The miniaturization of semiconductor chips has led to a steady decrease in the size and cost of electronic devices and improvements in their design. Similar progress has been more difficult to achieve in optical devices like lenses. In theory, metasurface structures for making nanometer-scale optics can be produced in the same semiconductor fabs that produce silicon chips. In practice, however, it is expensive and time-consuming to produce these metasurfaces, because of their structural complexity. An Arizona State University (ASU) team led by professor Chao Wang developed a scalable, multilayered approach to manufacturing metasurfaces that can be used to produce large-area functional structures for ultracompact optical, electronic, and quantum devices. Professor Chao Wang of Arizona State University (ASU) is developing an accessible manufacturing method for researchers to prototype and fabricate their designs. Courtesy of Marco Alexis-Chaira/ASU. The team used nanoimprint lithography (NIL), a nanofabrication technique that can produce results quickly over a large area, to enable functionality. To seamlessly align multilayered structures, the team used Moiré patterns. Using Moiré alignment markers and electron-beam writing, the researchers created two separate NIL molds over a patterning area greater than 20 sq mm. Both metasurface layers were engraved with the Moiré markers, and the second NIL mold was made optically transparent to allow the researchers to adjust the alignment during NIL processes. The interference patterns of the intentionally designed Moiré markers enabled the researchers to detect nanometer-scale alignment errors without needing visual aids. To simplify the multiple steps involved in nanofabrication and thus decrease the risk of structural damage to existing, bottom-layer metasurface structures, the researchers conceptualized a 3D scaffold. The 3D scaffold allows new metasurfaces to be built vertically, drastically reducing the time and cost of prototyping sophisticated devices. According to Wang, the 3D scaffold could enable researchers to complete a process that typically takes 24 hours in just minutes. The team tested the metasurface manufacturing method on a microscope. The new method helped condense the microscope’s analyzer from the size of a microwave to a microscopic chip. The researchers fabricated silicon and aluminum metasurfaces using nanolithography and the 3D pattern-transfer capabilities of NIL, respectively. The metasurfaces demonstrated nanometer-scale linewidth uniformity, sub-200 nm translational overlay accuracy, and a less than 0.017 rotational alignment error. Fabrication complexity and surface roughness were significantly reduced. The multilayer metasurfaces demonstrated circular polarization extinction ratios as large as approximately 20 and 80 in the blue and red wavelengths. The metasurface, chip-integrated CMOS imager was highly accurate in broad-band, full Stokes parameter analysis in the visible wavelength ranges and in single-shot polarimetric imaging. Wang was motivated to create the accessible manufacturing method by the dearth of resources available for researchers to test their theories and develop prototypes. “Researchers at universities need an established and accessible method for manufacturing metasurface products precisely over nanometer scale and, at the same time, produce them over millimeter scale or larger,” he said. “For many electronic or photonic devices, they require multiple layers of materials to perform their function. Only a few foundries in the world have access to the tools to make this; most university researchers don’t have access.” Wang’s collaborator, professor Yu Yao, believes that scalable nanomanufacturing of nanophotonic structures and metasurfaces is essential for technology transfers from lab to commercial applications. “So far, most researchers in the field have been using fabrication methods with costs that outweigh applications,” Yao said. “The NIL manufacturing method provides a fast and economical solution for fabrication and can be readily extended to large-scale production of various devices and systems, greatly shortening the time from lab demonstration to commercial product.” In addition to prototyping, production, and the development of new optical applications, the accessible manufacturing method from the ASU team could be used for printing, imaging, and information processing. Wang hopes that the method will be used to help sustain the demand for microelectronics from the energy, defense, and medical device industries. “We plan to explore how these processes can be used for advancing semiconductor electronic devices,” Wang said. “This research provided a preliminary demonstration of what is feasible, and we anticipate more interesting things to come. The research was published in Advanced Functional Materials (www.doi.org/10.1002/adfm.202404852).
