a silicon-based mold was fabricated through electron beam lithography, and multiple IPS sub-molds were replicated via hot nanoimprint. Then, a PET-IPS composite working mold was prepared, and a wafer-scale silicon-based metasurface mold was efficiently produced through multiple cross-patterned nanoimprint lithography. Based on relative adhesion control, high-precision metal metasurface structures were transferred to the target substrate through steps including mold replication, thermal evaporation, and imprinting. Finally, the structural depth uniformity of the mold and the transfer integrity of the metal nanostructures were analyzed through experiments such as mold morphology characterization and metal metasurface transfer rate testing.
A 3 mm×30 mm silicon master mold was exposed at a dose of 0.7 μC/cm2, achieving a minimum nanostructure size of 100 nm with a dimensional error below 4%, demonstrating excellent consistency. Cross-patterned nanoimprint lithography enabled the fabrication of 4-inch wafer-scale metal metasurfaces. Compared to traditional step-and-repeat nanoimprint, this technique improved the metasurface nanopattern fabrication efficiency by 7.6 times. The depth variation of the 4-inch silicon-based metasurface mold was below 7%. Systematic investigation of process parameters revealed that transfer temperature and duration significantly affected the transfer efficiency. At temperatures below 90 ℃, the transfer rate remained consistently low (below 10%), indicating inadequate conditions for effective transfer. In contrast, temperatures of 90 ℃, 110 ℃, and 130 ℃ demonstrated dramatically improved performance, achieving stable transfer rates exceeding 98% within 5-10 minutes, suggesting thermally activated enhancement of the transfer process. Notably, the 130 ℃ condition exhibited the fastest kinetics and highest ultimate transfer efficiency, attributable to accelerated methoxy group alcoholysis and hydroxyl dehydration-condensation reactions at elevated temperatures. However, when the imprint temperature reached 150 ℃ (approaching the glass transition temperature, Tg, of the IPS material), the demolding process became severely compromised. Initial transfer rates of 20% at 2 minutes progressively decreased to complete transfer failure (0%) after 5-25 minutes, resulting from dramatic viscosity increase in the IPS working mold that caused: (1) significant deformation under pressure, and (2) uncontrolled adhesion enhancement at both mold-target and mold-substrate interfaces, ultimately preventing successful demolding.
The cross-patterned nanoimprint lithography technique effectively reduces the fabrication cost of wafer-scale metal metasurfaces, demonstrating broad application potential in optical imaging, optical communications, sensing, and other fields.
Key words
nanoimprint /
metasurfaces /
pattern transfer /
wafer-scale /
adhesion
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Funding
The National Key Research and Development Program of China (2020YFA0710100); The Guangdong Basic and Applied Basic Research Foundation (2021A1515110590); The Natural Science Foundation of Hubei Province (2024AFB020); The National Natural Science Foundation of China (62405225); The Fundamental Research Funds for the Central Universities (104972025KFYjc0038)
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