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Lithography Technique Precisely Controls Nanodevice Feature Sizes

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HONG KONG, April 19, 2022 — Researchers at the University of Hong Kong (HKU) developed an approach for nanodevice fabrication to increase the efficiency of building nanostructures that require spatially different dimensions. The researchers devised and demonstrated a practical, scalable approach to nanolithography that combines interference lithography and grayscale-patterned secondary exposure (IL-GPSE). The technique enables high-throughput nanopatterning on a wafer-scale area as well as on-demand spatial modulation of nanostructures.

In nanodevice fabrication, some applications require nanostructures with uniform feature sizes, while others require structures with spatially varying morphologies. In applications in optoelectronics, plasmonics, meta-optics, biosensing, and other technologies, precise control of feature size is essential.

In its approach, the team used high-throughput interference lithography (IL) to efficiently fabricate large-area periodic nanostructures. IL exposes a large-area periodic nanoscale pattern on the photoresist substrate. The researchers then applied a secondary exposure (SE) of UV light, which carries a designed intensity distribution of a grayscale pattern, to the IL-exposed photoresist. This step allows the researchers to spatially modulate the feature sizes of individual nanostructures.

To support wafer-scale nanostructure patterning with improved uniformity, IL-GPSE compensates for the linewidth variation caused by the nonuniform IL exposure field by using a specially designed SE intensity distribution.

The researchers tested the reliability of their technique through experiments and numerical simulation. Using UV contact photolithography, maskless projection photolithography, and direct laser writing for SE, they demonstrated feature size modulation with submicron resolution and for a wafer-scale area.

a-d: A comparison of nanogratings on 4-inch wafer fabricated by IL only, and by IL-GPSE, indicating that IL-GPSE allows wafer-scale nanostructure patterning with improved uniformity by compensating the linewidth variation caused by the non-uniform IL exposure field. e-f: A demonstration of the 3-inch grayscale painting of Along the River During the Qingming Festival in the photoresist patterned by IL-GPSE. Courtesy of  Zhuofei Gan, Hongtao Feng, Liyang Chen, Siyi Min, Chuwei Liang, Menghong Xu, Zijie Jiang, Zhao Sun, Chuying Sun, Dehu Cui, and Wen-Di Li.
(a-d): A comparison of nanogratings on 4-in. wafer fabricated by IL only, and by IL-GPSE, indicating that IL-GPSE allows wafer-scale nanostructure patterning with improved uniformity by compensating the linewidth variation caused by the nonuniform IL exposure field. (e-f): A demonstration of the 3-in. grayscale of a painting in the photoresist patterned by IL-GPSE. Courtesy of  Zhuofei Gan, Hongtao Feng, Liyang Chen, Siyi Min, Chuwei Liang, Menghong Xu, Zijie Jiang, Zhao Sun, Chuying Sun, Dehu Cui, and Wen-Di Li.
The team demonstrated the successful fabrication of 4-in. wafer-scale nanogratings with uniform, 125-nm linewidths with less than 5% variation, using grayscale-patterned SE to compensate for the linewidth difference caused by the Gaussian distribution of the laser beams in the IL. The linewidth uniformity showed an improvement of 1100% over IL-only exposure.


Additionally, the researchers demonstrated a 3-in. wafer-scale structural color painting by spatially modulating the filling ratio of the nanostructures to achieve gradient grayscale color using SE. IL-GPSE enabled precise grayscale control, demonstrating that it could be used for potential applications in flat optical devices and structural color encryption.

The researchers said that traditional methods for controlling feature size in nanostructures are unable to simultaneously provide high throughput, control over a large area, and precise feature size control in nanopatterning. 

The IL-GPSE process portfolio could significantly improve the process of fabricating nanostructures that require spatially varying dimensions.

“Wafer-scale nanodevices fabricated by the presented technique can benefit a wide variety of applications,” the researchers said. “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.”

The IL-GPSE lithographic portfolio could also be applied to the fabrication of metasurfaces and metalenses that employ nanostructures with spatially modulated filling ratios. By separating the high-resolution patterning of metasurface building blocks and the size modulation, IL-GPSE has the potential to improve the patterning efficiency for these devices by orders of magnitude, compared to electron beam lithography.

The research was led by HKU professor Wen-Di Li. The research was published in Light: Science & Applications (www./doi.org/10.1038/s41377-022-00774-z).

Published: April 2022
Glossary
nano
An SI prefix meaning one billionth (10-9). Nano can also be used to indicate the study of atoms, molecules and other structures and particles on the nanometer scale. Nano-optics (also referred to as nanophotonics), for example, is the study of how light and light-matter interactions behave on the nanometer scale. See nanophotonics.
metasurfaces
Metasurfaces are two-dimensional arrays of subwavelength-scale artificial structures, often referred to as meta-atoms or meta-elements, arranged in a specific pattern to manipulate the propagation of light or other electromagnetic waves at subwavelength scales. These structures can control the phase, amplitude, and polarization of incident light across a planar surface, enabling unprecedented control over the wavefront of light. Key features and characteristics of metasurfaces include: ...
lithography
Lithography is a key process used in microfabrication and semiconductor manufacturing to create intricate patterns on the surface of substrates, typically silicon wafers. It involves the transfer of a desired pattern onto a photosensitive material called a resist, which is coated onto the substrate. The resist is then selectively exposed to light or other radiation using a mask or reticle that contains the pattern of interest. The lithography process can be broadly categorized into several...
wafer
In the context of electronics and semiconductor manufacturing, a wafer refers to a thin, flat disk or substrate made of a semiconducting material, usually crystalline silicon. Wafers serve as the foundation for the fabrication of integrated circuits (ICs), microelectromechanical systems (MEMS), and other microdevices. Here are key points regarding wafers: Material: Silicon is the most commonly used material for wafer fabrication due to its excellent semiconductor properties, high purity,...
photoresist
Photoresist is a light-sensitive material used in photolithography processes, particularly in the fabrication of semiconductor devices, integrated circuits, and microelectromechanical systems (MEMS). It is a crucial component in the patterning of semiconductor wafers during the manufacturing process. The primary function of photoresist is to undergo a chemical or physical change when exposed to light, making it selectively soluble or insoluble in a subsequent development step. The general...
meta-optics
Meta-optics, also known as metasurface optics or flat optics, is a branch of optics that involves the design, fabrication, and utilization of artificial structures called metasurfaces to control and manipulate light at the nanoscale level. Unlike traditional optics, which typically involve bulky lenses and mirrors, meta-optics aims to achieve similar functionalities using ultrathin, planar structures composed of subwavelength nanostructures. Metasurfaces are typically composed of arrays of...
manufacturingnanonanodevicesdevicesmetasurfacessurfaceslithographynanolithographymetalensesdevice fabricationnanodevice fabricationspatialindustrialwaferOpticsResearch & TechnologyeducationAsia Pacificphotoresistmeta-opticsLight SourcesUniversity of Hong Kong

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