Metalens Cuts Complexities for Semiconductor Manufacturing

Rice University researchers have developed a metalens that transforms incoming longwave UV light (UVA) into a focused output of vacuum UV (VUV) radiation. VUV is used in semiconductor manufacturing, photochemistry, and materials science. Historically it has been costly to work with, in part because it is absorbed by almost all types of glass used to make conventional lenses.

The team led by Naomi Halas, the Stanley C. Moore Professor in Electrical and Computer Engineering and a professor of biomedical engineering, chemistry, and physics at Rice University, developed a metalens that converts 394-nm UV into a focused output of 197-nm VUV. The disc-shaped metalens is composed of a transparent sheet of zinc oxide that is thinner than a sheet of paper and just 45 millionths of a meter in diameter.

To demonstrate this conversion, the researchers shined a 394-nm UVA laser at the back of the disc and measured the light that emerged from the other side.

"This work is particularly promising in light of recent demonstrations that chip manufacturers can scale up the production of metasurfaces with CMOS-compatible processes," Halas said. "This is a fundamental study, but it clearly points to a new strategy for high-throughput manufacturing of compact VUV optical components and devices."

The scientists built the metalens by precisely etching hundreds of tiny triangles on the surface of the microscopic film of zinc oxide. According to study co-first author Catherine Arndt, an applied physics graduate student in Halas' research group, the key feature of the metalens is its interface, a front surface that is studded with concentric circles of the tiny triangles.

"The interface is where all of the physics is happening," Arndt said. "We're actually imparting a phase shift, changing both how quickly the light is moving and the direction it's traveling. We don't have to collect the light output because we use electrodynamics to redirect it at the interface where we generate it."

Violet light has the lowest wavelength visible to humans. Ultraviolet has even lower wavelengths, which range from 400 to 10 nm. VUV, with wavelengths between 100 and 200 nm, is so named because it is strongly absorbed by oxygen.

By precisely etching hundreds of tiny triangular nanoresonators in precisely configured concentric circles on a microscopic film of zinc oxide, photonics researchers at Rice University created a metalens that converted 394-nm ultraviolet light (blue) into 197-nm vacuum UV (pink) and simultaneously focuses the VUV output on a small spot less than 2 millionths of a meter in diameter. Courtesy of M. Semmlinger/Rice University.

Using VUV light today typically requires a vacuum chamber or other specialized environment, as well as machinery to generate and focus VUV.

"Conventional materials usually don't generate VUV," Arndt said. "It's made today with nonlinear crystals, which are bulky, expensive, and often export-controlled. The upshot is that VUV is quite expensive."

In previous work, Halas and a team demonstrated that they could transform 394-nm UV into 197-nm VUV with a zinc oxide metasurface. Like the metalens, the metasurface was a transparent film of zinc oxide with a patterned surface. In this instance the required pattern wasn't as complex since it didn't need to focus the light output, Arndt said.

"Metalenses take advantage of the fact that the properties of light change when it hits a surface," she said. "For example, light travels faster through air than it does through water. That's why you get reflections on the surface of a pond. The surface of the water is the interface, and when sunlight hits the interface, a little of it reflects off."

The prior work showed that a metasurface could produce VUV by upconverting longwave UV via second-harmonic generation. But VUV is costly, in part because it is expensive to manipulate after it's produced. Commercially available systems can fill cabinets as large as refrigerators or compact cars and cost tens of thousands of dollars, she said.

"For a metalens, you're trying to both generate the light and manipulate it," Arndt said. "In the visible wavelength regime, metalens technology has become very efficient."

Arndt said that virtual reality headsets use this effect and that metalenses have recently been demonstrated for visible and infrared wavelengths, though not yet at shorter wavelengths.

To make the metalens, Arndt worked with co-corresponding author Din Ping Tsai of City University of Hong Kong, who helped produce the intricate metalens surface, and with three co-first authors.

Tests ultimately showed that the metalens could focus its 197-nm output onto a spot measuring 1.7 μm in diameter, increasing the power density of the light output by 21 times.

"It's really fundamental at this stage," Arndt said. She added that the metalens could be made to be more efficient.

The research was published in Science Advances (www.science.org/doi/10.1126/sciadv.abn5644).